Bpffs is a pseudo file system that persists bpf objects. Previously
bpf objects can only be pinned in bpffs, this patchset extends pinning
to allow bpf objects to be pinned (or exposed) to other file systems.
In particular, this patchset allows pinning bpf objects in kernfs. This
creates a new file entry in the kernfs file system and the created file
is able to reference the bpf object. By doing so, bpf can be used to
customize the file's operations, such as seq_show.
As a concrete usecase of this feature, this patchset introduces a
simple new program type called 'bpf_view', which can be used to format
a seq file by a kernel object's state. By pinning a bpf_view program
into a cgroup directory, userspace is able to read the cgroup's state
from file in a format defined by the bpf program.
Different from bpffs, kernfs doesn't have a callback when a kernfs node
is freed, which is problem if we allow the kernfs node to hold an extra
reference of the bpf object, because there is no chance to dec the
object's refcnt. Therefore the kernfs node created by pinning doesn't
hold reference of the bpf object. The lifetime of the kernfs node
depends on the lifetime of the bpf object. Rather than "pinning in
kernfs", it is "exposing to kernfs". We require the bpf object to be
pinned in bpffs first before it can be pinned in kernfs. When the
object is unpinned from bpffs, their kernfs nodes will be removed
automatically. This somehow treats a pinned bpf object as a persistent
"device".
We rely on fsnotify to monitor the inode events in bpffs. A new function
bpf_watch_inode() is introduced. It allows registering a callback
function at inode destruction. For the kernfs case, a callback that
removes kernfs node is registered at the destruction of bpffs inodes.
For other file systems such as sockfs, bpf_watch_inode() can monitor the
destruction of sockfs inodes and the created file entry can hold the bpf
object's reference. In this case, it is truly "pinning".
File operations other than seq_show can also be implemented using bpf.
For example, bpf may be of help for .poll and .mmap in kernfs.
Patch organization:
- patch 1/8 and 2/8 are preparations. 1/8 implements bpf_watch_inode();
2/8 records bpffs inode in bpf object.
- patch 3/8 and 4/8 implement generic logic for creating bpf backed
kernfs file.
- patch 5/8 and 6/8 add a new program type for formatting output.
- patch 7/8 implements cgroup seq_show operation using bpf.
- patch 8/8 adds selftest.
Hao Luo (8):
bpf: Support pinning in non-bpf file system.
bpf: Record back pointer to the inode in bpffs
bpf: Expose bpf object in kernfs
bpf: Support removing kernfs entries
bpf: Introduce a new program type bpf_view.
libbpf: Support of bpf_view prog type.
bpf: Add seq_show operation for bpf in cgroupfs
selftests/bpf: Test exposing bpf objects in kernfs
include/linux/bpf.h | 9 +-
include/uapi/linux/bpf.h | 2 +
kernel/bpf/Makefile | 2 +-
kernel/bpf/bpf_view.c | 190 ++++++++++++++
kernel/bpf/bpf_view.h | 25 ++
kernel/bpf/inode.c | 219 ++++++++++++++--
kernel/bpf/inode.h | 54 ++++
kernel/bpf/kernfs_node.c | 165 ++++++++++++
kernel/bpf/syscall.c | 3 +
kernel/bpf/verifier.c | 6 +
kernel/trace/bpf_trace.c | 12 +-
tools/include/uapi/linux/bpf.h | 2 +
tools/lib/bpf/libbpf.c | 21 ++
.../selftests/bpf/prog_tests/pinning_kernfs.c | 245 ++++++++++++++++++
.../selftests/bpf/progs/pinning_kernfs.c | 72 +++++
15 files changed, 995 insertions(+), 32 deletions(-)
create mode 100644 kernel/bpf/bpf_view.c
create mode 100644 kernel/bpf/bpf_view.h
create mode 100644 kernel/bpf/inode.h
create mode 100644 kernel/bpf/kernfs_node.c
create mode 100644 tools/testing/selftests/bpf/prog_tests/pinning_kernfs.c
create mode 100644 tools/testing/selftests/bpf/progs/pinning_kernfs.c
--
2.34.1.448.ga2b2bfdf31-goog
Introduce a new API called bpf_watch_inode() to watch the destruction of
an inode and calls a registered callback function. With the help of this
new API, one can implement pinning bpf objects in a non-bpf file system
such as sockfs. The ability of pinning bpf objects in an external file
system has potential uses: for example, allow using bpf programs to
customize file behaviors, as we can see in the following patches.
Extending the pinning logic in bpf_obj_do_pin() to associate bpf objects
to inodes of another file system is relatively straightforward. The
challenge is how to notify the bpf object when the associated inode is
gone so that the object's refcnt can be decremented at that time. Bpffs
uses .free_inode() callback in super_operations to drop object's refcnt.
But not every file system implements .free_inode() and inserting bpf
notification to every target file system can be too instrusive.
Thanks to fsnotify, there is a generic callback in VFS that can be
used to notify the events of an inode. bpf_watch_inode() implements on
top of that. bpf_watch_inode() allows the caller to pass in a callback
(for example, decrementing an object's refcnt), which will be called
when the inode is about to be freed. So typically, one can implement
exposing bpf objects to other file systems in the following steps:
1. extend bpf_obj_do_pin() to create a new entry in the target file
system.
2. call bpf_watch_inode() to register bpf object put operation at
the destruction of the newly created inode.
Of course, on a system with no fsnotify support, pinning bpf object in
non-bpf file system will not be available.
Signed-off-by: Hao Luo <[email protected]>
---
kernel/bpf/inode.c | 118 ++++++++++++++++++++++++++++++++++++++++-----
kernel/bpf/inode.h | 33 +++++++++++++
2 files changed, 140 insertions(+), 11 deletions(-)
create mode 100644 kernel/bpf/inode.h
diff --git a/kernel/bpf/inode.c b/kernel/bpf/inode.c
index 80da1db47c68..b4066dd986a8 100644
--- a/kernel/bpf/inode.c
+++ b/kernel/bpf/inode.c
@@ -16,18 +16,13 @@
#include <linux/fs.h>
#include <linux/fs_context.h>
#include <linux/fs_parser.h>
+#include <linux/fsnotify_backend.h>
#include <linux/kdev_t.h>
#include <linux/filter.h>
#include <linux/bpf.h>
#include <linux/bpf_trace.h>
#include "preload/bpf_preload.h"
-
-enum bpf_type {
- BPF_TYPE_UNSPEC = 0,
- BPF_TYPE_PROG,
- BPF_TYPE_MAP,
- BPF_TYPE_LINK,
-};
+#include "inode.h"
static void *bpf_any_get(void *raw, enum bpf_type type)
{
@@ -67,6 +62,95 @@ static void bpf_any_put(void *raw, enum bpf_type type)
}
}
+#ifdef CONFIG_FSNOTIFY
+/* Notification mechanism based on fsnotify, used in bpf to watch the
+ * destruction of an inode. This inode could an inode in bpffs or any
+ * other file system.
+ */
+
+struct notify_mark {
+ struct fsnotify_mark fsn_mark;
+ const struct notify_ops *ops;
+ void *object;
+ enum bpf_type type;
+ void *priv;
+};
+
+struct fsnotify_group *bpf_notify_group;
+struct kmem_cache *bpf_notify_mark_cachep __read_mostly;
+
+/* Handler for any inode event. */
+int handle_inode_event(struct fsnotify_mark *mark, u32 mask,
+ struct inode *inode, struct inode *dir,
+ const struct qstr *file_name, u32 cookie)
+{
+ return 0;
+}
+
+/* Handler for freeing marks. This is called when the watched inode is being
+ * freed.
+ */
+static void notify_freeing_mark(struct fsnotify_mark *mark, struct fsnotify_group *group)
+{
+ struct notify_mark *b_mark;
+
+ b_mark = container_of(mark, struct notify_mark, fsn_mark);
+
+ if (b_mark->ops && b_mark->ops->free_inode)
+ b_mark->ops->free_inode(b_mark->object, b_mark->type, b_mark->priv);
+}
+
+static void notify_free_mark(struct fsnotify_mark *mark)
+{
+ struct notify_mark *b_mark;
+
+ b_mark = container_of(mark, struct notify_mark, fsn_mark);
+
+ kmem_cache_free(bpf_notify_mark_cachep, b_mark);
+}
+
+struct fsnotify_ops bpf_notify_ops = {
+ .handle_inode_event = handle_inode_event,
+ .freeing_mark = notify_freeing_mark,
+ .free_mark = notify_free_mark,
+};
+
+static int bpf_inode_type(const struct inode *inode, enum bpf_type *type);
+
+/* Watch the destruction of an inode and calls the callbacks in the given
+ * notify_ops.
