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 <l...@intel.com>
Closes:
https://lore.kernel.org/oe-kbuild-all/202312050324.0kv4pnfz-...@intel.com/
Suggested-by: Timur Tabi <tt...@nvidia.com>
Signed-off-by: Jacob Keller <jacob.e.kel...@intel.com>
---
Changes in v2:
- Just increase the buffer size
- Link to v1:
https://lore.kernel.org/r/20250604-jk-nouveua-drm-bl-snprintf-fix-v1-1-79b1593ad...@intel.com
---
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
d47442125fa183146135f3725eae161c68e2a900..9aae26eb7d8fba54c8a989bfe7ecc2b10ccf7f61
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],
---
base-commit: 90b83efa6701656e02c86e7df2cb1765ea602d07
change-id: 20250604-jk-nouveua-drm-bl-snprintf-fix-c2ca525a3d2f
Best regards,
--
Jacob Keller <jacob.e.kel...@intel.com>
