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Subject: [PATCH 03/11] vfs: Allow fsinfo() to query what's in an fs_context
 [ver #15]
From:   David Howells <dhowells@redhat.com>
To:     viro@zeniv.linux.org.uk
Cc:     dhowells@redhat.com, raven@themaw.net, mszeredi@redhat.com,
        christian@brauner.io, linux-api@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Date:   Fri, 28 Jun 2019 16:44:28 +0100
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In-Reply-To: <156173661696.14042.17822154531324224780.stgit@warthog.procyon.org.uk>
References: <156173661696.14042.17822154531324224780.stgit@warthog.procyon.org.uk>
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Allow fsinfo() to be used to query the filesystem attached to an fs_context
once a superblock has been created or if it comes from fspick().

The caller must specify AT_FSINFO_FROM_FSOPEN in the parameters and must
supply the fd from fsopen() as dfd and must set filename to NULL.

This is done with something like:

	fd = fsopen("ext4", 0);
	...
	struct fsinfo_params params = {
		.at_flags = AT_FSINFO_FROM_FSOPEN;
		...
	};
	fsinfo(fd, NULL, &params, ...);

Signed-off-by: David Howells <dhowells@redhat.com>
---

 fs/fsinfo.c                |   44 +++++++++++++++++++++++++++++++++++++++++++-
 fs/statfs.c                |    2 +-
 include/uapi/linux/fcntl.h |    2 ++
 3 files changed, 46 insertions(+), 2 deletions(-)

diff --git a/fs/fsinfo.c b/fs/fsinfo.c
index 09e743b16235..d419cccbf3db 100644
--- a/fs/fsinfo.c
+++ b/fs/fsinfo.c
@@ -13,6 +13,7 @@
 #include <linux/security.h>
 #include <linux/uaccess.h>
 #include <linux/fsinfo.h>
+#include <linux/fs_context.h>
 #include <uapi/linux/mount.h>
 #include "internal.h"
 
@@ -347,6 +348,42 @@ static int vfs_fsinfo_fd(unsigned int fd, struct fsinfo_kparams *params)
 	return ret;
 }
 
+/*
+ * Allow access to an fs_context object as created by fsopen() or fspick().
+ */
+static int vfs_fsinfo_fscontext(int fd, struct fsinfo_kparams *params)
+{
+	struct fs_context *fc;
+	struct fd f = fdget(fd);
+	int ret;
+
+	if (!f.file)
+		return -EBADF;
+
+	ret = -EINVAL;
+	if (f.file->f_op != &fscontext_fops)
+		goto out_f;
+	ret = -EOPNOTSUPP;
+	if (fc->ops == &legacy_fs_context_ops)
+		goto out_f;
+
+	ret = mutex_lock_interruptible(&fc->uapi_mutex);
+	if (ret == 0) {
+		ret = -EBADFD;
+		if (fc->root) {
+			struct path path = { .dentry = fc->root };
+
+			ret = vfs_fsinfo(&path, params);
+		}
+
+		mutex_unlock(&fc->uapi_mutex);
+	}
+
+out_f:
+	fdput(f);
+	return ret;
+}
+
 /*
  * Return buffer information by requestable attribute.
  *
@@ -452,6 +489,9 @@ SYSCALL_DEFINE5(fsinfo,
 		kparams.request = user_params.request;
 		kparams.Nth = user_params.Nth;
 		kparams.Mth = user_params.Mth;
+
+		if ((kparams.at_flags & AT_FSINFO_FROM_FSOPEN) && pathname)
+			return -EINVAL;
 	} else {
 		kparams.request = FSINFO_ATTR_STATFS;
 	}
@@ -508,7 +548,9 @@ SYSCALL_DEFINE5(fsinfo,
 	if (!kparams.buffer)
 		goto error_scratch;
 
-	if (pathname)
+	if (kparams.at_flags & AT_FSINFO_FROM_FSOPEN)
+		ret = vfs_fsinfo_fscontext(dfd, &kparams);
+	else if (pathname)
 		ret = vfs_fsinfo_path(dfd, pathname, &kparams);
 	else
 		ret = vfs_fsinfo_fd(dfd, &kparams);
diff --git a/fs/statfs.c b/fs/statfs.c
index eea7af6f2f22..b9b63d9f4f24 100644
--- a/fs/statfs.c
+++ b/fs/statfs.c
@@ -86,7 +86,7 @@ int vfs_statfs(const struct path *path, struct kstatfs *buf)
 	int error;
 
 	error = statfs_by_dentry(path->dentry, buf);
-	if (!error)
+	if (!error && path->mnt)
 		buf->f_flags = calculate_f_flags(path->mnt);
 	return error;
 }
diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
index 1d338357df8a..6a2402a8fa30 100644
--- a/include/uapi/linux/fcntl.h
+++ b/include/uapi/linux/fcntl.h
@@ -91,6 +91,8 @@
 #define AT_STATX_FORCE_SYNC	0x2000	/* - Force the attributes to be sync'd with the server */
 #define AT_STATX_DONT_SYNC	0x4000	/* - Don't sync attributes with the server */
 
+#define AT_FSINFO_FROM_FSOPEN	0x2000	/* Examine the fs_context attached to dfd by fsopen() */
+
 #define AT_RECURSIVE		0x8000	/* Apply to the entire subtree */