+ */
+int bpf_watch_inode(struct inode *inode, const struct notify_ops *ops, void *priv)
+{
+ enum bpf_type type;
+ struct notify_mark *b_mark;
+ int ret;
+
+ if (IS_ERR(bpf_notify_group) || unlikely(!bpf_notify_mark_cachep))
+ return -ENOMEM;
+
+ b_mark = kmem_cache_alloc(bpf_notify_mark_cachep, GFP_KERNEL_ACCOUNT);
+ if (unlikely(!b_mark))
+ return -ENOMEM;
+
+ fsnotify_init_mark(&b_mark->fsn_mark, bpf_notify_group);
+ b_mark->ops = ops;
+ b_mark->priv = priv;
+ b_mark->object = inode->i_private;
+ bpf_inode_type(inode, &type);
+ b_mark->type = type;
+
+ ret = fsnotify_add_inode_mark(&b_mark->fsn_mark, inode,
+ /*allow_dups=*/1);
+
+ fsnotify_put_mark(&b_mark->fsn_mark); /* match get in fsnotify_init_mark */
+
+ return ret;
+}
+#endif
+
+/* bpffs */
+
static void *bpf_fd_probe_obj(u32 ufd, enum bpf_type *type)
{
void *raw;
@@ -435,11 +519,15 @@ static int bpf_iter_link_pin_kernel(struct dentry *parent,
return ret;
}
+static bool dentry_is_bpf_dir(struct dentry *dentry)
+{
+ return d_inode(dentry)->i_op == &bpf_dir_iops;
+}
+
static int bpf_obj_do_pin(const char __user *pathname, void *raw,
enum bpf_type type)
{
struct dentry *dentry;
- struct inode *dir;
struct path path;
umode_t mode;
int ret;
@@ -454,8 +542,7 @@ static int bpf_obj_do_pin(const char __user *pathname, void *raw,
if (ret)
goto out;
- dir = d_inode(path.dentry);
- if (dir->i_op != &bpf_dir_iops) {
+ if (!dentry_is_bpf_dir(path.dentry)) {
ret = -EPERM;
goto out;
}
@@ -821,8 +908,17 @@ static int __init bpf_init(void)
return ret;
ret = register_filesystem(&bpf_fs_type);
- if (ret)
+ if (ret) {
sysfs_remove_mount_point(fs_kobj, "bpf");
+ return ret;
+ }
+
+#ifdef CONFIG_FSNOTIFY
+ bpf_notify_mark_cachep = KMEM_CACHE(notify_mark, 0);
+ bpf_notify_group = fsnotify_alloc_group(&bpf_notify_ops);
+ if (IS_ERR(bpf_notify_group) || !bpf_notify_mark_cachep)
+ pr_warn("Failed to initialize bpf_notify system, user can not pin objects outside bpffs.\n");
+#endif
return ret;
}
diff --git a/kernel/bpf/inode.h b/kernel/bpf/inode.h
new file mode 100644
index 000000000000..3f53a4542028
--- /dev/null
+++ b/kernel/bpf/inode.h
@@ -0,0 +1,33 @@
+/* SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause) */
+/* Copyright (c) 2022 Google
+ */
+#ifndef __BPF_INODE_H_
+#define __BPF_INODE_H_
+
+enum bpf_type {
+ BPF_TYPE_UNSPEC = 0,
+ BPF_TYPE_PROG,
+ BPF_TYPE_MAP,
+ BPF_TYPE_LINK,
+};
+
+struct notify_ops {
+ void (*free_inode)(void *object, enum bpf_type type, void *priv);
+};
+
+#ifdef CONFIG_FSNOTIFY
+/* Watch the destruction of an inode and calls the callbacks in the given
+ * notify_ops.
+ */
+int bpf_watch_inode(struct inode *inode, const struct notify_ops *ops,
+ void *priv);
+#else
+static inline
+int bpf_watch_inode(struct inode *inode, const struct notify_ops *ops,
+ void *priv)
+{
+ return -EPERM;
+}
+#endif // CONFIG_FSNOTIFY
+
+#endif // __BPF_INODE_H_
--
2.34.1.448.ga2b2bfdf31-goog
When an object is pinned in bpffs, record the bpffs inode in the object.
The previous patch introduced bpf_watch_inode(), which can also be used
to watch the bpffs inode. This capability will be used in the following
patches to expose bpf objects to file systems where the nodes in the
file system are not backed by an inode.
Signed-off-by: Hao Luo <[email protected]>
---
include/linux/bpf.h | 5 +++-
kernel/bpf/inode.c | 60 ++++++++++++++++++++++++++++++++++++++++++++-
kernel/bpf/inode.h | 9 +++++++
3 files changed, 72 insertions(+), 2 deletions(-)
diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 6e947cd91152..2ec693c3d6f6 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -184,7 +184,8 @@ struct bpf_map {
char name[BPF_OBJ_NAME_LEN];
bool bypass_spec_v1;
bool frozen; /* write-once; write-protected by freeze_mutex */
- /* 14 bytes hole */
+ struct inode *backing_inode; /* back pointer to the inode in bpffs */
+ /* 6 bytes hole */
/* The 3rd and 4th cacheline with misc members to avoid false sharing
* particularly with refcounting.
@@ -991,6 +992,7 @@ struct bpf_prog_aux {
struct work_struct work;
struct rcu_head rcu;
};
+ struct inode *backing_inode; /* back pointer to the inode in bpffs */
};
struct bpf_array_aux {
@@ -1018,6 +1020,7 @@ struct bpf_link {
const struct bpf_link_ops *ops;
struct bpf_prog *prog;
struct work_struct work;
+ struct inode *backing_inode; /* back pointer to the inode in bpffs */
};
struct bpf_link_ops {
diff --git a/kernel/bpf/inode.c b/kernel/bpf/inode.c
index b4066dd986a8..9ba10912cbf8 100644
--- a/kernel/bpf/inode.c
+++ b/kernel/bpf/inode.c
@@ -226,6 +226,57 @@ static int bpf_inode_type(const struct inode *inode, enum bpf_type *type)
return 0;
}
+/* Conditionally set an object's backing inode. */
+static void cond_set_backing_inode(void *obj, enum bpf_type type,
+ struct inode *old, struct inode *new)
+{
+ struct inode **ptr;
+
+ if (type == BPF_TYPE_PROG) {
+ struct bpf_prog *prog = obj;
+ ptr = &prog->aux->backing_inode;
+ } else if (type == BPF_TYPE_MAP) {
+ struct bpf_map *map = obj;
+ ptr = &map->backing_inode;
+ } else if (type == BPF_TYPE_LINK) {
+ struct bpf_link *link = obj;
+ ptr = &link->backing_inode;
+ } else {
+ return;
+ }
+
+ if (*ptr == old)
+ *ptr = new;
+}
+
+struct inode *get_backing_inode(void *obj, enum bpf_type type)
+{
+ struct inode *inode = NULL;
+
+ if (type == BPF_TYPE_PROG) {
+ struct bpf_prog *prog = obj;
+ inode = prog->aux->backing_inode;
+ } else if (type == BPF_TYPE_MAP) {
+ struct bpf_map *map = obj;
+ inode = map->backing_inode;
+ } else if (type == BPF_TYPE_LINK) {
+ struct bpf_link *link = obj;
+ inode = link->backing_inode;
+ }
+
+ if (!inode)
+ return NULL;
+
+ spin_lock(&inode->i_lock);
+ if (inode->i_state & (I_FREEING | I_WILL_FREE | I_NEW)) {
+ spin_unlock(&inode->i_lock);
+ return NULL;
+ }
+ __iget(inode);
+ spin_unlock(&inode->i_lock);
+ return inode;
+}
+
static void bpf_dentry_finalize(struct dentry *dentry, struct inode *inode,
struct inode *dir)
{
@@ -418,6 +469,8 @@ static int bpf_mkobj_ops(struct dentry *dentry, umode_t mode, void *raw,
{
struct inode *dir = dentry->d_parent->d_inode;
struct inode *inode = bpf_get_inode(dir->i_sb, dir, mode);
+ enum bpf_type type;
+
if (IS_ERR(inode))
return PTR_ERR(inode);
@@ -425,6 +478,9 @@ static int bpf_mkobj_ops(struct dentry *dentry, umode_t mode, void *raw,
inode->i_fop = fops;
inode->i_private = raw;
+ if (!bpf_inode_type(inode, &type))
+ cond_set_backing_inode(raw, type, NULL, inode);
+
bpf_dentry_finalize(dentry, inode, dir);
return 0;
}
@@ -703,8 +759,10 @@ static void bpf_free_inode(struct inode *inode)
if (S_ISLNK(inode->i_mode))
kfree(inode->i_link);
- if (!bpf_inode_type(inode, &type))
+ if (!bpf_inode_type(inode, &type)) {
+ cond_set_backing_inode(inode->i_private, type, inode, NULL);
bpf_any_put(inode->i_private, type);
+ }
free_inode_nonrcu(inode);
}
diff --git a/kernel/bpf/inode.h b/kernel/bpf/inode.h
index 3f53a4542028..e7fe8137be80 100644
--- a/kernel/bpf/inode.h
+++ b/kernel/bpf/inode.h
@@ -30,4 +30,13 @@ int bpf_watch_inode(struct inode *inode, const struct notify_ops *ops,
}
#endif // CONFIG_FSNOTIFY
+/* Get the backing inode of a bpf object. When an object is pinned in bpf
+ * file system, an inode is associated with the object. This function returns
+ * that inode.
+ *
+ * On success, the inode is returned with refcnt incremented.
+ * On failure, NULL is returned.
+ */
+struct inode *get_backing_inode(void *obj, enum bpf_type);
+
#endif // __BPF_INODE_H_
--
2.34.1.448.ga2b2bfdf31-goog
This patch extends bpf_obj_do_pin() to allow creating a new entry in
kernfs which references a bpf object. Different from pinning objects
in bpffs, the created kernfs node does not hold an extra reference to
the object, because kernfs by itself doesn't have a notification
mechanism to put the object when the kernfs node is gone. Therefore
this patch is not "pinning" the object, but rather "exposing" the
object in kernfs. The lifetime of the created kernfs node depends on
the lifetime of the bpf object, not the other way around.
