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>
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File | Mode | Size |
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auth_gss | ||
xprtrdma | ||
Kconfig | -rw-r--r-- | 1.0 KB |
Makefile | -rw-r--r-- | 541 bytes |
addr.c | -rw-r--r-- | 8.7 KB |
auth.c | -rw-r--r-- | 15.9 KB |
auth_generic.c | -rw-r--r-- | 4.5 KB |
auth_null.c | -rw-r--r-- | 2.5 KB |
auth_unix.c | -rw-r--r-- | 5.5 KB |
backchannel_rqst.c | -rw-r--r-- | 8.8 KB |
bc_svc.c | -rw-r--r-- | 2.0 KB |
cache.c | -rw-r--r-- | 42.9 KB |
clnt.c | -rw-r--r-- | 44.4 KB |
netns.h | -rw-r--r-- | 353 bytes |
rpc_pipe.c | -rw-r--r-- | 24.9 KB |
rpcb_clnt.c | -rw-r--r-- | 27.6 KB |
sched.c | -rw-r--r-- | 25.0 KB |
socklib.c | -rw-r--r-- | 4.5 KB |
stats.c | -rw-r--r-- | 6.8 KB |
sunrpc.h | -rw-r--r-- | 1.7 KB |
sunrpc_syms.c | -rw-r--r-- | 2.2 KB |
svc.c | -rw-r--r-- | 30.8 KB |
svc_xprt.c | -rw-r--r-- | 33.3 KB |
svcauth.c | -rw-r--r-- | 3.7 KB |
svcauth_unix.c | -rw-r--r-- | 22.4 KB |
svcsock.c | -rw-r--r-- | 42.6 KB |
sysctl.c | -rw-r--r-- | 3.5 KB |
timer.c | -rw-r--r-- | 2.9 KB |
xdr.c | -rw-r--r-- | 30.8 KB |
xprt.c | -rw-r--r-- | 31.1 KB |
xprtsock.c | -rw-r--r-- | 75.7 KB |
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