Currently, displaying the btrfs subvol mount option doesn't escape `,`.
This makes parsing /proc/self/mounts and /proc/self/mountinfo
ambigious for subvolume names that contain commas. The text after the
comma could be mistaken for another option (think "subvol=foo,ro", where
ro is actually part of the subvolumes name).

This patch replaces the manual escape characters list with a call to
seq_show_option. Thanks to Calvin Walton for suggesting this approach.

Fixes: c8d3fe028f64 ("Btrfs: show subvol= and subvolid= in /proc/mounts")
Suggested-by: Calvin Walton <calvin.walton@kepstin.ca>
Signed-off-by: Johannes Kimmel <kernel@bareminimum.eu>
---
 fs/btrfs/super.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index dc4fee519ca6..a5f29ff3fbc2 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -1139,8 +1139,7 @@ static int btrfs_show_options(struct seq_file *seq, struct dentry *dentry)
 	subvol_name = btrfs_get_subvol_name_from_objectid(info,
 			btrfs_root_id(BTRFS_I(d_inode(dentry))->root));
 	if (!IS_ERR(subvol_name)) {
-		seq_puts(seq, ",subvol=");
-		seq_escape(seq, subvol_name, " \t\n\\");
+		seq_show_option(seq, "subvol", subvol_name);
 		kfree(subvol_name);
 	}
 	return 0;
-- 
2.49.0

On Wed, Mar 19, 2025 at 10:49:00PM +0100, Johannes Kimmel wrote:
> Currently, displaying the btrfs subvol mount option doesn't escape `,`.
> This makes parsing /proc/self/mounts and /proc/self/mountinfo
> ambigious for subvolume names that contain commas. The text after the
> comma could be mistaken for another option (think "subvol=foo,ro", where
> ro is actually part of the subvolumes name).

The subvol= option was intentionally last so the path does not mix with
other options but yeah it still can be confused if it's parsed in a
generic way and not assuming anything about the ordering.

I've checked util-linux/libmount, seems that it understands the "\," as
part of the option value so it should not break anything.

> This patch replaces the manual escape characters list with a call to
> seq_show_option. Thanks to Calvin Walton for suggesting this approach.
> 
> Fixes: c8d3fe028f64 ("Btrfs: show subvol= and subvolid= in /proc/mounts")
> Suggested-by: Calvin Walton <calvin.walton@kepstin.ca>
> Signed-off-by: Johannes Kimmel <kernel@bareminimum.eu>

Added to for-next, thanks.

On 20.03.25 00:52, David Sterba wrote:
> On Wed, Mar 19, 2025 at 10:49:00PM +0100, Johannes Kimmel wrote:
>> Currently, displaying the btrfs subvol mount option doesn't escape `,`.
>> This makes parsing /proc/self/mounts and /proc/self/mountinfo
>> ambigious for subvolume names that contain commas. The text after the
>> comma could be mistaken for another option (think "subvol=foo,ro", where
>> ro is actually part of the subvolumes name).
> 
> The subvol= option was intentionally last so the path does not mix with
> other options but yeah it still can be confused if it's parsed in a
> generic way and not assuming anything about the ordering.
> 
> I've checked util-linux/libmount, seems that it understands the "\," as
> part of the option value so it should not break anything.
> 
>> This patch replaces the manual escape characters list with a call to
>> seq_show_option. Thanks to Calvin Walton for suggesting this approach.
>>
>> Fixes: c8d3fe028f64 ("Btrfs: show subvol= and subvolid= in /proc/mounts")
>> Suggested-by: Calvin Walton <calvin.walton@kepstin.ca>
>> Signed-off-by: Johannes Kimmel <kernel@bareminimum.eu>
> 
> Added to for-next, thanks.

Thank you for taking care of this so quickly.