More specifically, we allow a bpf object to be exposed to kernfs only
after it becomes "persistent" by pinning in bpffs. So the lifetime of
the created kernfs node is tied to the bpffs inode. When the object
is unpinned from bpffs, the kernfs nodes exposing the bpf object will
be removed automatically. It uses the bpf_watch_inode() interface
introduced in the previous patches. Because the kernfs nodes do not
hold extra references to the object, we can remove the nodes at any
time without worrying about reference leak.
Signed-off-by: Hao Luo <[email protected]>
---
kernel/bpf/Makefile | 2 +-
kernel/bpf/inode.c | 43 +++++++++++++-------
kernel/bpf/inode.h | 11 ++++-
kernel/bpf/kernfs_node.c | 87 ++++++++++++++++++++++++++++++++++++++++
4 files changed, 126 insertions(+), 17 deletions(-)
create mode 100644 kernel/bpf/kernfs_node.c
diff --git a/kernel/bpf/Makefile b/kernel/bpf/Makefile
index c1a9be6a4b9f..b1abf0d94b5b 100644
--- a/kernel/bpf/Makefile
+++ b/kernel/bpf/Makefile
@@ -8,7 +8,7 @@ CFLAGS_core.o += $(call cc-disable-warning, override-init) $(cflags-nogcse-yy)
obj-$(CONFIG_BPF_SYSCALL) += syscall.o verifier.o inode.o helpers.o tnum.o bpf_iter.o map_iter.o task_iter.o prog_iter.o
obj-$(CONFIG_BPF_SYSCALL) += hashtab.o arraymap.o percpu_freelist.o bpf_lru_list.o lpm_trie.o map_in_map.o bloom_filter.o
-obj-$(CONFIG_BPF_SYSCALL) += local_storage.o queue_stack_maps.o ringbuf.o
+obj-$(CONFIG_BPF_SYSCALL) += local_storage.o queue_stack_maps.o ringbuf.o kernfs_node.o
obj-$(CONFIG_BPF_SYSCALL) += bpf_local_storage.o bpf_task_storage.o
obj-${CONFIG_BPF_LSM} += bpf_inode_storage.o
obj-$(CONFIG_BPF_SYSCALL) += disasm.o
diff --git a/kernel/bpf/inode.c b/kernel/bpf/inode.c
index 9ba10912cbf8..7e93e477b57c 100644
--- a/kernel/bpf/inode.c
+++ b/kernel/bpf/inode.c
@@ -580,6 +580,21 @@ static bool dentry_is_bpf_dir(struct dentry *dentry)
return d_inode(dentry)->i_op == &bpf_dir_iops;
}
+static int bpf_obj_do_pin_generic(struct dentry *dentry, umode_t mode,
+ void *obj, enum bpf_type type)
+{
+ switch (type) {
+ case BPF_TYPE_PROG:
+ return vfs_mkobj(dentry, mode, bpf_mkprog, obj);
+ case BPF_TYPE_MAP:
+ return vfs_mkobj(dentry, mode, bpf_mkmap, obj);
+ case BPF_TYPE_LINK:
+ return vfs_mkobj(dentry, mode, bpf_mklink, obj);
+ default:
+ return -EPERM;
+ }
+}
+
static int bpf_obj_do_pin(const char __user *pathname, void *raw,
enum bpf_type type)
{
@@ -598,22 +613,20 @@ static int bpf_obj_do_pin(const char __user *pathname, void *raw,
if (ret)
goto out;
- if (!dentry_is_bpf_dir(path.dentry)) {
- ret = -EPERM;
- goto out;
- }
+ if (dentry_is_kernfs_dir(path.dentry)) {
+ ret = bpf_obj_do_pin_kernfs(dentry, mode, raw, type);
- switch (type) {
- case BPF_TYPE_PROG:
- ret = vfs_mkobj(dentry, mode, bpf_mkprog, raw);
- break;
- case BPF_TYPE_MAP:
- ret = vfs_mkobj(dentry, mode, bpf_mkmap, raw);
- break;
- case BPF_TYPE_LINK:
- ret = vfs_mkobj(dentry, mode, bpf_mklink, raw);
- break;
- default:
+ /* Match bpf_fd_probe_obj(). bpf objects exposed to kernfs
+ * do not hold an active reference. The lifetime of the
+ * created kernfs node is tied to an inode in bpffs. So the
+ * kernfs node gets destroyed automatically when the object
+ * is unpinned from bpffs.
+ */
+ if (ret == 0)
+ bpf_any_put(raw, type);
+ } else if (dentry_is_bpf_dir(path.dentry)) {
+ ret = bpf_obj_do_pin_generic(dentry, mode, raw, type);
+ } else {
ret = -EPERM;
}
out:
diff --git a/kernel/bpf/inode.h b/kernel/bpf/inode.h
index e7fe8137be80..c12d385a3e2a 100644
--- a/kernel/bpf/inode.h
+++ b/kernel/bpf/inode.h
@@ -4,8 +4,10 @@
#ifndef __BPF_INODE_H_
#define __BPF_INODE_H_
+#include <linux/fs.h>
+
enum bpf_type {
- BPF_TYPE_UNSPEC = 0,
+ BPF_TYPE_UNSPEC = 0,
BPF_TYPE_PROG,
BPF_TYPE_MAP,
BPF_TYPE_LINK,
@@ -39,4 +41,11 @@ int bpf_watch_inode(struct inode *inode, const struct notify_ops *ops,
*/
struct inode *get_backing_inode(void *obj, enum bpf_type);
+/* Test whether a given dentry is a kernfs entry. */
+bool dentry_is_kernfs_dir(struct dentry *dentry);
+
+/* Expose bpf object to kernfs. Requires dentry to be in kernfs. */
+int bpf_obj_do_pin_kernfs(struct dentry *dentry, umode_t mode, void *obj,
+ enum bpf_type type);
+
#endif // __BPF_INODE_H_
diff --git a/kernel/bpf/kernfs_node.c b/kernel/bpf/kernfs_node.c
new file mode 100644
index 000000000000..c1c45f7b948b
--- /dev/null
+++ b/kernel/bpf/kernfs_node.c
@@ -0,0 +1,87 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Expose eBPF objects in kernfs file system.
+ */
+
+#include <linux/fs.h>
+#include <linux/kernfs.h>
+#include "inode.h"
+
+/* file_operations for kernfs file system */
+
+/* Handler when the watched inode is freed. */
+static void kn_watch_free_inode(void *obj, enum bpf_type type, void *kn)
+{
+ kernfs_remove(kn);
+
+ /* match get in bpf_obj_do_pin_kernfs */
+ kernfs_put(kn);
+}
+
+static const struct notify_ops notify_ops = {
+ .free_inode = kn_watch_free_inode,
+};
+
+/* Kernfs file operations for bpf created files. */
+static const struct kernfs_ops bpf_generic_ops = {
+};
+
+/* Test whether a given dentry is a kernfs entry. */
+bool dentry_is_kernfs_dir(struct dentry *dentry)
+{
+ return kernfs_node_from_dentry(dentry) != NULL;
+}
+
+/* Expose bpf object to kernfs. Requires dentry to exist in kernfs. */
+int bpf_obj_do_pin_kernfs(struct dentry *dentry, umode_t mode, void *obj,
+ enum bpf_type type)
+{
+ struct dentry *parent_dentry;
+ struct super_block *sb;
+ struct kernfs_node *parent_kn, *kn;
+ struct kernfs_root *root;
+ const struct kernfs_ops *ops;
+ struct inode *inode;
+ int ret;
+
+ sb = dentry->d_sb;
+ root = kernfs_root_from_sb(sb);
+ if (!root) /* Not a kernfs file system. */
+ return -EPERM;
+
+ parent_dentry = dentry->d_parent;
+ parent_kn = kernfs_node_from_dentry(parent_dentry);
+ if (WARN_ON(!parent_kn))
+ return -EPERM;
+
+ inode = get_backing_inode(obj, type);
+ if (!inode)
+ return -ENXIO;
+
+ ops = &bpf_generic_ops;
+ kn = __kernfs_create_file(parent_kn, dentry->d_iname, mode,
+ GLOBAL_ROOT_UID, GLOBAL_ROOT_GID,
+ 0, ops, inode, NULL, NULL);
+ if (IS_ERR(kn)) {
+ iput(inode);
+ return PTR_ERR(kn);
+ }
+
+ /* hold an active kn by bpffs inode. */
+ kernfs_get(kn);
+
+ /* Watch the backing inode of the object in bpffs. When the backing
+ * inode is freed, the created kernfs entry will be removed as well.
+ */
+ ret = bpf_watch_inode(inode, ¬ify_ops, kn);
+ if (ret) {
+ kernfs_put(kn);
+ kernfs_remove(kn);
+ iput(inode);
+ return ret;
+ }
+
+ kernfs_activate(kn);
+ iput(inode);
+ return 0;
+}
--
2.34.1.448.ga2b2bfdf31-goog
When a bpf object has been exposed in kernfs, there should be a way
to remove it. Kernfs doesn't implement unlink, therefore one can not
remove the entry in a normal way. To remove the file, we can allow
writing a special command to the new entry, which can trigger a
remove_self() for removal.
So far there are two ways to remove an entry that is created by pinning
bpf objects in kernfs:
1. unpin the object from bpffs.
2. write a special command to the kernfs entry.
