Revision 8aef18845266f5c05904c610088f2d1ed58f6be3 authored by Al Viro on 16 June 2011, 14:10:06 UTC, committed by Al Viro on 16 June 2011, 15:28:16 UTC
[Kudos to dhowells for tracking that crap down] If two processes attempt to cause automounting on the same mountpoint at the same time, the vfsmount holding the mountpoint will be left with one too few references on it, causing a BUG when the kernel tries to clean up. The problem is that lock_mount() drops the caller's reference to the mountpoint's vfsmount in the case where it finds something already mounted on the mountpoint as it transits to the mounted filesystem and replaces path->mnt with the new mountpoint vfsmount. During a pathwalk, however, we don't take a reference on the vfsmount if it is the same as the one in the nameidata struct, but do_add_mount() doesn't know this. The fix is to make sure we have a ref on the vfsmount of the mountpoint before calling do_add_mount(). However, if lock_mount() doesn't transit, we're then left with an extra ref on the mountpoint vfsmount which needs releasing. We can handle that in follow_managed() by not making assumptions about what we can and what we cannot get from lookup_mnt() as the current code does. The callers of follow_managed() expect that reference to path->mnt will be grabbed iff path->mnt has been changed. follow_managed() and follow_automount() keep track of whether such reference has been grabbed and assume that it'll happen in those and only those cases that'll have us return with changed path->mnt. That assumption is almost correct - it breaks in case of racing automounts and in even harder to hit race between following a mountpoint and a couple of mount --move. The thing is, we don't need to make that assumption at all - after the end of loop in follow_manage() we can check if path->mnt has ended up unchanged and do mntput() if needed. The BUG can be reproduced with the following test program: #include <stdio.h> #include <sys/types.h> #include <sys/stat.h> #include <unistd.h> #include <sys/wait.h> int main(int argc, char **argv) { int pid, ws; struct stat buf; pid = fork(); stat(argv[1], &buf); if (pid > 0) wait(&ws); return 0; } and the following procedure: (1) Mount an NFS volume that on the server has something else mounted on a subdirectory. For instance, I can mount / from my server: mount warthog:/ /mnt -t nfs4 -r On the server /data has another filesystem mounted on it, so NFS will see a change in FSID as it walks down the path, and will mark /mnt/data as being a mountpoint. This will cause the automount code to be triggered. !!! Do not look inside the mounted fs at this point !!! (2) Run the above program on a file within the submount to generate two simultaneous automount requests: /tmp/forkstat /mnt/data/testfile (3) Unmount the automounted submount: umount /mnt/data (4) Unmount the original mount: umount /mnt At this point the kernel should throw a BUG with something like the following: BUG: Dentry ffff880032e3c5c0{i=2,n=} still in use (1) [unmount of nfs4 0:12] Note that the bug appears on the root dentry of the original mount, not the mountpoint and not the submount because sys_umount() hasn't got to its final mntput_no_expire() yet, but this isn't so obvious from