linux-kernel - Re: [PATCH] drm/i195: Fix format string truncation warning

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Message-ID: <f33b29bb-72bd-4925-adb6-e8cd5267c142@ursulin.net>
Date: Fri, 5 Dec 2025 11:13:15 +0000
From: Tvrtko Ursulin <tursulin@...ulin.net>
To: Ard Biesheuvel <ardb@...nel.org>
Cc: Ard Biesheuvel <ardb+git@...gle.com>, linux-kernel@...r.kernel.org,
 Jani Nikula <jani.nikula@...ux.intel.com>,
 Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
 Rodrigo Vivi <rodrigo.vivi@...el.com>, David Airlie <airlied@...il.com>,
 Simona Vetter <simona@...ll.ch>, intel-gfx@...ts.freedesktop.org,
 dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH] drm/i195: Fix format string truncation warning


On 05/12/2025 10:48, Ard Biesheuvel wrote:
> On Sun, 9 Nov 2025 at 19:00, Ard Biesheuvel <ardb@...nel.org> wrote:
>>
>> On Sat, 8 Nov 2025 at 01:27, Tvrtko Ursulin <tursulin@...ulin.net> wrote:
>>>
>>>
>>> On 07/11/2025 16:42, Ard Biesheuvel wrote:
>>>> From: Ard Biesheuvel <ardb@...nel.org>
>>>>
>>>> GCC notices that the 16-byte uabi_name field could theoretically be too
>>>> small for the formatted string if the instance number exceeds 100.
>>>>
>>>> Given that there are apparently ABI concerns here, this is the minimal
>>>> fix that shuts up the compiler without changing the output or the
>>>> maximum length for existing values < 100.
>>>
>>> What would be those ABI concerns? I don't immediately see any.
>>>> drivers/gpu/drm/i915/intel_memory_region.c: In function ‘intel_memory_region_create’:
>>>> drivers/gpu/drm/i915/intel_memory_region.c:273:61: error: ‘%u’ directive output may be truncated writing between 1 and 5 bytes into a region of size between 3 and 11 [-Werror=format-truncation=]
>>>>     273 |         snprintf(mem->uabi_name, sizeof(mem->uabi_name), "%s%u",
>>>>         |                                                             ^~
>>>> drivers/gpu/drm/i915/intel_memory_region.c:273:58: note: directive argument in the range [0, 65535]
>>>>     273 |         snprintf(mem->uabi_name, sizeof(mem->uabi_name), "%s%u",
>>>>         |                                                          ^~~~~~
>>>> drivers/gpu/drm/i915/intel_memory_region.c:273:9: note: ‘snprintf’ output between 7 and 19 bytes into a destination of size 16
>>>>     273 |         snprintf(mem->uabi_name, sizeof(mem->uabi_name), "%s%u",
>>>>         |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>>>     274 |                  intel_memory_type_str(type), instance);
>>>>         |                  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>>>
>>>> Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
>>>> ---
>>>> Cc: Jani Nikula <jani.nikula@...ux.intel.com>
>>>> Cc: Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>
>>>> Cc: Rodrigo Vivi <rodrigo.vivi@...el.com>
>>>> Cc: Tvrtko Ursulin <tursulin@...ulin.net>
>>>> Cc: David Airlie <airlied@...il.com>
>>>> Cc: Simona Vetter <simona@...ll.ch>
>>>> Cc: intel-gfx@...ts.freedesktop.org
>>>> Cc: dri-devel@...ts.freedesktop.org
>>>>
>>>> This is unlikely to be the right fix, but sending a wrong patch is
>>>> usually a better way to elicit a response than just sending a bug
>>>> report.
>>>>
>>>>    drivers/gpu/drm/i915/intel_memory_region.c | 2 +-
>>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/i915/intel_memory_region.c b/drivers/gpu/drm/i915/intel_memory_region.c
>>>> index 59bd603e6deb..ad4afcf0c58a 100644
>>>> --- a/drivers/gpu/drm/i915/intel_memory_region.c
>>>> +++ b/drivers/gpu/drm/i915/intel_memory_region.c
>>>> @@ -271,7 +271,7 @@ intel_memory_region_create(struct drm_i915_private *i915,
>>>>        mem->instance = instance;
>>>>
>>>>        snprintf(mem->uabi_name, sizeof(mem->uabi_name), "%s%u",
>>>> -              intel_memory_type_str(type), instance);
>>>> +              intel_memory_type_str(type), instance % 100);
>>> It's a theoretical issue only since there is no hardware with a double
>>> digit number of instances.
>>>
>>> But I guess much prettier fix would be to simply grow the buffer.
>>>
>>
> 
> OK, so something like
> 
> --- a/drivers/gpu/drm/i915/intel_memory_region.h
> +++ b/drivers/gpu/drm/i915/intel_memory_region.h
> @@ -72,7 +72,7 @@ struct intel_memory_region {
>          u16 instance;
>          enum intel_region_id id;
>          char name[16];
> -       char uabi_name[16];
> +       char uabi_name[20];
>          bool private; /* not for userspace */
> 
>          struct {

Yes please. There is only two of those objects at majority of systems, 
and 3-4 on a few discrete cards supported by i915, so no big deal to 
grow them a tiny bit.

>>> Also, hm, how come gcc does not find the mem->name vsnprintf from
>>> intel_memory_region_set_name?
>>>
>>
> 
> AFAICT, intel_memory_region_set_name() is never called with a format
> string that could produce more than 15/16 bytes of output.

Right, I reminded myself that the non-uabi visible name does not have 
the instance number in it.

Regards,

Tvrtko