Subject: bug#16539: More details on df command output for you

From: Pádraig Brady <[email-masked]>
Date: Mon, 27 Jan 2014 01:06:16 +0000

On 01/26/2014 11:35 PM, Bernhard Voelker wrote:
> On 01/26/2014 12:28 PM, Pádraig Brady wrote:
>> On 01/25/2014 11:55 PM, Bernhard Voelker wrote:
>>> However, I remember some other corner cases with eclipsed file
>>> systems in the Fedora bug tracker. I think we're quite close
>>> to solve them all this time (hopefully).
>>> The idea was to trust the order of mount entries returned by
>>> the kernel, i.e. in the loop over the mount entries, if the
>>> mount point is the same one as a previous one, then we should
>>> process the one mounted later.
>>>
>>> E.g. the situation where 2 file systems are mounted on the
>>> same mount point:
>>>
>>>   $ findmnt | grep loop
>>>   └─/mnt                           /dev/loop0     ext4                rw,relatime,data=ordered
>>>     └─/mnt/dir                     /dev/loop1     ext4                rw,relatime,data=ordered
>>>       └─/mnt/dir                   /dev/loop2     ext4                rw,relatime,data=ordered
>>>
>>> df - the new one with your patch - still shows the wrong device:
>>>
>>>   $ src/df | grep loop
>>>   /dev/loop0        122835      1551    112110   2% /mnt
>>>   /dev/loop1        122835      1550    112111   2% /mnt/dir
>>>
>>> It should say /dev/loop2 here. BTW the numbers are correct.
> 
> BTW: the fstype is wrong, too (which can only be seen with -T or --output,
> and if it differs, of course).
> 
>> Right, that could be handled easy enough.
>> loop1 is not accessible above and so should be hidden.
>> But consider a bind mount resulting in something like:
>>
>>>   └─/mnt                           /dev/loop0     ext4                rw,relatime,data=ordered
>>>     └─/mnt/dir                     /dev/loop1     ext4                rw,relatime,data=ordered
>>>       └─/some/place/else           /dev/loop1     ext4                rw,relatime,data=ordered
>>>       └─/mnt/dir                   /dev/loop2     ext4                rw,relatime,data=ordered
>>
>> If we did a linear scan through that, we'd lose the /some/place/else
>> due to it being a longer mount dir, and then also the original loop1
>> as we took /dev/loop2 for /mnt/dir.
>> Seems like when discarding we would need to see if this was the
>> last entry for a device and then see if there are any other candidate
>> mount points for that device?
> 
> Hi Padraig,
> 
> thanks.
> Again, mount_list is a little beast - more below.
> 
> The following patch (on top of yours) would handle both cases
> without a problem.  Feel free to squash it in, if you like.
> 
> diff --git a/src/df.c b/src/df.c
> index 23b5156..78768cc 100644
> --- a/src/df.c
> +++ b/src/df.c
> @@ -631,9 +631,20 @@ filter_mount_list (void)
>        else
>          {
>            /* If we've already seen this device...  */
> +          struct devlist *d = NULL;
>            for (devlist = devlist_head; devlist; devlist = devlist->next)
>              if (devlist->dev_num == buf.st_dev)
> -              break;
> +              {
> +                d = devlist;
> +                if (!STREQ (devlist->me->me_devname, me->me_devname))
> +                  {
> +                    /* Fix the devname if the mount dir has been
> +                       mounted over by a different devname.  */
> +                    free (devlist->me->me_devname);
> +                    devlist->me->me_devname = xstrdup (me->me_devname);
> +                  }
> +              }
> +          devlist = d;
> 
>            if (devlist)
>              {
> 
> But there is yet another issue with the -a mode for such
> over-mounted and therefore eclipsed file systems:
> 
>   # Create 2 file system images: 1 ext4, 1 xfs.
>   $ dd if=/dev/zero bs=1M status=none count=128 of=img1
>   $ dd if=/dev/zero bs=1M status=none count=256 of=img2