Signed-off-by: Hao Luo <[email protected]>
---
kernel/bpf/kernfs_node.c | 22 ++++++++++++++++++++++
1 file changed, 22 insertions(+)
diff --git a/kernel/bpf/kernfs_node.c b/kernel/bpf/kernfs_node.c
index c1c45f7b948b..3d331d8357db 100644
--- a/kernel/bpf/kernfs_node.c
+++ b/kernel/bpf/kernfs_node.c
@@ -9,6 +9,9 @@
/* file_operations for kernfs file system */
+/* Command for removing a kernfs entry */
+#define REMOVE_CMD "rm"
+
/* Handler when the watched inode is freed. */
static void kn_watch_free_inode(void *obj, enum bpf_type type, void *kn)
{
@@ -22,8 +25,27 @@ static const struct notify_ops notify_ops = {
.free_inode = kn_watch_free_inode,
};
+static ssize_t bpf_generic_write(struct kernfs_open_file *of, char *buf,
+ size_t bytes, loff_t off)
+{
+ if (sysfs_streq(buf, REMOVE_CMD)) {
+ kernfs_remove_self(of->kn);
+ return bytes;
+ }
+
+ return -EINVAL;
+}
+
+static ssize_t bpf_generic_read(struct kernfs_open_file *of, char *buf,
+ size_t bytes, loff_t off)
+{
+ return -EIO;
+}
+
/* Kernfs file operations for bpf created files. */
static const struct kernfs_ops bpf_generic_ops = {
+ .write = bpf_generic_write,
+ .read = bpf_generic_read,
};
/* Test whether a given dentry is a kernfs entry. */
--
2.34.1.448.ga2b2bfdf31-goog
Introduce a new program type called "bpf_view", which can be used to
print out a kernel object's state to a seq file. So the signature of
this program consists of two parameters: a seq file and a kernel object.
Currently only 'struct cgroup' is supported.
The following patches will introduce a call site for this program type
and allow users to customize the format of printing out the state of
kernel objects to userspace.
Signed-off-by: Hao Luo <[email protected]>
---
include/linux/bpf.h | 4 +
include/uapi/linux/bpf.h | 2 +
kernel/bpf/Makefile | 2 +-
kernel/bpf/bpf_view.c | 179 +++++++++++++++++++++++++++++++++
kernel/bpf/bpf_view.h | 24 +++++
kernel/bpf/syscall.c | 3 +
kernel/bpf/verifier.c | 6 ++
kernel/trace/bpf_trace.c | 12 ++-
tools/include/uapi/linux/bpf.h | 2 +
9 files changed, 230 insertions(+), 4 deletions(-)
create mode 100644 kernel/bpf/bpf_view.c
create mode 100644 kernel/bpf/bpf_view.h
diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 2ec693c3d6f6..16f582dfff7e 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -1622,6 +1622,10 @@ void bpf_iter_map_show_fdinfo(const struct bpf_iter_aux_info *aux,
int bpf_iter_map_fill_link_info(const struct bpf_iter_aux_info *aux,
struct bpf_link_info *info);
+bool bpf_view_prog_supported(struct bpf_prog *prog);
+int bpf_view_link_attach(const union bpf_attr *attr, bpfptr_t uattr,
+ struct bpf_prog *prog);
+
int map_set_for_each_callback_args(struct bpf_verifier_env *env,
struct bpf_func_state *caller,
struct bpf_func_state *callee);
diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index b0383d371b9a..efa0f21d13ba 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -982,6 +982,7 @@ enum bpf_attach_type {
BPF_MODIFY_RETURN,
BPF_LSM_MAC,
BPF_TRACE_ITER,
+ BPF_TRACE_VIEW,
BPF_CGROUP_INET4_GETPEERNAME,
BPF_CGROUP_INET6_GETPEERNAME,
BPF_CGROUP_INET4_GETSOCKNAME,
@@ -1009,6 +1010,7 @@ enum bpf_link_type {
BPF_LINK_TYPE_NETNS = 5,
BPF_LINK_TYPE_XDP = 6,
BPF_LINK_TYPE_PERF_EVENT = 7,
+ BPF_LINK_TYPE_VIEW = 8,
MAX_BPF_LINK_TYPE,
};
diff --git a/kernel/bpf/Makefile b/kernel/bpf/Makefile
index b1abf0d94b5b..c662734d83c5 100644
--- a/kernel/bpf/Makefile
+++ b/kernel/bpf/Makefile
@@ -8,7 +8,7 @@ CFLAGS_core.o += $(call cc-disable-warning, override-init) $(cflags-nogcse-yy)
obj-$(CONFIG_BPF_SYSCALL) += syscall.o verifier.o inode.o helpers.o tnum.o bpf_iter.o map_iter.o task_iter.o prog_iter.o
obj-$(CONFIG_BPF_SYSCALL) += hashtab.o arraymap.o percpu_freelist.o bpf_lru_list.o lpm_trie.o map_in_map.o bloom_filter.o
-obj-$(CONFIG_BPF_SYSCALL) += local_storage.o queue_stack_maps.o ringbuf.o kernfs_node.o
+obj-$(CONFIG_BPF_SYSCALL) += local_storage.o queue_stack_maps.o ringbuf.o kernfs_node.o bpf_view.o
obj-$(CONFIG_BPF_SYSCALL) += bpf_local_storage.o bpf_task_storage.o
obj-${CONFIG_BPF_LSM} += bpf_inode_storage.o
obj-$(CONFIG_BPF_SYSCALL) += disasm.o
diff --git a/kernel/bpf/bpf_view.c b/kernel/bpf/bpf_view.c
new file mode 100644
index 000000000000..967a9240bab4
--- /dev/null
+++ b/kernel/bpf/bpf_view.c
@@ -0,0 +1,179 @@
+// SPDX-License-Identifier: GPL-2.0-only
+#include <linux/bpf.h>
+#include <linux/btf_ids.h>
+#include <linux/cgroup.h>
+#include <linux/filter.h>
+#include "bpf_view.h"
+
+static struct list_head targets = LIST_HEAD_INIT(targets);
+
+/* bpf_view_link operations */
+
+struct bpf_view_target_info {
+ struct list_head list;
+ const char *target;
+ u32 ctx_arg_info_size;
+ struct bpf_ctx_arg_aux ctx_arg_info[BPF_VIEW_CTX_ARG_MAX];
+ u32 btf_id;
+};
+
+struct bpf_view_link {
+ struct bpf_link link;
+ struct bpf_view_target_info *tinfo;
+};
+
+static void bpf_view_link_release(struct bpf_link *link)
+{
+}
+
+static void bpf_view_link_dealloc(struct bpf_link *link)
+{
+ struct bpf_view_link *view_link =
+ container_of(link, struct bpf_view_link, link);
+ kfree(view_link);
+}
+
+static void bpf_view_link_show_fdinfo(const struct bpf_link *link,
+ struct seq_file *seq)
+{
+ struct bpf_view_link *view_link =
+ container_of(link, struct bpf_view_link, link);
+
+ seq_printf(seq, "attach_target:\t%s\n", view_link->tinfo->target);
+}
+
+static const struct bpf_link_ops bpf_view_link_lops = {
+ .release = bpf_view_link_release,
+ .dealloc = bpf_view_link_dealloc,
+ .show_fdinfo = bpf_view_link_show_fdinfo,
+};
+
+bool bpf_link_is_view(struct bpf_link *link)
+{
+ return link->ops == &bpf_view_link_lops;
+}
+
+int bpf_view_link_attach(const union bpf_attr *attr, bpfptr_t uattr,
+ struct bpf_prog *prog)
+{
+ struct bpf_link_primer link_primer;
+ struct bpf_view_target_info *tinfo;
+ struct bpf_view_link *link;
+ u32 prog_btf_id;
+ bool existed = false;
+ int err;
+
+ prog_btf_id = prog->aux->attach_btf_id;
+ list_for_each_entry(tinfo, &targets, list) {
+ if (tinfo->btf_id == prog_btf_id) {
+ existed = true;
+ break;
+ }
+ }
+ if (!existed)
+ return -ENOENT;
+
+ link = kzalloc(sizeof(*link), GFP_USER | __GFP_NOWARN);
+ if (!link)
+ return -ENOMEM;
+
+ bpf_link_init(&link->link, BPF_LINK_TYPE_VIEW, &bpf_view_link_lops, prog);
+ link->tinfo = tinfo;
+ err = bpf_link_prime(&link->link, &link_primer);
+ if (err) {
+ kfree(link);
+ return err;
+ }
+
+ return bpf_link_settle(&link_primer);
+}
+
+int run_view_prog(struct bpf_prog *prog, void *ctx)
+{
+ int ret;
+
+ rcu_read_lock();
+ migrate_disable();
+ ret = bpf_prog_run(prog, ctx);
+ migrate_enable();
+ rcu_read_unlock();
+
+ return ret;
+}
+
+bool bpf_view_prog_supported(struct bpf_prog *prog)
+{
+ const char *attach_fname = prog->aux->attach_func_name;
+ const char *prefix = BPF_VIEW_FUNC_PREFIX;
+ u32 prog_btf_id = prog->aux->attach_btf_id;
+ struct bpf_view_target_info *tinfo;
+ int prefix_len = strlen(prefix);
+ bool supported = false;
+
+ if (strncmp(attach_fname, prefix, prefix_len))
+ return false;
+
+ list_for_each_entry(tinfo, &targets, list) {
+ if (tinfo->btf_id && tinfo->btf_id == prog_btf_id) {
+ supported = true;
+ break;
+ }
+ if (!strcmp(attach_fname + prefix_len, tinfo->target)) {
+ tinfo->btf_id = prog->aux->attach_btf_id;
+ supported = true;
+ break;
+ }
+ }
+ if (supported) {
+ prog->aux->ctx_arg_info_size = tinfo->ctx_arg_info_size;
+ prog->aux->ctx_arg_info = tinfo->ctx_arg_info;
+ }
+ return supported;
+}
+
+/* Generate BTF_IDs */
+BTF_ID_LIST(bpf_view_btf_ids)
+BTF_ID(struct, seq_file)
+BTF_ID(struct, cgroup)
+
+/* Index of bpf_view_btf_ids */
+enum {
+ BTF_ID_SEQ_FILE = 0,
+ BTF_ID_CGROUP,
+};
+
+static void register_bpf_view_target(struct bpf_view_target_info *target,
+ int idx[BPF_VIEW_CTX_ARG_MAX])
+{
+ int i;
+
