Re: drivers/platform/surface/surface3_power.c:243:2: warning: 'snprintf' will always be truncated; specified size is 10, but format string expands to at least 12

[Nathan Chancellor <nathan@kernel.org>]

On Wed, Jan 29, 2025 at 02:45:38PM +0200, Andy Shevchenko wrote:
> On Wed, Jan 29, 2025 at 10:09 AM kernel test robot <lkp@intel.com> wrote:
> >
> > tree:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
> > head:   05dbaf8dd8bf537d4b4eb3115ab42a5fb40ff1f5
> > commit: 85f7582cd484dbf491b6d9bb2af6ef1467a024d2 platform/surface: Move Surface 3 Power OpRegion driver to platform/surface
> > date:   4 years, 3 months ago
> > config: x86_64-randconfig-071-20241231 (https://download.01.org/0day-ci/archive/20250129/202501291646.VtwF98qd-lkp@intel.com/config)
> > compiler: clang version 19.1.3 (https://github.com/llvm/llvm-project ab51eccf88f5321e7c60591c5546b254b6afab99)
> > reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20250129/202501291646.VtwF98qd-lkp@intel.com/reproduce)
> >
> > If you fix the issue in a separate patch/commit (i.e. not just a new version of
> > the same patch/commit), kindly add following tags
> > | Reported-by: kernel test robot <lkp@intel.com>
> > | Closes: https://lore.kernel.org/oe-kbuild-all/202501291646.VtwF98qd-lkp@intel.com/
> >
> > All warnings (new ones prefixed by >>):
> >
> > >> drivers/platform/surface/surface3_power.c:243:2: warning: 'snprintf' will always be truncated; specified size is 10, but format string expands to at least 12 [-Wformat-truncation-non-kprintf]
> >      243 |         snprintf(bix->serial, ARRAY_SIZE(bix->serial), "%3pE%6pE", buf + 7, buf);
> 
> 3 + 'E' + 6 + 'E' + NUL, that's how the compiler thinks about this
> formatting string...
> 
> >          |         ^
> >    1 warning generated.
> 
> I do not know how to properly address this because it's false positive
> due to the compiler not being aware of pointer extensions used in
> Linux kernel. The w/a is to increase the buffer if we can, but is it
> practical? OTOH, this warning will be translated to a compilation
> error in case of default WERROR settings on `make W=1` builds which is
> also annoying to people what want have that clean.

Hmmm, I missed that -Wformat-truncation-non-kprintf was only disabled in
the non-W=1 block in scripts/Makefile.extrawarn. We never want to see
these warnings at any level because they will almost always be a false
positive, that's why the "non-kprintf" flags even exist in the first
place:

https://github.com/llvm/llvm-project/pull/65969

I suspect we want something like this.

diff --git a/scripts/Makefile.extrawarn b/scripts/Makefile.extrawarn
index 1d13cecc7cc7..8878f71463fc 100644
--- a/scripts/Makefile.extrawarn
+++ b/scripts/Makefile.extrawarn
@@ -31,6 +31,11 @@ KBUILD_CFLAGS-$(CONFIG_CC_NO_ARRAY_BOUNDS) += -Wno-array-bounds
 ifdef CONFIG_CC_IS_CLANG
 # The kernel builds with '-std=gnu11' so use of GNU extensions is acceptable.
 KBUILD_CFLAGS += -Wno-gnu
+
+# Clang checks for overflow/truncation with '%p', while GCC does not:
+# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111219
+KBUILD_CFLAGS += $(call cc-disable-warning, format-overflow-non-kprintf)
+KBUILD_CFLAGS += $(call cc-disable-warning, format-truncation-non-kprintf)
 else
 
 # gcc inanely warns about local variables called 'main'
@@ -102,11 +107,6 @@ KBUILD_CFLAGS += $(call cc-disable-warning, packed-not-aligned)
 KBUILD_CFLAGS += $(call cc-disable-warning, format-overflow)
 ifdef CONFIG_CC_IS_GCC
 KBUILD_CFLAGS += $(call cc-disable-warning, format-truncation)
-else
-# Clang checks for overflow/truncation with '%p', while GCC does not:
-# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111219
-KBUILD_CFLAGS += $(call cc-disable-warning, format-overflow-non-kprintf)
-KBUILD_CFLAGS += $(call cc-disable-warning, format-truncation-non-kprintf)
 endif
 KBUILD_CFLAGS += $(call cc-disable-warning, stringop-truncation)