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.
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.
Refactor the function to avoid the fixed-length buffer entirely. Use
kasprintf to allocate a buffer of appropriate size. This avoids the need
for BL_NAME_SIZE and resolves the -Wformat-truncation warning, fixing W=1
builds.
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/
Signed-off-by: Jacob Keller <jacob.e.kel...@intel.com>
---
This could also be fixed by simply increasing BL_NAME_SIZE to 24, making it
large enough to fit any size integer into its format string, or by checking
the return value of snprintf.
---
drivers/gpu/drm/nouveau/nouveau_backlight.c | 25 +++++++++++++------------
1 file changed, 13 insertions(+), 12 deletions(-)
diff --git a/drivers/gpu/drm/nouveau/nouveau_backlight.c
b/drivers/gpu/drm/nouveau/nouveau_backlight.c
index
d47442125fa183146135f3725eae161c68e2a900..d3cf69fe2eeb33c24ee698db19b34a56a72f51cb
100644
--- a/drivers/gpu/drm/nouveau/nouveau_backlight.c
+++ b/drivers/gpu/drm/nouveau/nouveau_backlight.c
@@ -42,22 +42,21 @@
#include "nouveau_acpi.h"
static struct ida bl_ida;
-#define BL_NAME_SIZE 15 // 12 for name + 2 for digits + 1 for '\0'
-static bool
-nouveau_get_backlight_name(char backlight_name[BL_NAME_SIZE],
- struct nouveau_backlight *bl)
+static char *
+nouveau_get_backlight_name(struct nouveau_backlight *bl)
{
const int nb = ida_alloc_max(&bl_ida, 99, GFP_KERNEL);
if (nb < 0)
- return false;
- if (nb > 0)
- snprintf(backlight_name, BL_NAME_SIZE, "nv_backlight%d", nb);
- else
- snprintf(backlight_name, BL_NAME_SIZE, "nv_backlight");
+ return NULL;
+
bl->id = nb;
- return true;
+
+ if (nb > 0)
+ return kasprintf(GFP_KERNEL, "nv_backlight%d", nb);
+ else
+ return kasprintf(GFP_KERNEL, "nv_backlight");
}
static int
@@ -293,9 +292,9 @@ nouveau_backlight_init(struct drm_connector *connector)
struct nouveau_backlight *bl;
struct nouveau_encoder *nv_encoder = NULL;
struct nvif_device *device = &drm->client.device;
- char backlight_name[BL_NAME_SIZE];
struct backlight_properties props = {0};
const struct backlight_ops *ops;
+ char *backlight_name;
int ret;
if (apple_gmux_present()) {
@@ -348,7 +347,8 @@ nouveau_backlight_init(struct drm_connector *connector)
goto fail_alloc;
}
- if (!nouveau_get_backlight_name(backlight_name, bl)) {
+ backlight_name = nouveau_get_backlight_name(bl);
+ if (!backlight_name) {
NV_ERROR(drm, "Failed to retrieve a unique name for the
backlight interface\n");
goto fail_alloc;
}
@@ -356,6 +356,7 @@ nouveau_backlight_init(struct drm_connector *connector)
props.type = BACKLIGHT_RAW;
bl->dev = backlight_device_register(backlight_name, connector->kdev,
nv_encoder, ops, &props);
+ kfree(backlight_name);
if (IS_ERR(bl->dev)) {
if (bl->id >= 0)
ida_free(&bl_ida, bl->id);
---
base-commit: 90b83efa6701656e02c86e7df2cb1765ea602d07
change-id: 20250604-jk-nouveua-drm-bl-snprintf-fix-c2ca525a3d2f
Best regards,
--
Jacob Keller <jacob.e.kel...@intel.com>
