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Subject: Re: [PATCH] xenbus: avoid stack overflow warning
To:     =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= <jgross@suse.com>,
        Arnd Bergmann <arnd@arndb.de>
Cc:     Stefano Stabellini <sstabellini@kernel.org>,
        Yan Yankovskyi <yyankovskyi@gmail.com>, Wei Liu <wl@xen.org>,
        xen-devel <xen-devel@lists.xenproject.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>
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From:   Boris Ostrovsky <boris.ostrovsky@oracle.com>
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Message-ID: <656d0dc4-6c4f-29df-be7b-4cb70c2c0a5e@oracle.com>
Date:   Tue, 5 May 2020 12:12:00 -0400
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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@suse.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.


-boris