[PATCH] kbuild: Disable clang's -Wformat-{overflow,truncation}-non-kprintf

From: Nathan Chancellor <nathan@kernel.org>
To: masahiroy@kernel.org, ndesaulniers@google.com
Cc: nicolas@fjasle.eu, trix@redhat.com, linux-kbuild@vger.kernel.org,
	 llvm@lists.linux.dev, patches@lists.linux.dev,
	 Nathan Chancellor <nathan@kernel.org>
Subject: [PATCH] kbuild: Disable clang's -Wformat-{overflow,truncation}-non-kprintf
Date: Mon, 02 Oct 2023 12:52:28 -0700	[thread overview]
Message-ID: <20231002-disable-wformat-truncation-overflow-non-kprintf-v1-1-35179205c8d9@kernel.org> (raw)

Recently, clang added support for -Wformat-overflow and
-Wformat-truncation. When building the kernel, it was discovered that
clang's implementation of these warnings handles the '%p' specifier,
which differs from GCC's implementation. This results in false positive
warnings due to the kernel's various '%p' extensions. Fortunately, the
clang developers placed this warning difference into a separate flag,
allowing the kernel to turn off the warning for '%p' unconditionally.

This is not currently an issue for a normal build, as -Wformat-overflow
and -Wformat-truncation are unconditionally disabled, which includes
this sub-warning. However, ever since commit 6d4ab2e97dcf ("extrawarn:
enable format and stringop overflow warnings in W=1"), these warnings
are in W=1 and the goal is to enable them in the normal build once they
are all eliminated. Disable the warnings for W=1 to avoid false
positives. This block should move with -Wformat-overflow and
-Wformat-truncation when they are enabled for a normal build.

Link: https://github.com/ClangBuiltLinux/linux/issues/1923
Link: https://github.com/llvm/llvm-project/issues/64871
Link: https://github.com/llvm/llvm-project/pull/65969
Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111219
Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78512
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
---
 scripts/Makefile.extrawarn | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/scripts/Makefile.extrawarn b/scripts/Makefile.extrawarn
index 2fe6f2828d37..bad1baa2cfb4 100644
--- a/scripts/Makefile.extrawarn
+++ b/scripts/Makefile.extrawarn
@@ -106,6 +106,13 @@ KBUILD_CFLAGS += $(call cc-option, -Wunused-const-variable)
 KBUILD_CFLAGS += $(call cc-option, -Wpacked-not-aligned)
 KBUILD_CFLAGS += $(call cc-option, -Wformat-overflow)
 KBUILD_CFLAGS += $(call cc-option, -Wformat-truncation)
+# Clang checks for overflow/truncation with '%p', while GCC does not:
+# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111219
+# The kernel has many extensions to '%p' that clang does not understand, so
+# always disable these warnings when '-Wformat-truncation' and
+# '-Wformat-overflow' are enabled.
+KBUILD_CFLAGS += $(call cc-disable-warning, format-overflow-non-kprintf)
+KBUILD_CFLAGS += $(call cc-disable-warning, format-truncation-non-kprintf)
 KBUILD_CFLAGS += $(call cc-option, -Wstringop-overflow)
 KBUILD_CFLAGS += $(call cc-option, -Wstringop-truncation)
 

---
base-commit: 8a749fd1a8720d4619c91c8b6e7528c0a355c0aa
change-id: 20231002-disable-wformat-truncation-overflow-non-kprintf-033e8d8b4de8

Best regards,
-- 
Nathan Chancellor <nathan@kernel.org>

