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Date:   Tue, 5 May 2020 18:34:10 +0200
From:   Jürgen Groß <jgross@...e.com>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Arnd Bergmann <arnd@...db.de>
Cc:     Stefano Stabellini <sstabellini@...nel.org>,
        Yan Yankovskyi <yyankovskyi@...il.com>, Wei Liu <wl@....org>,
        xen-devel <xen-devel@...ts.xenproject.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] xenbus: avoid stack overflow warning

On 05.05.20 18:12, Boris Ostrovsky wrote:
> 
> On 5/5/20 12:02 PM, Jürgen Groß wrote:
>> On 05.05.20 17:01, Arnd Bergmann wrote:
>>> On Tue, May 5, 2020 at 4:34 PM Jürgen Groß <jgross@...e.com> wrote:
>>>> On 05.05.20 16:15, Arnd Bergmann wrote:
>>>>> The __xenbus_map_ring() function has two large arrays, 'map' and
>>>>> 'unmap' on its stack. When clang decides to inline it into its caller,
>>>>> xenbus_map_ring_valloc_hvm(), the total stack usage exceeds the
>>>>> warning
>>>>> limit for stack size on 32-bit architectures.
>>>>>
>>>>> drivers/xen/xenbus/xenbus_client.c:592:12: error: stack frame size
>>>>> of 1104 bytes in function 'xenbus_map_ring_valloc_hvm'
>>>>> [-Werror,-Wframe-larger-than=]
>>>>>
>>>>> As far as I can tell, other compilers don't inline it here, so we get
>>>>> no warning, but the stack usage is actually the same. It is possible
>>>>> for both arrays to use the same location on the stack, but the
>>>>> compiler
>>>>> cannot prove that this is safe because they get passed to external
>>>>> functions that may end up using them until they go out of scope.
>>>>>
>>>>> Move the two arrays into separate basic blocks to limit the scope
>>>>> and force them to occupy less stack in total, regardless of the
>>>>> inlining decision.
>>>>
>>>> Why don't you put both arrays into a union?
>>>
>>> I considered that as well, and don't really mind either way. I think
>>> it does
>>> get a bit ugly whatever we do. If you prefer the union, I can respin the
>>> patch that way.
>>
>> Hmm, thinking more about it I think the real clean solution would be to
>> extend struct map_ring_valloc_hvm to cover the pv case, too, to add the
>> map and unmap arrays (possibly as a union) to it and to allocate it
>> dynamically instead of having it on the stack.
>>
>> Would you be fine doing this?
> 
> 
> 
> Another option might be to factor out/modify code from
> xenbus_unmap_ring() and call the resulting code from
> __xenbus_map_ring()'s fail path.

This will still allocate large arrays on the stack. If we ever increase
the max ring page order it will explode.


Juergen

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