+ for (i = 0; i < target->ctx_arg_info_size; ++i)
+ target->ctx_arg_info[i].btf_id = bpf_view_btf_ids[idx[i]];
+
+ INIT_LIST_HEAD(&target->list);
+ list_add(&target->list, &targets);
+}
+
+DEFINE_BPF_VIEW_FUNC(cgroup, struct seq_file *seq, struct cgroup *cgroup)
+
+static struct bpf_view_target_info cgroup_view_tinfo = {
+ .target = "cgroup",
+ .ctx_arg_info_size = 2,
+ .ctx_arg_info = {
+ { offsetof(struct bpf_view_cgroup_ctx, seq), PTR_TO_BTF_ID },
+ { offsetof(struct bpf_view_cgroup_ctx, cgroup), PTR_TO_BTF_ID },
+ },
+ .btf_id = 0,
+};
+
+static int __init bpf_view_init(void)
+{
+ int cgroup_view_idx[BPF_VIEW_CTX_ARG_MAX] = {
+ BTF_ID_SEQ_FILE, BTF_ID_CGROUP };
+
+ register_bpf_view_target(&cgroup_view_tinfo, cgroup_view_idx);
+
+ return 0;
+}
+late_initcall(bpf_view_init);
+
diff --git a/kernel/bpf/bpf_view.h b/kernel/bpf/bpf_view.h
new file mode 100644
index 000000000000..1a1110a5727f
--- /dev/null
+++ b/kernel/bpf/bpf_view.h
@@ -0,0 +1,24 @@
+// SPDX-License-Identifier: GPL-2.0-only
+#ifndef _BPF_VIEW_H_
+#define _BPF_VIEW_H_
+
+#include <linux/bpf.h>
+
+#define BPF_VIEW_FUNC_PREFIX "bpf_view_"
+#define DEFINE_BPF_VIEW_FUNC(target, args...) \
+ extern int bpf_view_ ## target(args); \
+ int __init bpf_view_ ## target(args) { return 0; }
+
+#define BPF_VIEW_CTX_ARG_MAX 2
+
+struct bpf_view_cgroup_ctx {
+ __bpf_md_ptr(struct seq_file *, seq);
+ __bpf_md_ptr(struct cgroup *, cgroup);
+};
+
+bool bpf_link_is_view(struct bpf_link *link);
+
+/* Run a bpf_view program */
+int run_view_prog(struct bpf_prog *prog, void *ctx);
+
+#endif // _BPF_VIEW_H_
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index fa4505f9b611..32ac84d3ac0b 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -3175,6 +3175,7 @@ attach_type_to_prog_type(enum bpf_attach_type attach_type)
case BPF_CGROUP_SETSOCKOPT:
return BPF_PROG_TYPE_CGROUP_SOCKOPT;
case BPF_TRACE_ITER:
+ case BPF_TRACE_VIEW:
return BPF_PROG_TYPE_TRACING;
case BPF_SK_LOOKUP:
return BPF_PROG_TYPE_SK_LOOKUP;
@@ -4235,6 +4236,8 @@ static int tracing_bpf_link_attach(const union bpf_attr *attr, bpfptr_t uattr,
if (prog->expected_attach_type == BPF_TRACE_ITER)
return bpf_iter_link_attach(attr, uattr, prog);
+ else if (prog->expected_attach_type == BPF_TRACE_VIEW)
+ return bpf_view_link_attach(attr, uattr, prog);
else if (prog->type == BPF_PROG_TYPE_EXT)
return bpf_tracing_prog_attach(prog,
attr->link_create.target_fd,
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index bfb45381fb3f..ce7816519c93 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -9770,6 +9770,7 @@ static int check_return_code(struct bpf_verifier_env *env)
case BPF_MODIFY_RETURN:
return 0;
case BPF_TRACE_ITER:
+ case BPF_TRACE_VIEW:
break;
default:
return -ENOTSUPP;
@@ -13971,6 +13972,7 @@ int bpf_check_attach_target(struct bpf_verifier_log *log,
break;
case BPF_TRACE_ITER:
+ case BPF_TRACE_VIEW:
if (!btf_type_is_func(t)) {
bpf_log(log, "attach_btf_id %u is not a function\n",
btf_id);
@@ -14147,6 +14149,10 @@ static int check_attach_btf_id(struct bpf_verifier_env *env)
if (!bpf_iter_prog_supported(prog))
return -EINVAL;
return 0;
+ } else if (prog->expected_attach_type == BPF_TRACE_VIEW) {
+ if (!bpf_view_prog_supported(prog))
+ return -EINVAL;
+ return 0;
}
if (prog->type == BPF_PROG_TYPE_LSM) {
diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index 21aa30644219..9413b5af6e2c 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -1630,6 +1630,12 @@ raw_tp_prog_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
}
}
+static inline bool prog_support_seq_helpers(const struct bpf_prog *prog)
+{
+ return prog->expected_attach_type == BPF_TRACE_ITER ||
+ prog->expected_attach_type == BPF_TRACE_VIEW;
+}
+
const struct bpf_func_proto *
tracing_prog_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
{
@@ -1663,15 +1669,15 @@ tracing_prog_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
return &bpf_get_socket_ptr_cookie_proto;
#endif
case BPF_FUNC_seq_printf:
- return prog->expected_attach_type == BPF_TRACE_ITER ?
+ return prog_support_seq_helpers(prog) ?
&bpf_seq_printf_proto :
NULL;
case BPF_FUNC_seq_write:
- return prog->expected_attach_type == BPF_TRACE_ITER ?
+ return prog_support_seq_helpers(prog) ?
&bpf_seq_write_proto :
NULL;
case BPF_FUNC_seq_printf_btf:
- return prog->expected_attach_type == BPF_TRACE_ITER ?
+ return prog_support_seq_helpers(prog) ?
&bpf_seq_printf_btf_proto :
NULL;
case BPF_FUNC_d_path:
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index b0383d371b9a..efa0f21d13ba 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -982,6 +982,7 @@ enum bpf_attach_type {
BPF_MODIFY_RETURN,
BPF_LSM_MAC,
BPF_TRACE_ITER,
+ BPF_TRACE_VIEW,
BPF_CGROUP_INET4_GETPEERNAME,
BPF_CGROUP_INET6_GETPEERNAME,
BPF_CGROUP_INET4_GETSOCKNAME,
@@ -1009,6 +1010,7 @@ enum bpf_link_type {
BPF_LINK_TYPE_NETNS = 5,
BPF_LINK_TYPE_XDP = 6,
BPF_LINK_TYPE_PERF_EVENT = 7,
+ BPF_LINK_TYPE_VIEW = 8,
MAX_BPF_LINK_TYPE,
};
--
2.34.1.448.ga2b2bfdf31-goog
The previous patch introdued a new program type bpf_view. This
patch adds support for bpf_view in libbpf.
Signed-off-by: Hao Luo <[email protected]>
---
tools/lib/bpf/libbpf.c | 21 +++++++++++++++++++++
1 file changed, 21 insertions(+)
diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 7f10dd501a52..0d458e34d82c 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -8570,6 +8570,7 @@ static struct bpf_link *attach_raw_tp(const struct bpf_program *prog, long cooki
static struct bpf_link *attach_trace(const struct bpf_program *prog, long cookie);
static struct bpf_link *attach_lsm(const struct bpf_program *prog, long cookie);
static struct bpf_link *attach_iter(const struct bpf_program *prog, long cookie);
+static struct bpf_link *attach_view(const struct bpf_program *prog, long cookie);
static const struct bpf_sec_def section_defs[] = {
SEC_DEF("socket", SOCKET_FILTER, 0, SEC_NONE | SEC_SLOPPY_PFX),
@@ -8599,6 +8600,7 @@ static const struct bpf_sec_def section_defs[] = {
SEC_DEF("lsm/", LSM, BPF_LSM_MAC, SEC_ATTACH_BTF, attach_lsm),
SEC_DEF("lsm.s/", LSM, BPF_LSM_MAC, SEC_ATTACH_BTF | SEC_SLEEPABLE, attach_lsm),
SEC_DEF("iter/", TRACING, BPF_TRACE_ITER, SEC_ATTACH_BTF, attach_iter),
+ SEC_DEF("view/", TRACING, BPF_TRACE_VIEW, SEC_ATTACH_BTF, attach_view),
SEC_DEF("syscall", SYSCALL, 0, SEC_SLEEPABLE),
SEC_DEF("xdp_devmap/", XDP, BPF_XDP_DEVMAP, SEC_ATTACHABLE),
SEC_DEF("xdp_cpumap/", XDP, BPF_XDP_CPUMAP, SEC_ATTACHABLE),
@@ -8896,6 +8898,7 @@ static int bpf_object__collect_st_ops_relos(struct bpf_object *obj,
#define BTF_TRACE_PREFIX "btf_trace_"
#define BTF_LSM_PREFIX "bpf_lsm_"
#define BTF_ITER_PREFIX "bpf_iter_"
+#define BTF_VIEW_PREFIX "bpf_view_"
#define BTF_MAX_NAME_SIZE 128
void btf_get_kernel_prefix_kind(enum bpf_attach_type attach_type,
@@ -8914,6 +8917,10 @@ void btf_get_kernel_prefix_kind(enum bpf_attach_type attach_type,
*prefix = BTF_ITER_PREFIX;
*kind = BTF_KIND_FUNC;
break;
+ case BPF_TRACE_VIEW:
+ *prefix = BTF_VIEW_PREFIX;
+ *kind = BTF_KIND_FUNC;
+ break;
default:
*prefix = "";
*kind = BTF_KIND_FUNC;
@@ -10575,6 +10582,20 @@ struct bpf_link *bpf_program__attach_freplace(const struct bpf_program *prog,
}
}
+static struct bpf_link *attach_view(const struct bpf_program *prog, long cookie)
+{
+ const char *target_name;
+ const char *prefix = "view/";
+ int btf_id;
+
+ target_name = prog->sec_name + strlen(prefix);
+ btf_id = libbpf_find_vmlinux_btf_id(target_name, BPF_TRACE_VIEW);
+ if (btf_id < 0)
+ return libbpf_err_ptr(btf_id);
+
+ return bpf_program__attach_fd(prog, 0, btf_id, "view");
+}
+
struct bpf_link *
bpf_program__attach_iter(const struct bpf_program *prog,
const struct bpf_iter_attach_opts *opts)
--
2.34.1.448.ga2b2bfdf31-goog
Previous patches allow exposing bpf objects to kernfs file system.
