Commit 7e06de7c authored by Qu Wenruo's avatar Qu Wenruo Committed by David Sterba
Browse files

btrfs: canonicalize the device path before adding it 



[PROBLEM]
Currently btrfs accepts any file path for its device, resulting some
weird situation:

 # ./mount_by_fd /dev/test/scratch1  /mnt/btrfs/

The program has the following source code:

 #include <fcntl.h>
 #include <stdio.h>
 #include <sys/mount.h>

 int main(int argc, char *argv[]) {
	int fd = open(argv[1], O_RDWR);
	char path[256];
	snprintf(path, sizeof(path), "/proc/self/fd/%d", fd);
	return mount(path, argv[2], "btrfs", 0, NULL);
 }

Then we can have the following weird device path:

 BTRFS: device fsid 2378be81-fe12-46d2-a9e8-68cf08dd98d5 devid 1 transid 7 /proc/self/fd/3 (253:2) scanned by mount_by_fd (18440)

Normally it's not a big deal, and later udev can trigger a device path
rename. But if udev didn't trigger, the device path "/proc/self/fd/3"
will show up in mtab.

[CAUSE]
For filename "/proc/self/fd/3", it means the opened file descriptor 3.
In above case, it's exactly the device we want to open, aka points to
"/dev/test/scratch1" which is another symlink pointing to "/dev/dm-2".

Inside kernel we solve the mount source using LOOKUP_FOLLOW, which
follows the symbolic link and grab the proper block device.

But inside btrfs we also save the filename into btrfs_device::name, and
utilize that member to report our mount source, which leads to the above
situation.

[FIX]
Instead of unconditionally trust the path, check if the original file
(not following the symbolic link) is inside "/dev/", if not, then
manually lookup the path to its final destination, and use that as our
device path.

This allows us to still use symbolic links, like
"/dev/mapper/test-scratch" from LVM2, which is required for fstests runs
with LVM2 setup.

And for really weird names, like the above case, we solve it to
"/dev/dm-2" instead.

Reviewed-by: default avatarFilipe Manana <fdmanana@suse.com>
Link: https://bugzilla.suse.com/show_bug.cgi?id=1230641


Reported-by: default avatarFabian Vogt <fvogt@suse.com>
Signed-off-by: default avatarQu Wenruo <wqu@suse.com>
Signed-off-by: default avatarDavid Sterba <dsterba@suse.com>
parent 2e8b6bc0

fs/btrfs/volumes.c

+86 −1
Original line number Diff line number Diff line
@@ -732,6 +732,78 @@ const u8 *btrfs_sb_fsid_ptr(const struct btrfs_super_block *sb) 
	return has_metadata_uuid ? sb->metadata_uuid : sb->fsid;

}



/*

 * We can have very weird soft links passed in.

 * One example is "/proc/self/fd/<fd>", which can be a soft link to

 * a block device.

 *

 * But it's never a good idea to use those weird names.

 * Here we check if the path (not following symlinks) is a good one inside

 * "/dev/".

 */

static bool is_good_dev_path(const char *dev_path)

{

	struct path path = { .mnt = NULL, .dentry = NULL };

	char *path_buf = NULL;

	char *resolved_path;

	bool is_good = false;

	int ret;



	if (!dev_path)

		goto out;



	path_buf = kmalloc(PATH_MAX, GFP_KERNEL);

	if (!path_buf)

		goto out;



	/*

	 * Do not follow soft link, just check if the original path is inside

	 * "/dev/".

	 */

	ret = kern_path(dev_path, 0, &path);

	if (ret)

		goto out;

	resolved_path = d_path(&path, path_buf, PATH_MAX);

	if (IS_ERR(resolved_path))

		goto out;

	if (strncmp(resolved_path, "/dev/", strlen("/dev/")))

		goto out;

	is_good = true;

out:

	kfree(path_buf);

	path_put(&path);

	return is_good;

}



static int get_canonical_dev_path(const char *dev_path, char *canonical)

{

	struct path path = { .mnt = NULL, .dentry = NULL };

	char *path_buf = NULL;

	char *resolved_path;

	int ret;



	if (!dev_path) {

		ret = -EINVAL;

		goto out;

	}



	path_buf = kmalloc(PATH_MAX, GFP_KERNEL);

	if (!path_buf) {

		ret = -ENOMEM;

		goto out;

	}



	ret = kern_path(dev_path, LOOKUP_FOLLOW, &path);

	if (ret)

		goto out;

	resolved_path = d_path(&path, path_buf, PATH_MAX);

	ret = strscpy(canonical, resolved_path, PATH_MAX);

out:

	kfree(path_buf);

	path_put(&path);

	return ret;

}



static bool is_same_device(struct btrfs_device *device, const char *new_path)

{

	struct path old = { .mnt = NULL, .dentry = NULL };
@@ -1419,12 +1491,23 @@ struct btrfs_device *btrfs_scan_one_device(const char *path, blk_mode_t flags, 
	bool new_device_added = false;

	struct btrfs_device *device = NULL;

	struct file *bdev_file;

	char *canonical_path = NULL;

	u64 bytenr;

	dev_t devt;

	int ret;



	lockdep_assert_held(&uuid_mutex);



	if (!is_good_dev_path(path)) {

		canonical_path = kmalloc(PATH_MAX, GFP_KERNEL);

		if (canonical_path) {

			ret = get_canonical_dev_path(path, canonical_path);

			if (ret < 0) {

				kfree(canonical_path);

				canonical_path = NULL;

			}

		}

	}

	/*

	 * Avoid an exclusive open here, as the systemd-udev may initiate the

	 * device scan which may race with the user's mount or mkfs command,
@@ -1469,7 +1552,8 @@ struct btrfs_device *btrfs_scan_one_device(const char *path, blk_mode_t flags, 
		goto free_disk_super;

	}



	device = device_list_add(path, disk_super, &new_device_added);

	device = device_list_add(canonical_path ? : path, disk_super,

				 &new_device_added);

	if (!IS_ERR(device) && new_device_added)

		btrfs_free_stale_devices(device->devt, device);
@@ -1478,6 +1562,7 @@ struct btrfs_device *btrfs_scan_one_device(const char *path, blk_mode_t flags, 


error_bdev_put:

	fput(bdev_file);

	kfree(canonical_path);



	return device;

}