This introduces a function which can be used to fetch a file, given an
arbitrary task. As long as the user holds a reference (refcnt) to the
task_struct it is safe to call, and will either return NULL on failure,
or a pointer to the file, with a refcnt.
Signed-off-by: Sargun Dhillon <[email protected]>
---
fs/file.c | 19 +++++++++++++++++++
include/linux/fdtable.h | 10 ++++++++++
2 files changed, 29 insertions(+)
diff --git a/fs/file.c b/fs/file.c
index 3da91a112bab..98601a503a0f 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -1015,3 +1015,22 @@ int iterate_fd(struct files_struct *files, unsigned n,
return res;
}
EXPORT_SYMBOL(iterate_fd);
+
+struct file *get_task_file(struct task_struct *task, unsigned int fd)
+{
+ struct file *file = NULL;
+
+ task_lock(task);
+ rcu_read_lock();
+
+ if (task->files) {
+ file = fcheck_files(task->files, fd);
+ if (file && !get_file_rcu(file))
+ file = NULL;
+ }
+
+ rcu_read_unlock();
+ task_unlock(task);
+
+ return file;
+}
diff --git a/include/linux/fdtable.h b/include/linux/fdtable.h
index f07c55ea0c22..eacb1a56df44 100644
--- a/include/linux/fdtable.h
+++ b/include/linux/fdtable.h
@@ -115,6 +115,16 @@ int iterate_fd(struct files_struct *, unsigned,
int (*)(const void *, struct file *, unsigned),
const void *);
+/*
+ * get_task_file - get a reference to a file from another task
+ * @task: the task to get the file descriptor from
+ * @fd: the file descriptor number to fetch
+ *
+ * returns NULL on failure, or pointer to the file on success, with a reference
+ * It requires that the task is pinned prior to calling it.
+ */
+struct file *get_task_file(struct task_struct *task, unsigned int fd);
+
extern int __alloc_fd(struct files_struct *files,
unsigned start, unsigned end, unsigned flags);
extern void __fd_install(struct files_struct *files,
--
2.20.1
On 12/09, Sargun Dhillon wrote:
>
> +struct file *get_task_file(struct task_struct *task, unsigned int fd)
> +{
> + struct file *file = NULL;
> +
> + task_lock(task);
> + rcu_read_lock();
> +
> + if (task->files) {
> + file = fcheck_files(task->files, fd);
> + if (file && !get_file_rcu(file))
> + file = NULL;
> + }
On second thought this is not exactly right, get_file_rcu() can fail if
get_task_file() races with dup2(), in this case we need to do fcheck_files()
again. And this is what __fget() already does, so may be the patch below
makes more sense?
I will leave this to other reviewers, but suddenly I recall that I have
already sent the patch which adds a similar helper a while ago.
See https://lore.kernel.org/lkml/[email protected]/
In short, get_files_struct() should be avoided because it can race with
exec() and break POSIX locks which use ->fl_owner = files_struct.
Oleg.
--- x/fs/file.c
+++ x/fs/file.c
@@ -706,9 +706,9 @@ void do_close_on_exec(struct files_struc
spin_unlock(&files->file_lock);
}
-static struct file *__fget(unsigned int fd, fmode_t mask, unsigned int refs)
+static struct file *__fget_files(struct files_struct *files, unsigned int fd,
+ fmode_t mask, unsigned int refs)
{
- struct files_struct *files = current->files;
struct file *file;
rcu_read_lock();
@@ -729,6 +729,23 @@ loop:
return file;
}
+struct file *fget_task(struct task_struct *task, unsigned int fd)
+{
+ struct file *file;
+
+ task_lock(task);
+ if (task->files)
+ file = __fget_files(task->files, fd, 0, 1);
+ task_unlock(task);
+
+ return file;
+}
+
+static struct file *__fget(unsigned int fd, fmode_t mask, unsigned int refs)
+{
+ return __fget_files(current->files, fd, mask, refs);
+}
+
struct file *fget_many(unsigned int fd, unsigned int refs)
{
return __fget(fd, FMODE_PATH, refs);