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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hgpk.h
/*
* OLPC HGPK (XO-1) touchpad PS/2 mouse driver
*/
#ifndef _HGPK_H
#define _HGPK_H
#define HGPK_GS 0xff /* The GlideSensor */
#define HGPK_PT 0xcf /* The PenTablet */
enum hgpk_model_t {
HGPK_MODEL_PREA = 0x0a, /* pre-B1s */
HGPK_MODEL_A = 0x14, /* found on B1s, PT disabled in hardware */
HGPK_MODEL_B = 0x28, /* B2s, has capacitance issues */
HGPK_MODEL_C = 0x3c,
HGPK_MODEL_D = 0x50, /* C1, mass production */
};
enum hgpk_spew_flag {
NO_SPEW,
MAYBE_SPEWING,
SPEW_DETECTED,
RECALIBRATING,
};
#define SPEW_WATCH_COUNT 42 /* at 12ms/packet, this is 1/2 second */
enum hgpk_mode {
HGPK_MODE_MOUSE,
HGPK_MODE_GLIDESENSOR,
HGPK_MODE_PENTABLET,
HGPK_MODE_INVALID
};
struct hgpk_data {
struct psmouse *psmouse;
enum hgpk_mode mode;
bool powered;
enum hgpk_spew_flag spew_flag;
int spew_count, x_tally, y_tally; /* spew detection */
unsigned long recalib_window;
struct delayed_work recalib_wq;
int abs_x, abs_y;
int dupe_count;
int xbigj, ybigj, xlast, ylast; /* jumpiness detection */
int xsaw_secondary, ysaw_secondary; /* jumpiness detection */
};
#define hgpk_dbg(psmouse, format, arg...) \
dev_dbg(&(psmouse)->ps2dev.serio->dev, format, ## arg)
#define hgpk_err(psmouse, format, arg...) \
dev_err(&(psmouse)->ps2dev.serio->dev, format, ## arg)
#define hgpk_info(psmouse, format, arg...) \
dev_info(&(psmouse)->ps2dev.serio->dev, format, ## arg)
#define hgpk_warn(psmouse, format, arg...) \
dev_warn(&(psmouse)->ps2dev.serio->dev, format, ## arg)
#define hgpk_notice(psmouse, format, arg...) \
dev_notice(&(psmouse)->ps2dev.serio->dev, format, ## arg)
#ifdef CONFIG_MOUSE_PS2_OLPC
void hgpk_module_init(void);
int hgpk_detect(struct psmouse *psmouse, bool set_properties);
int hgpk_init(struct psmouse *psmouse);
#else
static inline void hgpk_module_init(void)
{
}
static inline int hgpk_detect(struct psmouse *psmouse, bool set_properties)
{
return -ENODEV;
}
static inline int hgpk_init(struct psmouse *psmouse)
{
return -ENODEV;
}
#endif
#endif
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