They allow creating file entries in kernfs, which can reference bpf
objects. The referred bpf objects can be used to customize the new
entry's file operations.
In particular, this patch introduces one concrete use case of this
feature. It implements the .seq_show file operation for the cgroup
file system. The seq_show handler takes the bpf object and use it to
format its output seq file. The bpf object needs to be a link to the
newly introduced "bpf_view" program type.
Signed-off-by: Hao Luo <[email protected]>
---
kernel/bpf/bpf_view.c | 11 ++++++++
kernel/bpf/bpf_view.h | 1 +
kernel/bpf/inode.c | 4 +--
kernel/bpf/inode.h | 3 +++
kernel/bpf/kernfs_node.c | 58 +++++++++++++++++++++++++++++++++++++++-
5 files changed, 73 insertions(+), 4 deletions(-)
diff --git a/kernel/bpf/bpf_view.c b/kernel/bpf/bpf_view.c
index 967a9240bab4..8f035c5a9b6a 100644
--- a/kernel/bpf/bpf_view.c
+++ b/kernel/bpf/bpf_view.c
@@ -166,6 +166,17 @@ static struct bpf_view_target_info cgroup_view_tinfo = {
.btf_id = 0,
};
+bool bpf_link_is_cgroup_view(struct bpf_link *link)
+{
+ struct bpf_view_link *view_link;
+
+ if (!bpf_link_is_view(link))
+ return false;
+
+ view_link = container_of(link, struct bpf_view_link, link);
+ return view_link->tinfo == &cgroup_view_tinfo;
+}
+
static int __init bpf_view_init(void)
{
int cgroup_view_idx[BPF_VIEW_CTX_ARG_MAX] = {
diff --git a/kernel/bpf/bpf_view.h b/kernel/bpf/bpf_view.h
index 1a1110a5727f..a02564e529cb 100644
--- a/kernel/bpf/bpf_view.h
+++ b/kernel/bpf/bpf_view.h
@@ -17,6 +17,7 @@ struct bpf_view_cgroup_ctx {
};
bool bpf_link_is_view(struct bpf_link *link);
+bool bpf_link_is_cgroup_view(struct bpf_link *link);
/* Run a bpf_view program */
int run_view_prog(struct bpf_prog *prog, void *ctx);
diff --git a/kernel/bpf/inode.c b/kernel/bpf/inode.c
index 7e93e477b57c..1ae4a7b8c732 100644
--- a/kernel/bpf/inode.c
+++ b/kernel/bpf/inode.c
@@ -115,8 +115,6 @@ struct fsnotify_ops bpf_notify_ops = {
.free_mark = notify_free_mark,
};
-static int bpf_inode_type(const struct inode *inode, enum bpf_type *type);
-
/* Watch the destruction of an inode and calls the callbacks in the given
* notify_ops.
*/
@@ -211,7 +209,7 @@ static struct inode *bpf_get_inode(struct super_block *sb,
return inode;
}
-static int bpf_inode_type(const struct inode *inode, enum bpf_type *type)
+int bpf_inode_type(const struct inode *inode, enum bpf_type *type)
{
*type = BPF_TYPE_UNSPEC;
if (inode->i_op == &bpf_prog_iops)
diff --git a/kernel/bpf/inode.h b/kernel/bpf/inode.h
index c12d385a3e2a..dea78341549b 100644
--- a/kernel/bpf/inode.h
+++ b/kernel/bpf/inode.h
@@ -17,6 +17,9 @@ struct notify_ops {
void (*free_inode)(void *object, enum bpf_type type, void *priv);
};
+/* Get the type of bpf object from bpffs inode. */
+int bpf_inode_type(const struct inode *inode, enum bpf_type *type);
+
#ifdef CONFIG_FSNOTIFY
/* Watch the destruction of an inode and calls the callbacks in the given
* notify_ops.
diff --git a/kernel/bpf/kernfs_node.c b/kernel/bpf/kernfs_node.c
index 3d331d8357db..7b58bfc1951e 100644
--- a/kernel/bpf/kernfs_node.c
+++ b/kernel/bpf/kernfs_node.c
@@ -3,15 +3,33 @@
* Expose eBPF objects in kernfs file system.
*/
+#include <linux/bpf.h>
#include <linux/fs.h>
#include <linux/kernfs.h>
+#include <linux/btf_ids.h>
+#include <linux/magic.h>
+#include <linux/seq_file.h>
#include "inode.h"
+#include "bpf_view.h"
/* file_operations for kernfs file system */
/* Command for removing a kernfs entry */
#define REMOVE_CMD "rm"
+static const struct kernfs_ops bpf_generic_ops;
+static const struct kernfs_ops bpf_cgroup_ops;
+
+/* Choose the right kernfs_ops for different kernfs. */
+static const struct kernfs_ops *bpf_kernfs_ops(struct super_block *sb)
+{
+ if (sb->s_magic == CGROUP_SUPER_MAGIC ||
+ sb->s_magic == CGROUP2_SUPER_MAGIC)
+ return &bpf_cgroup_ops;
+
+ return &bpf_generic_ops;
+}
+
/* Handler when the watched inode is freed. */
static void kn_watch_free_inode(void *obj, enum bpf_type type, void *kn)
{
@@ -80,7 +98,7 @@ int bpf_obj_do_pin_kernfs(struct dentry *dentry, umode_t mode, void *obj,
if (!inode)
return -ENXIO;
- ops = &bpf_generic_ops;
+ ops = bpf_kernfs_ops(sb);
kn = __kernfs_create_file(parent_kn, dentry->d_iname, mode,
GLOBAL_ROOT_UID, GLOBAL_ROOT_GID,
0, ops, inode, NULL, NULL);
@@ -107,3 +125,41 @@ int bpf_obj_do_pin_kernfs(struct dentry *dentry, umode_t mode, void *obj,
iput(inode);
return 0;
}
+
+/* file_operations for cgroup file system */
+static int bpf_cgroup_seq_show(struct seq_file *seq, void *v)
+{
+ struct bpf_view_cgroup_ctx ctx;
+ struct kernfs_open_file *of;
+ struct kernfs_node *kn;
+ struct cgroup *cgroup;
+ struct inode *inode;
+ struct bpf_link *link;
+ enum bpf_type type;
+
+ of = seq->private;
+ kn = of->kn;
+ cgroup = kn->parent->priv;
+
+ inode = kn->priv;
+ if (bpf_inode_type(inode, &type))
+ return -ENXIO;
+
+ if (type != BPF_TYPE_LINK)
+ return -EACCES;
+
+ link = inode->i_private;
+ if (!bpf_link_is_cgroup_view(link))
+ return -EACCES;
+
+ ctx.seq = seq;
+ ctx.cgroup = cgroup;
+
+ return run_view_prog(link->prog, &ctx);
+}
+
+static const struct kernfs_ops bpf_cgroup_ops = {
+ .seq_show = bpf_cgroup_seq_show,
+ .write = bpf_generic_write,
+ .read = bpf_generic_read,
+};
--
2.34.1.448.ga2b2bfdf31-goog
Add selftests for exposing bpf objects in kernfs. Basically the added
test tests two functionalities:
1. the ability to expose generic bpf objects in kernfs.
2. the ability to expose bpf_view programs to cgroup file system and
read from the created cgroupfs entry.
