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

> [ 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
>
> It's a single bit corruption - but the hardware in question has a
> good track record with thousands of bootups, so it might be a
> reference count related corruption as well.
>
> It started triggering in this merge window, so one of these might be
> a starting point:
>
> 3e3b5c0: tty: use prepare/finish_wait
> 5fc5b42: tty: remove sleep_on
> 26a2e20: tty: Untangle termios and mm mutex dependencies
> 0b4068a: tty: simplify buffer allocator cleanups
> c481c70: tty: remove buffer special casing
> 852e99d: tty: bring ldisc into CodingStyle
> f2c4c65: tty: Move ldisc_flush
> c65c9bc: tty: rewrite the ldisc locking
> e8b70e7: tty: Extract various bits of ldisc code
> 5f0878a: tty: Fix oops when scanning the polling list for kgdb
> 38db897: tty: throttling race fix
> 1ec739b: tty: Implement a drain delay in the tty port
> fcc8ac1: tty: Add carrier processing on close to the tty_port core
>
> (But ... if it's a low-probability bug then it might be an older bug
> as well.)
>
> I tried two other reboots and the bug did not trigger in a way
> visible in the log - so it's sporadic. I've started a reboot loop
> with this kernel on that box, to see whether it's repeatable within
> a reasonable amount of time.
>
> This is the -tip testbox that generally triggers SMP races very well
> (and as the first one amongst boxes) - so my first guess would be on
> some narrow (or not so narrow but config/timing dependent) SMP race
> window.
>
> Since it's not reproducible in any easy fashion, there's no
> bisection possible either, on this box. I've Cc:-ed all the
> tty/kmalloc/race experts, maybe the bug can be seen ...
>
> I've attached the config and the full bootlog.
>
> Ingo
>

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