stat(2) returning device ID not existing in mountinfo

[Tomasz Sterna <tomek@xiaoka.com>]

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Hi.

I have spotted an issue with stat(2) call on files on btrfs.
It is giving me dev_t st_dev number that does not correspond to any
mounted filesystem in proc's mountinfo.

A quick example:

$ grep btrfs /proc/self/mountinfo 
61 0 0:36 /root / rw,relatime shared:1 - btrfs /dev/bcache0 rw,ssd,space_cache,subvolid=535,subvol=/root
75 61 0:36 /home /home rw,relatime shared:30 - btrfs /dev/bcache0 rw,ssd,space_cache,subvolid=258,subvol=/home

As you can see both btrfs subvolumes are 0:36, but files on these:

$ stat -c "%d" /etc/passwd
38
$ stat -c "%d" /home/smoku/test.txt
44

Passing these through major(3)/minor(3) give: 0:38 and 0:44

There is clearly something fishy going on. :-)
Simple one-liner shows that only btrfs and autofs misbehave like this:

$ </proc/self/mountinfo cut -d' ' -f3,5,9 | while read DEV PATH FS; do echo $DEV $(/usr/bin/stat -c "%d" $PATH) $FS; done
0:16 16 sysfs
0:4 4 proc
0:6 6 devtmpfs
0:17 17 securityfs
0:18 18 tmpfs
0:19 19 devpts
0:20 20 tmpfs
0:21 21 tmpfs
0:22 22 cgroup
0:23 23 pstore
0:24 24 efivarfs
0:25 25 cgroup
0:26 26 cgroup
0:27 27 cgroup
0:28 28 cgroup
0:29 29 cgroup
0:30 30 cgroup
0:31 31 cgroup
0:32 32 cgroup
0:33 33 cgroup
0:34 34 cgroup
0:35 35 configfs
0:36 38 btrfs
0:15 15 hugetlbfs
0:39 68 autofs
0:14 14 mqueue
0:40 40 tmpfs
0:7 7 debugfs
0:42 42 nfsd
0:36 44 btrfs
8:2 2050 ext3
8:1 2049 vfat
0:47 47 rpc_pipefs
0:50 50 fusectl
0:51 51 tmpfs
0:49 49 fuse.gvfsd-fuse
0:68 68 binfmt_misc


I already attempted a illinformed-patch in fs/btrfs/super.c:

@@ -1127,6 +1127,7 @@ static int btrfs_fill_super(struct super_block *sb,
 		goto fail_close;
 	}
 
+	sb->s_dev = inode->i_sb->s_dev;
 	sb->s_root = d_make_root(inode);
 	if (!sb->s_root) {
 		err = -ENOMEM;

but it didn't help.

I would like to dig deeper and fix it, but first I have to ask:
- Which number is wrong?
  The one returned by stat() or the one in mountinfo?

I am running:

$ uname -a
Linux lair.home.lan 4.7.3-200.pf3.fc24.x86_64 #1 SMP Tue Sep 13 12:34:03 CEST 2016 x86_64 x86_64 x86_64 GNU/Linux



-- 
smoku @ http://abadcafe.pl/ @ http://xiaoka.com/

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