The test assumes cgroup v2 is mounted at /sys/fs/cgroup/ and bpffs is
mounted at /sys/fs/bpf/
Signed-off-by: Hao Luo <[email protected]>
---
.../selftests/bpf/prog_tests/pinning_kernfs.c | 245 ++++++++++++++++++
.../selftests/bpf/progs/pinning_kernfs.c | 72 +++++
2 files changed, 317 insertions(+)
create mode 100644 tools/testing/selftests/bpf/prog_tests/pinning_kernfs.c
create mode 100644 tools/testing/selftests/bpf/progs/pinning_kernfs.c
diff --git a/tools/testing/selftests/bpf/prog_tests/pinning_kernfs.c b/tools/testing/selftests/bpf/prog_tests/pinning_kernfs.c
new file mode 100644
index 000000000000..aa702d05bf25
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/pinning_kernfs.c
@@ -0,0 +1,245 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <fcntl.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <test_progs.h>
+#include <time.h>
+#include <unistd.h>
+#include "pinning_kernfs.skel.h"
+
+/* remove pinned object from kernfs */
+static void do_unpin(const char *kernfs_path, const char *msg)
+{
+ struct stat statbuf = {};
+ const char cmd[] = "rm";
+ int fd;
+
+ fd = open(kernfs_path, O_WRONLY);
+ if (fd < 0)
+ return;
+ ASSERT_GE(write(fd, cmd, sizeof(cmd)), 0, "fail_unpin_cgroup_entry");
+ close(fd);
+
+ ASSERT_ERR(stat(kernfs_path, &statbuf), msg);
+}
+
+static void do_pin(int fd, const char *pinpath)
+{
+ struct stat statbuf = {};
+
+ if (!ASSERT_OK(bpf_obj_pin(fd, pinpath), "bpf_obj_pin"))
+ return;
+
+ ASSERT_OK(stat(pinpath, &statbuf), "stat");
+}
+
+static void check_pinning(const char *bpffs_rootpath,
+ const char *kernfs_rootpath)
+{
+ const char msg[] = "xxx";
+ char buf[8];
+ struct pinning_kernfs *skel;
+ struct bpf_link *link;
+ int prog_fd, map_fd, link_fd;
+ char bpffs_path[64];
+ char kernfs_path[64];
+ struct stat statbuf = {};
+ int err, fd;
+
+ skel = pinning_kernfs__open_and_load();
+ if (!ASSERT_OK_PTR(skel, "pinning_kernfs__open_and_load"))
+ return;
+
+ snprintf(kernfs_path, 64, "%s/bpf_obj", kernfs_rootpath);
+ snprintf(bpffs_path, 64, "%s/bpf_obj", bpffs_rootpath);
+
+ prog_fd = bpf_program__fd(skel->progs.wait_show);
+
+ /* test 1:
+ *
+ * - expose object in kernfs without pinning in bpffs in the first place.
+ */
+ ASSERT_ERR(bpf_obj_pin(prog_fd, kernfs_path), "pin_kernfs_first");
+
+ /* test 2:
+ *
+ * - expose bpf prog in kernfs.
+ * - read/write the newly creaded kernfs entry.
+ */
+ do_pin(prog_fd, bpffs_path);
+ do_pin(prog_fd, kernfs_path);
+ fd = open(kernfs_path, O_RDWR);
+ err = read(fd, buf, sizeof(buf));
+ if (!ASSERT_EQ(err, -1, "unexpected_successful_read"))
+ goto out;
+
+ err = write(fd, msg, sizeof(msg));
+ if (!ASSERT_EQ(err, -1, "unexpected_successful_write"))
+ goto out;
+
+ close(fd);
+ do_unpin(kernfs_path, "kernfs_unpin_prog");
+ ASSERT_OK(unlink(bpffs_path), "bpffs_unlink_prog");
+
+ /* test 3:
+ *
+ * - expose bpf map in kernfs.
+ * - read/write the newly created kernfs entry.
+ */
+ map_fd = bpf_map__fd(skel->maps.wait_map);
+ do_pin(map_fd, bpffs_path);
+ do_pin(map_fd, kernfs_path);
+ fd = open(kernfs_path, O_RDWR);
+ err = read(fd, buf, sizeof(buf));
+ if (!ASSERT_EQ(err, -1, "unexpected_successful_read"))
+ goto out;
+
+ err = write(fd, msg, sizeof(msg));
+ if (!ASSERT_EQ(err, -1, "unexpected_successful_write"))
+ goto out;
+
+ close(fd);
+ do_unpin(kernfs_path, "kernfs_unpin_map");
+ ASSERT_OK(unlink(bpffs_path), "bpffs_unlink_map");
+
+ /* test 4:
+ *
+ * - expose bpf link in kernfs.
+ * - read/write the newly created kernfs entry.
+ * - removing bpffs entry also removes kernfs entries.
+ */
+ link = bpf_program__attach(skel->progs.wait_record);
+ link_fd = bpf_link__fd(link);
+ do_pin(link_fd, bpffs_path);
+ do_pin(link_fd, kernfs_path);
+ fd = open(kernfs_path, O_RDWR);
+ err = read(fd, buf, sizeof(buf));
+ if (!ASSERT_EQ(err, -1, "unexpected_successful_read"))
+ goto destroy_link;
+
+ err = write(fd, msg, sizeof(msg));
+ if (!ASSERT_EQ(err, -1, "unexpected_successful_write"))
+ goto destroy_link;
+
+ ASSERT_OK(unlink(bpffs_path), "bpffs_unlink_link");
+ ASSERT_ERR(stat(kernfs_path, &statbuf), "unpin_bpffs_first");
+
+ /* cleanup */
+destroy_link:
+ bpf_link__destroy(link);
+out:
+ close(fd);
+ pinning_kernfs__destroy(skel);
+}
+
+static void spin_on_cpu(int seconds)
+{
+ time_t start, now;
+
+ start = time(NULL);
+ do {
+ now = time(NULL);
+ } while (now - start < seconds);
+}
+
+static void do_work(const char *cgroup)
+{
+ int cpu = 0, pid;
+ char cmd[128];
+
+ /* make cgroup threaded */
+ snprintf(cmd, 128, "echo threaded > %s/cgroup.type", cgroup);
+ system(cmd);
+
+ pid = fork();
+ if (pid == 0) {
+ /* attach to cgroup */
+ snprintf(cmd, 128, "echo %d > %s/cgroup.procs", getpid(), cgroup);
+ system(cmd);
+
+ /* pin process to target cpu */
+ snprintf(cmd, 128, "taskset -pc %d %d", cpu, getpid());
+ system(cmd);
+
+ spin_on_cpu(3); /* spin on cpu for 3 seconds */
+ exit(0);
+ }
+
+ /* pin process to target cpu */
+ snprintf(cmd, 128, "taskset -pc %d %d", cpu, getpid());
+ system(cmd);
+
+ spin_on_cpu(3); /* spin on cpu for 3 seconds */
+ wait(NULL);
+}
+
+void read_from_file(const char *path)
+{
+ int id = 0, lat;
+ char buf[64];
+ int fd;
+
+ fd = open(path, O_RDONLY);
+ if (fd < 0)
+ return;
+ ASSERT_GE(read(fd, buf, sizeof(buf)), 0, "fail_read_cgroup_entry");
+ ASSERT_EQ(sscanf(buf, "%d %d", &id, &lat), 2, "unexpected_seq_show_output");
+ close(fd);
+}
+
+static void check_cgroup_seq_show(const char *bpffs_dir,
+ const char *cgroup_dir)
+{
+ struct pinning_kernfs *skel;
+ char bpffs_path[64];
+ char cgroup_path[64];
+ int fd;
+
+ skel = pinning_kernfs__open_and_load();
+ if (!ASSERT_OK_PTR(skel, "pinning_kernfs__open_and_load"))
+ return;
+
+ pinning_kernfs__attach(skel);
+
+ snprintf(bpffs_path, 64, "%s/bpf_obj", bpffs_dir);
+ snprintf(cgroup_path, 64, "%s/bpf_obj", cgroup_dir);
+
+ /* generate wait events for the cgroup */
+ do_work(cgroup_dir);
+
+ /* expose wait_show prog to cgroupfs */
+ fd = bpf_link__fd(skel->links.wait_show);
+ ASSERT_OK(bpf_obj_pin(fd, bpffs_path), "pin_bpffs");
+ ASSERT_OK(bpf_obj_pin(fd, cgroup_path), "pin_cgroupfs");
+
+ /* read from cgroupfs and check results */
+ read_from_file(cgroup_path);
+
+ /* cleanup */
+ do_unpin(cgroup_path, "cgroup_unpin_seq_show");
+ ASSERT_OK(unlink(bpffs_path), "bpffs_unlink_seq_show");
+
+ pinning_kernfs__destroy(skel);
+}
+
+void test_pinning_kernfs(void)
+{
+ char kernfs_tmpl[] = "/sys/fs/cgroup/bpf_pinning_test_XXXXXX";
+ char bpffs_tmpl[] = "/sys/fs/bpf/pinning_test_XXXXXX";
+ char *kernfs_rootpath, *bpffs_rootpath;
+
+ kernfs_rootpath = mkdtemp(kernfs_tmpl);
+ bpffs_rootpath = mkdtemp(bpffs_tmpl);
+
+ /* check pinning map, prog and link in kernfs */
+ if (test__start_subtest("pinning"))
+ check_pinning(bpffs_rootpath, kernfs_rootpath);
+
+ /* check cgroup seq_show implemented using bpf */
+ if (test__start_subtest("cgroup_seq_show"))
+ check_cgroup_seq_show(bpffs_rootpath, kernfs_rootpath);
+
+ rmdir(kernfs_rootpath);
+ rmdir(bpffs_rootpath);
+}
diff --git a/tools/testing/selftests/bpf/progs/pinning_kernfs.c b/tools/testing/selftests/bpf/progs/pinning_kernfs.c
new file mode 100644
index 000000000000..ca03a9443794
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/pinning_kernfs.c
@@ -0,0 +1,72 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2022 Google */
+
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+
+struct bpf_map_def SEC("maps") wait_map = {
+ .type = BPF_MAP_TYPE_HASH,
+ .key_size = sizeof(__u64),
+ .value_size = sizeof(__u64),
+ .max_entries = 65532,
+};
+
+/* task_group() from kernel/sched/sched.h */
+static struct task_group *task_group(struct task_struct *p)
+{
+ return p->sched_task_group;
+}
+
+static struct cgroup *task_cgroup(struct task_struct *p)
+{
+ struct task_group *tg;
+
+ tg = task_group(p);
+ return tg->css.cgroup;
+}
+
+/* cgroup_id() from linux/cgroup.h */
+static __u64 cgroup_id(const struct cgroup *cgroup)
+{
+ return cgroup->kn->id;
+}
+
+SEC("tp_btf/sched_stat_wait")
+int BPF_PROG(wait_record, struct task_struct *p, __u64 delta)
+{
+ struct cgroup *cgrp;
+ __u64 *wait_ns;
+ __u64 id;
+
+ cgrp = task_cgroup(p);
+ if (!cgrp)
+ return 0;
+
+ id = cgroup_id(cgrp);
+ wait_ns = bpf_map_lookup_elem(&wait_map, &id);
+
+ /* record the max wait latency seen so far */
+ if (!wait_ns)
+ bpf_map_update_elem(&wait_map, &id, &delta, BPF_NOEXIST);
+ else if (*wait_ns < delta)
+ *wait_ns = delta;
+ return 0;
+}
+
+SEC("view/cgroup")
+int BPF_PROG(wait_show, struct seq_file *seq, struct cgroup *cgroup)
+{
+ __u64 id, *value;
+
+ id = cgroup_id(cgroup);
+ value = bpf_map_lookup_elem(&wait_map, &id);
+
+ if (value)
+ BPF_SEQ_PRINTF(seq, "%llu %llu\n", id, *value);
+ else
+ BPF_SEQ_PRINTF(seq, "%llu 0\n", id);
+ return 0;
+}
+
+char _license[] SEC("license") = "GPL";
--
2.34.1.448.ga2b2bfdf31-goog
Hello, Hao.
