6.6-stable review patch. If anyone has any objections, please let me know.
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From: Jacob Keller jacob.e.keller@intel.com
[ Upstream commit 61b2b3737499f1fb361a54a16828db24a8345e85 ]
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@intel.com Closes: https://lore.kernel.org/oe-kbuild-all/202312050324.0kv4PnfZ-lkp@intel.com/ Suggested-by: Timur Tabi ttabi@nvidia.com Signed-off-by: Jacob Keller jacob.e.keller@intel.com Link: https://lore.kernel.org/r/20250610-jk-nouveua-drm-bl-snprintf-fix-v2-1-7fdd4... Signed-off-by: Danilo Krummrich dakr@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- drivers/gpu/drm/nouveau/nouveau_backlight.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/drivers/gpu/drm/nouveau/nouveau_backlight.c b/drivers/gpu/drm/nouveau/nouveau_backlight.c index 91b5ecc575380..4e6f305c9e504 100644 --- a/drivers/gpu/drm/nouveau/nouveau_backlight.c +++ b/drivers/gpu/drm/nouveau/nouveau_backlight.c @@ -42,7 +42,7 @@ #include "nouveau_acpi.h"
static struct ida bl_ida; -#define BL_NAME_SIZE 15 // 12 for name + 2 for digits + 1 for '\0' +#define BL_NAME_SIZE 24 // 12 for name + 11 for digits + 1 for '\0'
static bool nouveau_get_backlight_name(char backlight_name[BL_NAME_SIZE],