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    SubjectRe: tty_ldisc_try_get(): BUG kmalloc-8: Poison overwritten

    * Pekka Enberg <penberg@cs.helsinki.fi> wrote:

    > Hi Ingo,
    >
    > Ingo Molnar wrote:
    >> Ok, this is one for those who like to look at weird crashes/bugs.
    >>
    >> Here's a new regression that popped up in this merge window, there's
    >> some sort of slab corruption going on in tty data structures:
    >>
    >> [ 74.900215] =============================================================================
    >> [ 74.908193] BUG kmalloc-8: Poison overwritten
    >> [ 74.908193] -----------------------------------------------------------------------------
    >> [ 74.908193] [ 74.908193] INFO: 0x5d883a14-0x5d883a14. First byte
    >> 0x6a instead of 0x6b
    >> [ 74.908193] INFO: Allocated in tty_ldisc_try_get+0x1a/0xb0 age=8015 cpu=0 pid=1
    >> [ 74.908193] INFO: Freed in tty_ldisc_put+0x48/0x50 age=4 cpu=3 pid=4236
    >> [ 74.908193] INFO: Slab 0x42c6eeb4 objects=73 used=61 fp=0x5d883a10 flags=0x1d0000c3
    >> [ 74.908193] INFO: Object 0x5d883a10 @offset=2576 fp=0x5d883d90
    >> [ 74.908193] [ 74.908193] Bytes b4 0x5d883a00: 01 00 00 00 de 04
    >> ff ff 5a 5a 5a 5a 5a 5a 5a 5a ....�.��ZZZZZZZZ
    >> [ 74.908193] Object 0x5d883a10: 6b 6b 6b 6b 6a 6b 6b a5
    >> kkkkjkk�
    >
    > This is struct tty_ldisc and the corruption happens in the first byte of
    > ->refcount. This probably just means that there's a race condition and
    > someone is doing tty_ldisc_deref() after tty_ldisc_put().
    >
    > You could add something like
    >
    > WARN_ON(ld->refcount == 0x6b)
    >
    > to tty_ldisc_deref() to see if that triggers.

    I've committed the debug hack below - will let you know if it
    triggers.

    Ingo

    ------------->
    From 2f5eb191a6fd392d8f66f5e249bd05b3444b5420 Mon Sep 17 00:00:00 2001
    From: Ingo Molnar <mingo@elte.hu>
    Date: Sun, 14 Jun 2009 10:30:31 +0200
    Subject: [PATCH] [not for upstream] tty: Add debug check to tty_ldisc_deref()
    MIME-Version: 1.0
    Content-Type: text/plain; charset=utf-8
    Content-Transfer-Encoding: 8bit

    Try to catch this SLAB corruption earlier:

