[PATCH v6 3/4] atomic: Add alignment check to instrumented atomic operations

[Finn Thain]

From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>

Add a Kconfig option for debug builds which logs a warning when an
instrumented atomic operation takes place that's misaligned.
Some platforms don't trap for this.

[fthain: added __DISABLE_BUG_TABLE macro.]

Cc: Sasha Levin <sashal@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: x86@xxxxxxxxxx
Cc: Ard Biesheuvel <ardb@xxxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Link: https://lore.kernel.org/lkml/20250901093600.GF4067720@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
Link: https://lore.kernel.org/linux-next/df9fbd22-a648-ada4-fee0-68fe4325ff82@xxxxxxxxxxxxxx/
Signed-off-by: Finn Thain <fthain@xxxxxxxxxxxxxx>
---
Checkpatch.pl says...
ERROR: Missing Signed-off-by: line by nominal patch author 'Peter Ziljstra <peterz@xxxxxxxxxxxxx>'
---
Changed since v5:
 - Add new __DISABLE_BUG_TABLE macro to prevent a build failure on those
architectures which use atomics in pre-boot code like the EFI stub loader:

x86_64-linux-gnu-ld: error: unplaced orphan section `__bug_table' from `arch/x86/boot/compressed/sev-handle-vc.o'

Changed since v2:
 - Always check for natural alignment.
---
 arch/x86/boot/compressed/Makefile     |  1 +
 drivers/firmware/efi/libstub/Makefile |  1 +
 include/linux/instrumented.h          | 10 ++++++++++
 lib/Kconfig.debug                     | 10 ++++++++++
 4 files changed, 22 insertions(+)

diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
index 68f9d7a1683b..122967c80e48 100644
--- a/arch/x86/boot/compressed/Makefile
+++ b/arch/x86/boot/compressed/Makefile
@@ -42,6 +42,7 @@ KBUILD_CFLAGS += -Wno-microsoft-anon-tag
 endif
 KBUILD_CFLAGS += -Wno-pointer-sign
 KBUILD_CFLAGS += -fno-asynchronous-unwind-tables
+KBUILD_CFLAGS += -D__DISABLE_BUG_TABLE
 KBUILD_CFLAGS += -D__DISABLE_EXPORTS
 # Disable relocation relaxation in case the link is not PIE.
 KBUILD_CFLAGS += $(call cc-option,-Wa$(comma)-mrelax-relocations=no)
diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile
index 7d15a85d579f..ac3e7c64aedb 100644
--- a/drivers/firmware/efi/libstub/Makefile
+++ b/drivers/firmware/efi/libstub/Makefile
@@ -42,6 +42,7 @@ KBUILD_CFLAGS			:= $(subst $(CC_FLAGS_FTRACE),,$(cflags-y)) \
 				   -ffreestanding \
 				   -fno-stack-protector \
 				   $(call cc-option,-fno-addrsig) \
+				   -D__DISABLE_BUG_TABLE \
 				   -D__DISABLE_EXPORTS
 
 #
diff --git a/include/linux/instrumented.h b/include/linux/instrumented.h
index 711a1f0d1a73..bcd1113b55a1 100644
--- a/include/linux/instrumented.h
+++ b/include/linux/instrumented.h
@@ -7,6 +7,7 @@
 #ifndef _LINUX_INSTRUMENTED_H
 #define _LINUX_INSTRUMENTED_H
 
+#include <linux/bug.h>
 #include <linux/compiler.h>
 #include <linux/kasan-checks.h>
 #include <linux/kcsan-checks.h>
@@ -67,6 +68,9 @@ static __always_inline void instrument_atomic_read(const volatile void *v, size_
 {
 	kasan_check_read(v, size);
 	kcsan_check_atomic_read(v, size);
+#ifndef __DISABLE_BUG_TABLE
+	WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & (size - 1)));
+#endif
 }
 
 /**
@@ -81,6 +85,9 @@ static __always_inline void instrument_atomic_write(const volatile void *v, size
 {
 	kasan_check_write(v, size);
 	kcsan_check_atomic_write(v, size);
+#ifndef __DISABLE_BUG_TABLE
+	WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & (size - 1)));
+#endif
 }
 
 /**
@@ -95,6 +102,9 @@ static __always_inline void instrument_atomic_read_write(const volatile void *v,
 {
 	kasan_check_write(v, size);
 	kcsan_check_atomic_read_write(v, size);
+#ifndef __DISABLE_BUG_TABLE
+	WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & (size - 1)));
+#endif
 }
 
 /**
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index ba36939fda79..4b4d1445ef9c 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1359,6 +1359,16 @@ config DEBUG_PREEMPT
 	  depending on workload as it triggers debugging routines for each
 	  this_cpu operation. It should only be used for debugging purposes.
 
+config DEBUG_ATOMIC
+	bool "Debug atomic variables"
+	depends on DEBUG_KERNEL
+	help
+	  If you say Y here then the kernel will add a runtime alignment check
+	  to atomic accesses. Useful for architectures that do not have trap on
+	  mis-aligned access.
+
+	  This option has potentially significant overhead.
+
 menu "Lock Debugging (spinlocks, mutexes, etc...)"
 
 config LOCK_DEBUGGING_SUPPORT
-- 
2.49.1