linux-kernel - Re: bio linked list corruption.

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Date:   Tue, 18 Oct 2016 18:05:57 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Chris Mason <clm@...com>, Jens Axboe <axboe@...com>,
        Dave Jones <davej@...emonkey.org.uk>,
        Al Viro <viro@...iv.linux.org.uk>, Josef Bacik <jbacik@...com>,
        David Sterba <dsterba@...e.com>,
        linux-btrfs <linux-btrfs@...r.kernel.org>,
        Linux Kernel <linux-kernel@...r.kernel.org>,
        Andrew Lutomirski <luto@...nel.org>
Subject: Re: bio linked list corruption.

On 10/18/2016 05:10 PM, Linus Torvalds wrote:
> On Tue, Oct 18, 2016 at 4:42 PM, Chris Mason <clm@...com> wrote:
>>
>> Seems to be the whole thing:
>
> Ahh. On lkml, so I do have it in my mailbox, but Dave changed the
> subject line when he tested on ext4 rather than btrfs..
>
> Anyway, the corrupted address is somewhat interesting. As Dave Jones
> said, he saw
>
>   list_add corruption. prev->next should be next (ffffe8ffff806648),
> but was ffffc9000067fcd8. (prev=ffff880503878b80).
>   list_add corruption. prev->next should be next (ffffe8ffffc05648),
> but was ffffc9000028bcd8. (prev=ffff880503a145c0).
>
> and Dave Chinner reports
>
>   list_add corruption. prev->next should be next (ffffe8ffffc02808),
> but was ffffc90005f6bda8. (prev=ffff88013363bb80).
>
> and it's worth noting that the "but was" is a remarkably consistent
> vmalloc address (the ffffc9000.. pattern gives it away). In fact, it's
> identical across two boots for DaveJ in the low 14 bits, and fairly
> high up in those low 14 bots (0x3cd8).
>
> DaveC has a different address, but it's also in the vmalloc space, and
> also looks like it is fairly high up in 14 bits (0x3da8). So in both
> cases it's almost certainly a stack address with a fairly empty stack.
> The differences are presumably due to different kernel configurations
> and/or just different filesystems calling the same function that does
> the same bad thing but now at different depths in the stack.
>
> Adding Andy to the cc, because this *might* be triggered by the
> vmalloc stack code itself. Maybe the re-use of stacks showing some
> problem? Maybe Chris (who can't see the problem) doesn't have
> CONFIG_VMAP_STACK enabled?

Wouldn't this cause the exact opposite problem?  If the warning is to be 
believed, then prev is *not* on the stack but somehow prev->next ended 
up pointing to the stack.  If stack reuse caused something to corrupt a 
value on the stack, then how would this cause a stack address to be 
written to a non-stack location?  All I can think of is that "prev" 
itself is corrupted somehow.

One possible debugging approach would be to change:

#define NR_CACHED_STACKS 2

to

#define NR_CACHED_STACKS 0

in kernel/fork.c and to set CONFIG_DEBUG_PAGEALLOC=y.  The latter will 
force an immediate TLB flush after vfree.

Also, CONFIG_DEBUG_VIRTUAL=y can be quite helpful for debugging stack 
issues.  I'm tempted to do something equivalent to hardwiring that 
option on for a while if CONFIG_VMAP_STACK=y.