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

[Pekka Enberg]

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 ....ï.ïïZZZZZZZZ
> [   74.908193]   Object 0x5d883a10:  6b 6b 6b 6b 6a 6b 6b a5                         kkkkjkkï        

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                                     ïïïï            
> [   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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