Re: kmemcheck caught read from freed memory (cfq_free_io_context)

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Tue, 2008-04-01 at 23:08 +0200, Vegard Nossum wrote:
> Hi,
> 
> This appeared in my logs:
> 
>  kmemcheck: Caught 32-bit read from freed memory (f7042348)
> 
>  Pid: 1374, comm: bash Not tainted (2.6.25-rc7 #92)
>  EIP: 0060:[<c0502f0d>] EFLAGS: 00210202 CPU: 0
>  EIP is at call_for_each_cic+0x2d/0x44
>  EAX: 00200286 EBX: 00000001 ECX: c200e908 EDX: f7042348
>  ESI: f6c26c60 EDI: c0503310 EBP: f70fff38 ESP: c082ec88
>   DS: 007b ES: 007b FS: 00d8 GS: 0033 SS: 0068
>  CR0: 8005003b CR2: f7826904 CR3: 36cd7000 CR4: 000006c0
>  DR0: 00000000 DR1: 00000000 DR2: 00000000 DR3: 00000000
>  DR6: ffff4ff0 DR7: 00000400
>   [<c041cff8>] kmemcheck_read+0xa8/0xe0
>   [<c041d1d5>] kmemcheck_access+0x1a5/0x244
>   [<c0668252>] do_page_fault+0x622/0x6fc
>   [<c06666aa>] error_code+0x72/0x78
>   [<c050323f>] cfq_free_io_context+0xf/0x70
>   [<c04fc4d7>] put_io_context+0x4f/0x58
>   [<c04fc568>] exit_io_context+0x60/0x6c
>   [<c042f871>] do_exit+0x4d9/0x6f0
>   [<c042fab1>] do_group_exit+0x29/0x88
>   [<c042fb1f>] sys_exit_group+0xf/0x14
>   [<c0406105>] sysenter_past_esp+0x6d/0xa4
>   [<ffffffff>] 0xffffffff
> 
> The error occurs in cfq_free_io_context()'s call to
> call_for_each_cic() which looks like this:
> 
>         rcu_read_lock();
>         hlist_for_each_entry_rcu(cic, n, &ioc->cic_list, cic_list) {
>                 func(ioc, cic);
>                 called++;
>         }
>         rcu_read_unlock();
> 
> The function that is called is cic_free_func(). It is postulated that
> hlist_for_each_entry_rcu() will dereference the previously freed list
> element to get the ->next pointer.
> 
> After a short discussion with Pekka Enberg and Peter Zijlstra, it
> seemed evident that this list traversal should use
> hlist_for_each_entry_safe_rcu() instead, which would buffer the next
> pointer before the object is freed.
> 
> Does this report seem to be valid?
> 
> The kernel is 2.6.25-rc7.

The missing hlist for loop would look something like so:

#define hlist_for_each_entry_safe_rcu(tpos, pos, n, head, member)        \
        for (pos = (head)->first;                                        \
             rcu_dereference(pos) && ({ n = pos->next; 1; }) &&          \
                ({ tpos = hlist_entry(pos, typeof(*tpos), member); 1;}); \
             pos = n)