linux-kernel - Re: [GIT PULL] Enable -Wstringop-overflow globally

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Message-ID: <90b41ce3-0eca-4bc9-bc04-ad68ba91d7ee@linux.intel.com>
Date: Tue, 30 Jan 2024 15:52:32 +0100
From: Thomas Hellström <thomas.hellstrom@...ux.intel.com>
To: "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
 David Laight <David.Laight@...LAB.COM>,
 'Linus Torvalds' <torvalds@...ux-foundation.org>,
 Kees Cook <keescook@...omium.org>, Lucas De Marchi
 <lucas.demarchi@...el.com>, Oded Gabbay <ogabbay@...nel.org>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>
Cc: "Gustavo A. R. Silva" <gustavoars@...nel.org>,
 "linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 intel-xe@...ts.freedesktop.org,
 "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>
Subject: Re: [GIT PULL] Enable -Wstringop-overflow globally

Hi,

On 1/27/24 20:53, Gustavo A. R. Silva wrote:
>
>
> On 1/27/24 09:11, David Laight wrote:
>> From: Linus Torvalds
>>> Sent: 26 January 2024 22:36
>>>
>>> On Fri, 26 Jan 2024 at 14:24, Kees Cook <keescook@...omium.org> wrote:
>>>>
>>>> I think xe has some other weird problems too. This may be related 
>>>> (under
>>>> allocating):
>>>>
>>>> ../drivers/gpu/drm/xe/xe_vm.c: In function 'xe_vma_create':
>>>> ../drivers/gpu/drm/xe/xe_vm.c:806:21: warning: allocation of 
>>>> insufficient size '224' for type
>>> 'struct xe_vma' with size '368' [-Walloc-size]
>>>>    806 |                 vma = kzalloc(sizeof(*vma) - sizeof(struct 
>>>> xe_userptr),
>>>>        |                     ^
>>>
>>> That code is indeed odd, but there's a comment in the xe_vma definition
>>>
>>>          /**
>>>           * @userptr: user pointer state, only allocated for VMAs 
>>> that are
>>>           * user pointers
>>>           */
>>>          struct xe_userptr userptr;
>>>
>>> although I agree that it should probably simply be made a final
>>> variably-sized array instead (and then you make that array size be
>>> 0/1).
>>
>> That entire code is odd.
>> It isn't obvious that the flag values that cause the short allocate
>> are the same ones that control whether the extra data is accessed.
>>
>> Never mind the oddities with the 'flags |= ' assignments int the
>> 'remap next' path.
>>
>> Anyone know how many of these actually get allocated (and their
>> lifetimes)?
>> How much difference would it make to allocate 368 (maybe 384?)
>> bytes instead of 224 (likely 256).
>
> [CC+ xen list and maintainers]
>
> Probably the xen maintainer can help us out here.

Unfortunately the number of these can be quite large, and with a long 
lifetime which I guess was the reason that size optimization was done in 
the first place.

Ideally IMO this should've been subclassed to an xe_userptr_vma, but 
until we have a chance to clean that up, We can look at the 
variable-sized array or simply allocate the full size until we get to that.

Thanks,

Thomas

>
> -- 
> Gustavo
>
>>
>>     David
>>
>> -
>> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, 
>> MK1 1PT, UK
>> Registration No: 1397386 (Wales)