>   $ mkfs -t ext4 -F img1 >/dev/null 2>&1
>   $ mkfs -t xfs  -f img2 >/dev/null 2>&1
>   $ mkdir /mnt{1,2}
> 
>   # Mount both on /mnt1.
>   $ mount -o loop img1 /mnt1
>   $ mount -o loop img2 /mnt1
> 
>   # Mount the former (ext4) also on /mnt2 via its loop device.
>   $ mount /dev/loop0 /mnt2
> 
>   # Result:
>   $ findmnt --output=TARGET,SOURCE,FSTYPE | grep loop
>   ├─/mnt1                          /dev/loop0     ext4
>   │ └─/mnt1                        /dev/loop1     xfs
>   └─/mnt2                          /dev/loop0     ext4
> 
> Everything is fine now with the filtered df run ...
> 
>   $ src/df --out -h | grep loop
>   /dev/loop1     xfs        256K     3  256K    1%  252M   13M  239M   6% -    /mnt1
>   /dev/loop0     ext4        32K    11   32K    1%  120M  1.6M  110M   2% -    /mnt2
> 
> ...but "df -a" prints the wrong statistics for the "over-mounted" /mnt1!
> 
>   $ src/df --out -h -a | grep loop
>   /dev/loop0     ext4                  256K     3  256K    1%  252M   13M  239M   6% -    /mnt1
>   /dev/loop1     xfs                   256K     3  256K    1%  252M   13M  239M   6% -    /mnt1
>   /dev/loop0     ext4                   32K    11   32K    1%  120M  1.6M  110M   2% -    /mnt2
> 
> Okay, this is nothing new.
> BTW: strictly speaking, also the output of today's "df -t rootfs -a"
> is wrong because the numbers are definitely not that of the early-boot
> rootfs file system.
> 
> Now, how should df handle this?
> 
> a)
> df silently filters out the mount entries of all eclipsed mount dirs,
> even with -a.
> --> Hmm, I think this would probably contradict to POSIX.
> 
> b)
> df prints an error diagnostic for each eclipsed mount dir, and exits
> non-Zero.
> --> Well, there are probably such mounts on every system, e.g. on my box:
> 
>   TARGET                       SOURCE         FSTYPE
>   /proc/sys/fs/binfmt_misc     systemd-1      autofs
>   /proc/sys/fs/binfmt_misc     binfmt_misc    binfmt_misc
> 
> Therefore, a "df -a" would always fail. ;-(
> At least on my system, there are
> 
> c)
> df prints a warning diagnostic for each eclipsed mount dir, and exits
> Zero (unless another error occurs).
> 
> --> Due to the same reason as in b), these warning might be messy
> and users will probably be irritated.
> 
> d)
> df outputs "-" for all numbers of such eclipsed file systems, e.g.
> 
>   $ src/df --out -h -a | grep mnt1
>   /dev/loop0     ext4                     -     -     -     -     -     -     -    - -    /mnt1
>   /dev/loop1     xfs                   256K     3  256K    1%  252M   13M  239M   6% -    /mnt1
> 
> 
> Maybe d) is the best solution, as it mirrors what df can know:
> it knows source, target and the file system type, but it doesn't
> have access to the block and inode numbers.
> 
> WDYT?

Thanks for the nice analysis and tests.
d) seems like the best option here, though we'd have to be careful
about cases where /proc/mounts was giving a system wide view,
while df wasn't privy to that due to mount namespaces or
overmounts etc. I'm not thinking of a specific issue here,
just the general problem.

wrt c) and annoying warnings, I also notice `df -a` on a default Fedora 20 install here,
giving multiple duplicate warnings like:
  df: ‘net:[4026532416]’: No such file or directory
  df: ‘net:[4026532416]’: No such file or directory
That's due to:
  $ grep net: /proc/mounts
  proc net:[4026532416] proc rw,nosuid,nodev,noexec,relatime 0 0
  proc net:[4026532416] proc rw,nosuid,nodev,noexec,relatime 0 0
Which is due to support for namespaces.
Seems like we should not try to lookup non absolute mount points?

Also a general point is that a lot of stuff has changed underneath us recently,
and perhaps we should be looking at abstracting that away somewhere
(like libmount that is part of util-linux). In the short term anyway
we should fix up the above warts within df.

thanks,
Pádraig.

thanks,
Pádraig.