shmem_file_setup creating SYSV00000000 files in (ext3) rootfilesystem

From: desrt
Date: Fri Dec 05 2003 - 22:56:05 EST

kernel version is 2.4.23.

the problem is caused by the fact that the checks that shmem_file_setup
does to determine the location of the mountpoint of tmpfs fail to take
into account the effect of pivot_root.

that is: if i mount a tmpfs as /, then mount a ext3 filesystem and
pivot_root into it with put_old as, say, /var/tmp, then shmem_file_setup
will still think that the tmpfs is mounted at / (and as a result creates
shared memory files on the root filesystem instead of on /var/tmp as it
ought to)

i'm not actually sure what the code in mm/shmem.c:shmem_file_setup()
does, but i assume the problem is this line:
root = shm_mnt->mnt_root;

and somehow as a result, this happens:
peloton:/proc# grep deleted */maps
17923/maps:4201a000-4207a000 rw-s 00000000 00:04 21233690
/SYSV00000000 (deleted)
17923/maps:424dd000-4253d000 rw-s 00000000 00:04 21266460
/SYSV00000000 (deleted)
17926/maps:4201a000-4207a000 rw-s 00000000 00:04 21233690
/SYSV00000000 (deleted)
17926/maps:424dd000-4253d000 rw-s 00000000 00:04 21266460
/SYSV00000000 (deleted)
17927/maps:4201a000-4207a000 rw-s 00000000 00:04 21233690
/SYSV00000000 (deleted)
[many many many lines follow]

if you have any advice or can confirm to me that this is actually a bug
in the kernel, please reply. i'm not on the list.


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