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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i8042.h
#ifndef _I8042_H
#define _I8042_H
/*
* Copyright (c) 1999-2002 Vojtech Pavlik
*
* This program is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 as published by
* the Free Software Foundation.
*/
/*
* Arch-dependent inline functions and defines.
*/
#if defined(CONFIG_MACH_JAZZ)
#include "i8042-jazzio.h"
#elif defined(CONFIG_SGI_HAS_I8042)
#include "i8042-ip22io.h"
#elif defined(CONFIG_SNI_RM)
#include "i8042-snirm.h"
#elif defined(CONFIG_PPC)
#include "i8042-ppcio.h"
#elif defined(CONFIG_SPARC)
#include "i8042-sparcio.h"
#elif defined(CONFIG_X86) || defined(CONFIG_IA64)
#include "i8042-x86ia64io.h"
#elif defined(CONFIG_UNICORE32)
#include "i8042-unicore32io.h"
#else
#include "i8042-io.h"
#endif
/*
* This is in 50us units, the time we wait for the i8042 to react. This
* has to be long enough for the i8042 itself to timeout on sending a byte
* to a non-existent mouse.
*/
#define I8042_CTL_TIMEOUT 10000
/*
* Status register bits.
*/
#define I8042_STR_PARITY 0x80
#define I8042_STR_TIMEOUT 0x40
#define I8042_STR_AUXDATA 0x20
#define I8042_STR_KEYLOCK 0x10
#define I8042_STR_CMDDAT 0x08
#define I8042_STR_MUXERR 0x04
#define I8042_STR_IBF 0x02
#define I8042_STR_OBF 0x01
/*
* Control register bits.
*/
#define I8042_CTR_KBDINT 0x01
#define I8042_CTR_AUXINT 0x02
#define I8042_CTR_IGNKEYLOCK 0x08
#define I8042_CTR_KBDDIS 0x10
#define I8042_CTR_AUXDIS 0x20
#define I8042_CTR_XLATE 0x40
/*
* Return codes.
*/
#define I8042_RET_CTL_TEST 0x55
/*
* Expected maximum internal i8042 buffer size. This is used for flushing
* the i8042 buffers.
*/
#define I8042_BUFFER_SIZE 16
/*
* Number of AUX ports on controllers supporting active multiplexing
* specification
*/
#define I8042_NUM_MUX_PORTS 4
/*
* Debug.
*/
#ifdef DEBUG
static unsigned long i8042_start_time;
#define dbg_init() do { i8042_start_time = jiffies; } while (0)
#define dbg(format, arg...) \
do { \
if (i8042_debug) \
printk(KERN_DEBUG KBUILD_MODNAME ": [%d] " format, \
(int) (jiffies - i8042_start_time), ##arg); \
} while (0)
#else
#define dbg_init() do { } while (0)
#define dbg(format, arg...) \
do { \
if (0) \
printk(KERN_DEBUG pr_fmt(format), ##arg); \
} while (0)
#endif
#endif /* _I8042_H */
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