compiler_types: Provide __no_kstack_erase to disable coverage only on Clang

In order to support Clang's stack depth tracking (for Linux's kstack_erase
feature), the coverage sanitizer needed to be disabled for __init (and
__head) section code. Doing this universally (i.e. for GCC too) created
a number of unexpected problems, ranging from changes to inlining logic
to failures to DCE code on earlier GCC versions.

Since this change is only needed for Clang, specialize it so that GCC
doesn't see the change as it isn't needed there (the GCC implementation
of kstack_erase uses a GCC plugin that removes stack depth tracking
instrumentation from __init sections during a late pass in the IR).

Successfully build and boot tested with GCC 12 and Clang 22.

Fixes: 381a38e ("init.h: Disable sanitizer coverage for __init and __head")
Reported-by: kernel test robot <[email protected]>
Closes: https://lore.kernel.org/oe-kbuild-all/[email protected]/
Reported-by: [email protected]
Closes: https://lore.kernel.org/all/[email protected]/
Reviewed-by: Nathan Chancellor <[email protected]>
Reviewed-by: Marco Elver <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Kees Cook <[email protected]>

Author: kees

arch/x86/include/asm/init.h

@@ -5,7 +5,7 @@
 #if defined(CONFIG_CC_IS_CLANG) && CONFIG_CLANG_VERSION < 170000
 #define __head	__section(".head.text") __no_sanitize_undefined __no_stack_protector
 #else
-#define __head	__section(".head.text") __no_sanitize_undefined __no_sanitize_coverage
+#define __head	__section(".head.text") __no_sanitize_undefined __no_kstack_erase
 #endif
 
 struct x86_mapping_info {

include/linux/compiler-clang.h

@@ -89,6 +89,9 @@
 #define __no_sanitize_coverage
 #endif
 
+/* Only Clang needs to disable the coverage sanitizer for kstack_erase. */
+#define __no_kstack_erase	__no_sanitize_coverage
+
 #if __has_feature(shadow_call_stack)
 # define __noscs	__attribute__((__no_sanitize__("shadow-call-stack")))
 #endif

include/linux/compiler_types.h

@@ -424,6 +424,10 @@ struct ftrace_likely_data {
 # define randomized_struct_fields_end
 #endif
 
+#ifndef __no_kstack_erase
+# define __no_kstack_erase
+#endif
+
 #ifndef __noscs
 # define __noscs
 #endif

include/linux/init.h

@@ -51,7 +51,7 @@
    discard it in modules) */
 #define __init		__section(".init.text") __cold __latent_entropy	\
 						__noinitretpoline	\
-						__no_sanitize_coverage
+						__no_kstack_erase
 #define __initdata	__section(".init.data")
 #define __initconst	__section(".init.rodata")
 #define __exitdata	__section(".exit.data")