Re: [PATCH v2] drm/nouveau/bl: increase buffer size to avoid truncate warning

[Danilo Krummrich]

On 6/10/25 11:54 PM, Jacob Keller wrote:
> The nouveau_get_backlight_name() function generates a unique name for the
> backlight interface, appending an id from 1 to 99 for all backlight devices
> after the first.
>
> GCC 15 (and likely other compilers) produce the following
> -Wformat-truncation warning:
>
> nouveau_backlight.c: In function ‘nouveau_backlight_init’:
> nouveau_backlight.c:56:69: error: ‘%d’ directive output may be truncated writing between 1 and 10 bytes into a region of size 3 [-Werror=format-truncation=]
>     56 |                 snprintf(backlight_name, BL_NAME_SIZE, "nv_backlight%d", nb);
>        |                                                                     ^~
> In function ‘nouveau_get_backlight_name’,
>      inlined from ‘nouveau_backlight_init’ at nouveau_backlight.c:351:7:
> nouveau_backlight.c:56:56: note: directive argument in the range [1, 2147483647]
>     56 |                 snprintf(backlight_name, BL_NAME_SIZE, "nv_backlight%d", nb);
>        |                                                        ^~~~~~~~~~~~~~~~
> nouveau_backlight.c:56:17: note: ‘snprintf’ output between 14 and 23 bytes into a destination of size 15
>     56 |                 snprintf(backlight_name, BL_NAME_SIZE, "nv_backlight%d", nb);
>        |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> The warning started appearing after commit ab244be47a8f ("drm/nouveau:
> Fix a potential theorical leak in nouveau_get_backlight_name()") This fix
> for the ida usage removed the explicit value check for ids larger than 99.
> The compiler is unable to intuit that the ida_alloc_max() limits the
> returned value range between 0 and 99.
>
> Because the compiler can no longer infer that the number ranges from 0 to
> 99, it thinks that it could use as many as 11 digits (10 + the potential -
> sign for negative numbers).
>
> The warning has gone unfixed for some time, with at least one kernel test
> robot report. The code breaks W=1 builds, which is especially frustrating
> with the introduction of CONFIG_WERROR.
>
> The string is stored temporarily on the stack and then copied into the
> device name. Its not a big deal to use 11 more bytes of stack rounding out
> to an even 24 bytes. Increase BL_NAME_SIZE to 24 to avoid the truncation
> warning. This fixes the W=1 builds that include this driver.
>
> Compile tested only.
>
> Fixes: ab244be47a8f ("drm/nouveau: Fix a potential theorical leak in nouveau_get_backlight_name()")
> Reported-by: kernel test robot <lkp@xxxxxxxxx>
> Closes: https://lore.kernel.org/oe-kbuild-all/202312050324.0kv4PnfZ-lkp@xxxxxxxxx/
> Suggested-by: Timur Tabi <ttabi@xxxxxxxxxx>
> Signed-off-by: Jacob Keller <jacob.e.keller@xxxxxxxxx>

Applied to drm-misc-fixes, thanks!