linux-kernel - [PATCH] kbuild: Use -fmin-function-alignment when available

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Message-Id: <20240212145355.1050-1-petr.pavlu@suse.com>
Date: Mon, 12 Feb 2024 15:53:55 +0100
From: Petr Pavlu <petr.pavlu@...e.com>
To: masahiroy@...nel.org,
	nathan@...nel.org,
	nicolas@...sle.eu,
	mark.rutland@....com
Cc: linux-kbuild@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Petr Pavlu <petr.pavlu@...e.com>
Subject: [PATCH] kbuild: Use -fmin-function-alignment when available

GCC recently added option -fmin-function-alignment, which should appear
in GCC 14. Unlike -falign-functions, this option causes all functions to
be aligned at the specified value, including the cold ones.

Detect availability of -fmin-function-alignment and use it instead of
-falign-functions when present. Introduce CC_HAS_SANE_FUNCTION_ALIGNMENT
and make the workarounds for the broken function alignment conditional
on this setting.

Signed-off-by: Petr Pavlu <petr.pavlu@...e.com>
---
 Makefile                       |  7 ++++++-
 arch/Kconfig                   |  8 ++++++++
 include/linux/compiler_types.h | 10 +++++-----
 kernel/exit.c                  |  5 ++++-
 4 files changed, 23 insertions(+), 7 deletions(-)

diff --git a/Makefile b/Makefile
index 7e0b2ad98905..9516e43f6e45 100644
--- a/Makefile
+++ b/Makefile
@@ -974,7 +974,12 @@ export CC_FLAGS_CFI
 endif
 
 ifneq ($(CONFIG_FUNCTION_ALIGNMENT),0)
-KBUILD_CFLAGS += -falign-functions=$(CONFIG_FUNCTION_ALIGNMENT)
+# Set the minimal function alignment. Try to use the newer GCC option
+# -fmin-function-alignment, or fall back to -falign-funtions. See also
+# CONFIG_CC_HAS_SANE_FUNCTION_ALIGNMENT.
+KBUILD_CFLAGS += $(call cc-option, \
+	-fmin-function-alignment=$(CONFIG_FUNCTION_ALIGNMENT), \
+	-falign-functions=$(CONFIG_FUNCTION_ALIGNMENT))
 endif
 
 # arch Makefile may override CC so keep this after arch Makefile is included
diff --git a/arch/Kconfig b/arch/Kconfig
index a5af0edd3eb8..e2448f927fae 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -1507,4 +1507,12 @@ config FUNCTION_ALIGNMENT
 	default 4 if FUNCTION_ALIGNMENT_4B
 	default 0
 
+config CC_HAS_SANE_FUNCTION_ALIGNMENT
+	# Detect availability of the GCC option -fmin-function-alignment which
+	# guarantees minimal alignment for all functions. GCC 13 and older have
+	# only -falign-functions which the compiler ignores for cold functions
+	# and this hence requires extra care in the kernel. Clang provides
+	# strict alignment always when using -falign-functions.
+	def_bool $(cc-option, -fmin-function-alignment=8) || CC_IS_CLANG
+
 endmenu
diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index 663d8791c871..f0152165e83c 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -99,17 +99,17 @@ static inline void __chk_io_ptr(const volatile void __iomem *ptr) { }
  *   gcc: https://gcc.gnu.org/onlinedocs/gcc/Label-Attributes.html#index-cold-label-attribute
  *
  * When -falign-functions=N is in use, we must avoid the cold attribute as
- * contemporary versions of GCC drop the alignment for cold functions. Worse,
- * GCC can implicitly mark callees of cold functions as cold themselves, so
- * it's not sufficient to add __function_aligned here as that will not ensure
- * that callees are correctly aligned.
+ * GCC drops the alignment for cold functions. Worse, GCC can implicitly mark
+ * callees of cold functions as cold themselves, so it's not sufficient to add
+ * __function_aligned here as that will not ensure that callees are correctly
+ * aligned.
  *
  * See:
  *
  *   https://lore.kernel.org/lkml/Y77%2FqVgvaJidFpYt@FVFF77S0Q05N
  *   https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88345#c9
  */
-#if !defined(CONFIG_CC_IS_GCC) || (CONFIG_FUNCTION_ALIGNMENT == 0)
+#if defined(CONFIG_CC_HAS_SANE_FUNCTION_ALIGNMENT) || (CONFIG_FUNCTION_ALIGNMENT == 0)
 #define __cold				__attribute__((__cold__))
 #else
 #define __cold
diff --git a/kernel/exit.c b/kernel/exit.c
index dfb963d2f862..5a6fed4ad3df 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -1920,7 +1920,10 @@ EXPORT_SYMBOL(thread_group_exited);
  *
  * See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88345#c11
  */
-__weak __function_aligned void abort(void)
+#ifndef CONFIG_CC_HAS_SANE_FUNCTION_ALIGNMENT
+__function_aligned
+#endif
+__weak void abort(void)
 {
 	BUG();
 

base-commit: 841c35169323cd833294798e58b9bf63fa4fa1de
-- 
2.35.3