Re: Using /proc/mounts in mountlist.c for linux

[Karel Zak <kzak@redhat.com>]

On Tue, May 31, 2011 at 10:31:17AM +0100, Pádraig Brady wrote:
> On 31/05/11 01:14, James Youngman wrote:
> > On Tue, Apr 5, 2011 at 1:36 PM, Philipp Thomas <pth@suse.de> wrote:
> >> GNU find will not recognize file systems of type autofs on newer Linux
> >> kernels as autofs entries are only listed in /proc/mounts and mountlist.c
> >> includes glibc mntent.h which takes the _PATH_MOUNTED from paths.h and that
> >> is /etc/mtab.
> >>
> >> After a longer discussion, we (SUSE) chose to patch mountlist.c in findutils
> >> to use proc/mounts instead of /etc/mtab which fixed ou problem.
> >>
> >> Would gnulib accept the attached patch to mountlist.c?
> > 
> > I don't know if this patch was accepted, but it shouldn't be.   The
> > problem is that /proc/mounts has incomplete data for /.   This will
> > break gnulib's mountlist, at least with the current form of the patch,
> > because mountlist will have an incorrect idea of the type of the root
> > filesystem.   Here's an example showing the problem:
> > 
> > ~$ cat tryit.sh
> > #! /bin/sh
> > f() {
> >     echo "$1"
> >     ( ls -l /etc/mtab; find / -maxdepth 0 -printf '%p %F\n' ) |
> >     sed -e 's_^_    _'
> > }
> > 
> > set -e
> > cd /etc
> > f "regular /etc/mtab"
> > 
> > mv mtab mtab.old; ln -s ../proc/mounts mtab
> > f "with /proc/mounts"
> > rm mtab; mv mtab.old mtab
> > ~$ sudo sh tryit.sh
> > regular /etc/mtab
> >     -rw-r--r-- 1 root root 1869 May 30 23:53 /etc/mtab
> >     / ext3
> > with /proc/mounts
> >     lrwxrwxrwx 1 root root 14 May 31 01:12 /etc/mtab -> ../proc/mounts
> >     / rootfs


 That's strange, why for "/" it does not search in the file (mtab) in reverse
 order?

example A)

         # mount -t ext3 /dev/sda6 /mnt/test
         # mount -t ext4 /dev/mapper/kzak-home /mnt/test

  ... so I have two entries for the same mountpoint:

         # grep /mnt/test /proc/mounts 
         /dev/sda6 /mnt/test ext3  rw,relatime,errors=continue,barrier=0,data=ordered 0 0
         /dev/mapper/kzak-home /mnt/test ext4 rw,relatime,barrier=1,data=ordered 0 0


  this is correct (ext4 is the second entry):

         # find /mnt/test -maxdepth 0 -printf '%p %F\n'
         /mnt/test ext4


example B)

  the same thing with root FS:

         # grep -E '(/dev/sda4|rootfs)' /proc/mounts 
         rootfs / rootfs rw 0 0
         /dev/sda4 / ext3  rw,noatime,errors=continue,user_xattr,acl,barrier=0,data=ordered 0 0

  ... so I have two entries for the same mountpoint:


         # find / -maxdepth 0 -printf '%p %F\n'
         / rootfs

  why does it return the first entry? It seems like obvious bug. You
  have to read entries in the mtab file in reverse order.


  Anyway, /proc/self/mountinfo is definitely more sexy... :-)

    Karel

-- 
 Karel Zak  <kzak@redhat.com>
 http://karelzak.blogspot.com