the call trace: [<ffffffff8117cd82>] shrink_dcache_for_umount+0x69/0x82 [<ffffffff8116160e>] generic_shutdown_super+0x37/0x15b [<ffffffffa00fae56>] ? nfs_super_return_all_delegations+0x2e/0x1b1 [nfs] [<ffffffff811617f3>] kill_anon_super+0x1d/0x7e [<ffffffffa00d0be1>] nfs4_kill_super+0x60/0xb6 [nfs] [<ffffffff81161c17>] deactivate_locked_super+0x34/0x83 [<ffffffff811629ff>] deactivate_super+0x6f/0x7b [<ffffffff81186261>] mntput_no_expire+0x18d/0x199 [<ffffffff811862a8>] mntput+0x3b/0x44 [<ffffffff81186d87>] release_mounts+0xa2/0xbf [<ffffffff811876af>] sys_umount+0x47a/0x4ba [<ffffffff8109e1ca>] ? trace_hardirqs_on_caller+0x1fd/0x22f [<ffffffff816ea86b>] system_call_fastpath+0x16/0x1b as do_umount() is inlined. However, you can see release_mounts() in there. Note also that it may be necessary to have multiple CPU cores to be able to trigger this bug. Tested-by: Jeff Layton <jlayton@redhat.com> Tested-by: Ian Kent <raven@themaw.net> Signed-off-by: David Howells <dhowells@redhat.com> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
1 parent 50338b8
File | Mode | Size |
---|---|---|
88pm860x-ts.c | -rw-r--r-- | 6.1 KB |
Kconfig | -rw-r--r-- | 19.6 KB |
Makefile | -rw-r--r-- | 2.8 KB |
ad7877.c | -rw-r--r-- | 21.1 KB |
ad7879-i2c.c | -rw-r--r-- | 3.1 KB |
ad7879-spi.c | -rw-r--r-- | 4.3 KB |
ad7879.c | -rw-r--r-- | 15.1 KB |
ad7879.h | -rw-r--r-- | 705 bytes |
ads7846.c | -rw-r--r-- | 33.8 KB |
atmel-wm97xx.c | -rw-r--r-- | 11.6 KB |
atmel_mxt_ts.c | -rw-r--r-- | 27.5 KB |
atmel_tsadcc.c | -rw-r--r-- | 11.3 KB |
bu21013_ts.c | -rw-r--r-- | 17.7 KB |
cy8ctmg110_ts.c | -rw-r--r-- | 8.4 KB |
da9034-ts.c | -rw-r--r-- | 8.9 KB |
dynapro.c | -rw-r--r-- | 4.8 KB |
eeti_ts.c | -rw-r--r-- | 7.9 KB |
elo.c | -rw-r--r-- | 8.9 KB |
fujitsu_ts.c | -rw-r--r-- | 4.2 KB |
gunze.c | -rw-r--r-- | 4.8 KB |
h3600_ts_input.c | -rw-r--r-- | 12.7 KB |
hampshire.c | -rw-r--r-- | 5.0 KB |
hp680_ts_input.c | -rw-r--r-- | 2.9 KB |
htcpen.c | -rw-r--r-- | 5.8 KB |
inexio.c | -rw-r--r-- | 4.8 KB |
intel-mid-touch.c | -rw-r--r-- | 15.4 KB |
jornada720_ts.c | -rw-r--r-- | 4.8 KB |
lpc32xx_ts.c | -rw-r--r-- | 10.3 KB |
mainstone-wm97xx.c | -rw-r--r-- | 7.7 KB |
max11801_ts.c | -rw-r--r-- | 7.5 KB |
mc13783_ts.c | -rw-r--r-- | 6.7 KB |
mcs5000_ts.c | -rw-r--r-- | 8.4 KB |
migor_ts.c | -rw-r--r-- | 7.0 KB |
mk712.c | -rw-r--r-- | 5.7 KB |
mtouch.c | -rw-r--r-- | 5.2 KB |
pcap_ts.c | -rw-r--r-- | 6.8 KB |
penmount.c | -rw-r--r-- | 3.8 KB |
s3c2410_ts.c | -rw-r--r-- | 11.2 KB |
st1232.c | -rw-r--r-- | 6.2 KB |
stmpe-ts.c | -rw-r--r-- | 9.7 KB |
tnetv107x-ts.c | -rw-r--r-- | 9.4 KB |
touchit213.c | -rw-r--r-- | 5.9 KB |
touchright.c | -rw-r--r-- | 4.1 KB |
touchwin.c | -rw-r--r-- | 4.3 KB |
tps6507x-ts.c | -rw-r--r-- | 8.7 KB |
tsc2005.c | -rw-r--r-- | 18.3 KB |
tsc2007.c | -rw-r--r-- | 9.6 KB |
ucb1400_ts.c | -rw-r--r-- | 12.9 KB |
usbtouchscreen.c | -rw-r--r-- | 39.5 KB |
w90p910_ts.c | -rw-r--r-- | 8.6 KB |
wacom_w8001.c | -rw-r--r-- | 14.0 KB |
wm831x-ts.c | -rw-r--r-- | 11.7 KB |
wm9705.c | -rw-r--r-- | 9.1 KB |
wm9712.c | -rw-r--r-- | 11.6 KB |
wm9713.c | -rw-r--r-- | 12.0 KB |
wm97xx-core.c | -rw-r--r-- | 21.5 KB |
zylonite-wm97xx.c | -rw-r--r-- | 5.8 KB |
![swh spinner](/static/img/swh-spinner.gif)
Computing file changes ...