On Wed, Jan 12, 2022 at 11:31:44AM -0800, Hao Luo wrote:
> As a concrete usecase of this feature, this patchset introduces a
> simple new program type called 'bpf_view', which can be used to format
> a seq file by a kernel object's state. By pinning a bpf_view program
> into a cgroup directory, userspace is able to read the cgroup's state
> from file in a format defined by the bpf program.
Both kernfs users - sysfs and cgroups - are hard APIs just as procfs, so
allowing bpf programs to add arbitrarily formatted files anywhere likely
isn't a good idea. Even ignoring the hard-defined interface problem,
allowing arbitrary files can cause surprising failures through namespace
collisions (which may be worked around by restricting where these files
reside or how they're named).
While the attraction is understandable, I think this is a misguided
direction. Text file interfaces are okay, or sometimes even good, for
certain things - communicating well established small piece of information.
They're easy to access and as long as the format stays really stable, the
million parsers that they end up spawning are mostly manageable although you
inevitably end up with "I was reading 3rd field of 4th line and you added a
new line above!".
The above also illustrates the problems with using these text file
interfaces. They're good for really static stuff or something really
provisional like for debugging where there's only one or very few consumers.
Outside of those extremes, they become pretty terrible. They are very
inefficient when the data volume isn't trivial. There's no good way to
synchronize accesses to multiple files. There are million ways to parse
them, many of them ever so subtly wrong. There's no good way to version the
interface (not that you can't). And if you throw these flexible files in the
midst of other hard-API files, it'll severely exacerbate confusion.
Also, for something which isn't stable, I think it's better to have some of
the access logic on the reader side. For example, let's say you've been
using data from a cgroup file on the system. One day, the system boots and
the file isn't there. How would you debug that? If it were a, say, py-bcc
script which fetched data through bpf, it wouldn't be difficult to track.
This isn't just happenstance - if you're reaching into a black box to get
data, you better keep that mechanism close to you as it's a fragile
temporary thing prone to breaking.
Yet another argument against it is that the kernel is a really bad place to
process and format data. We can't do real percentiles, or any kind of
non-trivial numeric analysis, or even handle and format real numbers. Given
that bpf already has an a lot more efficient way to transfer data between
kernel and user, it doesn't make sense to push data formatting into the
kernel. Export data to userspace, process and format there.
If there are reasons why this isn't very convenient using bpf. I think the
right thing to do is improving those. One issue that Song raised was that
there's no easy to allow non-root users to run specific bpf programs and
even if we do that with SUID binaries, each execution would be pretty
expensive involving full verification run and so on. But those problems are
solvable - maybe known BPF programs can be cached and made available to
other users - and I think concentrating on such direction would be way more
fruitful for wider purposes than trying to make BPF show text files in
existing fixed interfaces.
Thanks.
--
tejun
On Wed, Jan 12, 2022 at 2:37 PM Tejun Heo <[email protected]> wrote:
>
> Hello, Hao.
>
Thanks Tejun for your insights.
> On Wed, Jan 12, 2022 at 11:31:44AM -0800, Hao Luo wrote:
> > As a concrete usecase of this feature, this patchset introduces a
> > simple new program type called 'bpf_view', which can be used to format
> > a seq file by a kernel object's state. By pinning a bpf_view program
> > into a cgroup directory, userspace is able to read the cgroup's state
> > from file in a format defined by the bpf program.
>
> Both kernfs users - sysfs and cgroups - are hard APIs just as procfs, so
> allowing bpf programs to add arbitrarily formatted files anywhere likely
> isn't a good idea. Even ignoring the hard-defined interface problem,
> allowing arbitrary files can cause surprising failures through namespace
> collisions (which may be worked around by restricting where these files
> reside or how they're named).
>
> While the attraction is understandable, I think this is a misguided
> direction. Text file interfaces are okay, or sometimes even good, for
> certain things - communicating well established small piece of information.
> They're easy to access and as long as the format stays really stable, the
> million parsers that they end up spawning are mostly manageable although you
> inevitably end up with "I was reading 3rd field of 4th line and you added a
> new line above!".
>
> The above also illustrates the problems with using these text file
> interfaces. They're good for really static stuff or something really
> provisional like for debugging where there's only one or very few consumers.
> Outside of those extremes, they become pretty terrible. They are very
> inefficient when the data volume isn't trivial. There's no good way to
> synchronize accesses to multiple files. There are million ways to parse
> them, many of them ever so subtly wrong. There's no good way to version the
> interface (not that you can't). And if you throw these flexible files in the
> midst of other hard-API files, it'll severely exacerbate confusion.
>
I understand the importance of a set of hard APIs and appreciate the
effort maintainers put on maintaining them. I acknowledge the problems
of text file interfaces mentioned above. But there are situations
where the text file interface also provides great value, in a sense, I
think, outweighs its limitations. Bpf iter has many great
applications. Bpf iter could be made more efficient, providing greater
value.
I agree that mixing flexible files with hard-API files is a problem.
And my understanding is that, it's the key concern here. It would be
great if there is a way to separate the bpf files from the stable
APIs. I'm now thinking along this direction.
> Also, for something which isn't stable, I think it's better to have some of
> the access logic on the reader side. For example, let's say you've been
> using data from a cgroup file on the system. One day, the system boots and
> the file isn't there. How would you debug that? If it were a, say, py-bcc
> script which fetched data through bpf, it wouldn't be difficult to track.
> This isn't just happenstance - if you're reaching into a black box to get
> data, you better keep that mechanism close to you as it's a fragile
> temporary thing prone to breaking.
>
From the view of userspace, allowing bpf to define kernel interface is
giving the userspace full control. With everything controlled in
userspace, debugging is easier rather than harder in my understanding.
Access logic on the reader side is always needed of course. I've seen
bugs where even stable files in cgroupfs are seemingly missing, which
is harder to debug than bpf loading failure.
> Yet another argument against it is that the kernel is a really bad place to
> process and format data. We can't do real percentiles, or any kind of
> non-trivial numeric analysis, or even handle and format real numbers. Given
> that bpf already has an a lot more efficient way to transfer data between
> kernel and user, it doesn't make sense to push data formatting into the
> kernel. Export data to userspace, process and format there.
>
There is a plus side of processing and formatting data in the kernel.
Not every type of handling needs to process real percentiles or format
real numbers. A big saving in cpu cycles can be achieved by the power
of customizing data encoding inside the kernel.
Currently many data are exported in text, userspace parses them and
encodes them in a format suitable for network transmission (for
example, Protobuf). If the kernel can encode the data directly in its
final format, that would save the cpu cycles spent on encoding to and
decoding from text. Multiplying this saving by the frequency of data
collections and scale of the data center, the number can be
significant. Bpf can transfer data in binary, but there is currently
no good way to control data encoding and organize data in cgroups, as
far as I know.
> If there are reasons why this isn't very convenient using bpf. I think the
> right thing to do is improving those. One issue that Song raised was that
> there's no easy to allow non-root users to run specific bpf programs and
> even if we do that with SUID binaries, each execution would be pretty
> expensive involving full verification run and so on. But those problems are
> solvable - maybe known BPF programs can be cached and made available to
> other users - and I think concentrating on such direction would be way more
> fruitful for wider purposes than trying to make BPF show text files in
> existing fixed interfaces.
>
> Thanks.
>
> --
> tejun
Thanks,
Hao