Re: BUG in strnlen

[Eric Dumazet <dada1@cosmosbay.com>]

Randy Dunlap a écrit :
> Hi,
>
> All of my daily testing (x86_64, 4 CPUs, 8 GB RAM)
> since (after) 2.6.25 is seeing this BUG:
> (i.e., 2.6.25 does not do this)
>
>
> BUG: unable to handle kernel paging request at ffffffffa00b7551
> IP: [<ffffffff80357aac>] strnlen+0x15/0x1f
> PGD 203067 PUD 207063 PMD 27e44f067 PTE 0
> Oops: 0000 [1] SMP 
> CPU 3 
> Modules linked in: hp_ilo parport_pc lp parport tg3 cciss ehci_hcd ohci_hcd uhci_hcd [last unloaded: reiserfs]
> Pid: 20926, comm: cat Not tainted 2.6.25-git5 #1
> RIP: 0010:[<ffffffff80357aac>]  [<ffffffff80357aac>] strnlen+0x15/0x1f
> RSP: 0018:ffff810274981cc8  EFLAGS: 00010297
> RAX: ffffffffa00b7551 RBX: ffff810274981d38 RCX: ffffffff80603719
> RDX: ffff810274981d68 RSI: fffffffffffffffe RDI: ffffffffa00b7551
> RBP: ffff810274981cc8 R08: 00000000ffffffff R09: 00000000000000c8
> R10: 0000000000000050 R11: 0000000000000246 R12: ffff8102364600cc
> R13: ffffffffa00b7551 R14: 0000000000000011 R15: 0000000000000010
> FS:  00007f956375d6f0(0000) GS:ffff81027f808980(0000) knlGS:00000000f7f7f6c0
> CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
> CR2: ffffffffa00b7551 CR3: 00000002734d5000 CR4: 00000000000006e0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
> Process cat (pid: 20926, threadinfo ffff810274980000, task ffff81026d18ce20)
> Stack:  ffff810274981d28 ffffffff80358d5a ffff810274981d28 0000000000000f34
>  ffff8102364600cc ffff810236461000 ffffffff80603719 ffff81024ac14f00
>  ffff81024ac14f00 0000000000000004 0000000000000000 0000000000000000
> Call Trace:
>  [<ffffffff80358d5a>] vsnprintf+0x31b/0x592
>  [<ffffffff802a78eb>] seq_printf+0x7e/0xa7
>  [<ffffffff8024c6fe>] ? debug_mutex_free_waiter+0x46/0x4a
>  [<ffffffff8053aaa2>] ? __down_read+0x17/0x92
>  [<ffffffff80539c25>] ? __mutex_lock_slowpath+0x1d8/0x1e5
>  [<ffffffff802886c2>] ? count_partial+0x45/0x4d
>  [<ffffffff80289a6d>] s_show+0x7e/0xcb
>  [<ffffffff802a7dd9>] seq_read+0x10b/0x298
>  [<ffffffff802c7dbb>] proc_reg_read+0x7b/0x95
>  [<ffffffff8028ec0b>] vfs_read+0xab/0x154
>  [<ffffffff8028f015>] sys_read+0x47/0x6f
>  [<ffffffff8020c182>] tracesys+0xd5/0xda
>
>
> Code: 48 8d 44 11 ff 40 38 30 74 0a 48 ff c8 48 39 d0 73 f3 31 c0 c9 c3 55 48 89 f8 48 89 e5 eb 03 48 ff c0 48 ff ce 48 83 fe ff 74 05 <80> 38 00 75 ef c9 48 29 f8 c3 55 31 c0 48 89 e5 eb 13 41 38 c8 
> RIP  [<ffffffff80357aac>] strnlen+0x15/0x1f
>  RSP <ffff810274981cc8>
> CR2: ffffffffa00b7551
>
>
> ---
>
>   
My initial thoughts are :

Fault address is  0xffffffffa00b7551 which is in module mapping space on 
x86_64

strnlen() is OK

Some module created a kmem_cache (with kmem_cache_create()).
slub or slab kept a pointer to the cache name in their internal structures.
Module was unloaded but forgot to destroy kmem cache before unloading.

Fault happens while doing "cat /proc/slabinfo", when trying to 
dereference cache name since module was unloaded and its memory unmapped.

Next step is to find which module was unloaded ...