    [ 74.900215] =============================================================================
    [ 74.908193] BUG kmalloc-8: Poison overwritten
    [ 74.908193] -----------------------------------------------------------------------------
    [ 74.908193]
    [ 74.908193] INFO: 0x5d883a14-0x5d883a14. First byte 0x6a instead of 0x6b
    [ 74.908193] INFO: Allocated in tty_ldisc_try_get+0x1a/0xb0 age=8015 cpu=0 pid=1
    [ 74.908193] INFO: Freed in tty_ldisc_put+0x48/0x50 age=4 cpu=3 pid=4236
    [ 74.908193] INFO: Slab 0x42c6eeb4 objects=73 used=61 fp=0x5d883a10 flags=0x1d0000c3
    [ 74.908193] INFO: Object 0x5d883a10 @offset=2576 fp=0x5d883d90
    [ 74.908193]
    [ 74.908193] Bytes b4 0x5d883a00: 01 00 00 00 de 04 ff ff 5a 5a 5a 5a 5a 5a 5a 5a ....Þ.ÿÿZZZZZZZZ
    [ 74.908193] Object 0x5d883a10: 6b 6b 6b 6b 6a 6b 6b a5 kkkkjkk¥
    [ 74.908193] Redzone 0x5d883a18: bb bb bb bb »»»»
    [ 74.908193] Padding 0x5d883a40: 5a 5a 5a 5a 5a 5a 5a 5a ZZZZZZZZ
    [ 74.908193] Pid: 4230, comm: mingetty Not tainted 2.6.30-tip #744
    [ 74.908193] Call Trace:
    [ 74.908193] [<410ae628>] print_trailer+0xc8/0xd0
    [ 74.908193] [<410ae6a3>] check_bytes_and_report+0x73/0x90
    [ 74.908193] [<410ae941>] check_object+0xa1/0x130
    [ 74.908193] [<410aef1e>] alloc_debug_processing+0x5e/0xd0
    [ 74.908193] [<410af99e>] __slab_alloc+0x11e/0x150
    [ 74.908193] [<413d9c7a>] ? tty_ldisc_try_get+0x1a/0xb0
    [ 74.908193] [<410afcdb>] kmem_cache_alloc+0x7b/0x120
    [ 74.908193] [<413d9c7a>] ? tty_ldisc_try_get+0x1a/0xb0
    [ 74.908193] [<413d9c7a>] ? tty_ldisc_try_get+0x1a/0xb0
    [ 74.908193] [<413d9c7a>] tty_ldisc_try_get+0x1a/0xb0
    [ 74.908193] [<410b06a3>] ? __kmalloc+0x163/0x170
    [ 74.908193] [<413d9d77>] tty_ldisc_get+0x17/0x40
    [ 74.908193] [<413da63d>] tty_ldisc_init+0xd/0x30
    [ 74.908193] [<413d4098>] initialize_tty_struct+0x38/0x210
    [ 74.908193] [<413d5d6f>] tty_init_dev+0x4f/0xb0
    [ 74.908193] [<413d5f25>] __tty_open+0x155/0x2d0
    [ 74.908193] [<413d60b7>] tty_open+0x17/0x30
    [ 74.908193] [<410bb599>] chrdev_open+0xe9/0x100
    [ 74.908193] [<410b721e>] __dentry_open+0xbe/0x190
    [ 74.908193] [<410b813c>] nameidata_to_filp+0x2c/0x50
    [ 74.908193] [<410bb4b0>] ? chrdev_open+0x0/0x100
    [ 74.908193] [<410c2eba>] do_filp_open+0x2aa/0x580
    [ 74.908193] [<4100a1bb>] ? sched_clock+0xb/0x20
    [ 74.908193] [<410596c7>] ? put_lock_stats+0x17/0x30
    [ 74.908193] [<41059734>] ? lock_release_holdtime+0x54/0x60
    [ 74.908193] [<4105d4d9>] ? lock_release_nested+0x99/0xd0
    [ 74.908193] [<41377421>] ? debug_spin_unlock+0x21/0x80
    [ 74.908193] [<41377495>] ? _raw_spin_unlock+0x15/0x20
    [ 74.908193] [<410cad50>] ? alloc_fd+0xc0/0xd0
    [ 74.908193] [<410b7020>] do_sys_open+0x40/0x80
    [ 74.908193] [<410b70ae>] sys_open+0x1e/0x30
    [ 74.908193] [<4100388f>] sysenter_do_call+0x12/0x3c
    [ 74.908193] FIX kmalloc-8: Restoring 0x5d883a14-0x5d883a14=0x6b
    [ 74.908193]
    [ 74.908193] FIX kmalloc-8: Marking all objects used
    [ 75.188226] warning: `sudo' uses deprecated v2 capabilities in a way that may be insecure.

    Note, a value of 0x6b is legit for the refcount in theory, so this
    debug hack is not suitable for upstream.

    NOT-Signed-off-by: Ingo Molnar <mingo@elte.hu>
    ---
    drivers/char/tty_ldisc.c | 1 +
    1 files changed, 1 insertions(+), 0 deletions(-)

    diff --git a/drivers/char/tty_ldisc.c b/drivers/char/tty_ldisc.c
    index 39c8f86..dd1daf3 100644
    --- a/drivers/char/tty_ldisc.c
    +++ b/drivers/char/tty_ldisc.c
    @@ -361,6 +361,7 @@ void tty_ldisc_deref(struct tty_ldisc *ld)
    unsigned long flags;

    BUG_ON(ld == NULL);
    + WARN_ON_ONCE(ld->refcount == 0x6b);

    spin_lock_irqsave(&tty_ldisc_lock, flags);
    if (ld->refcount == 0)
    --
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