Messages in this thread Patch in this message | | | From | Mark Rutland <> | Subject | [PATCH 1/8] Compiler attributes: GCC function alignment workarounds | Date | Mon, 9 Jan 2023 13:58:21 +0000 |
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From local testing, contemporary versions of of GCC (e.g. GCC 12.2.0) don't respect '-falign-functions=N' in all cases. This is unfortunate, as (for non-zero values of N) CONFIG_FUNCTION_ALIGNMENT=N will set '-falign-functions=N', but this won't take effect for all functions. LLVM appears to respect '-falign-functions=N' in call cases.
Today, for x86 this turns out to be functionally benign, though it does somewhat undermine the CONFIG_FUNCTION_ALIGNMENT option, and it means that CONFIG_DEBUG_FORCE_FUNCTION_ALIGN_64B is not as robust as we'd like.
On arm64 we'd like to use CONFIG_FUNCTION_ALIGNMENT to implement ftrace functionality, and we'll require function alignment to be respected for functional reasons.
As far as I can tell, GCC doesn't respect '-falign-functions=N':
* When the __weak__ attribute is used
GCC seems to forget the alignment specified by '-falign-functions=N', but will respect the '__aligned__(N)' function attribute. Thus, we can work around this by explciitly setting the alignment for weak functions.
* When the __cold__ attribute is used
GCC seems to forget the alignment specified by '-falign-functions=N', and also doesn't seem to respect the '__aligned__(N)' function attribute. The only way to work around this is to not use the __cold__ attibute.
This patch implements workarounds for these two cases, using a function attribute to set the alignment of __weak__ functions, and preventing the use of the __cold__ attribute when CONFIG_FUNCTION_ALIGNMENT is non-zero.
I've tested this by selecting CONFIG_DEBUG_FORCE_FUNCTION_ALIGN_64B=y, building and booting a kernel, and looking for misaligned text symbols:
* arm64:
Before: # grep ' [Tt] ' /proc/kallsyms | grep -iv '[048c]0 [Tt] ' | wc -l 4939
After: # grep ' [Tt] ' /proc/kallsyms | grep -iv '[048c]0 [Tt] ' | wc -l 908
* x86_64:
Before: # grep ' [Tt] ' /proc/kallsyms | grep -iv '[048c]0 [Tt] ' | wc -l 7969
After: # grep ' [Tt] ' /proc/kallsyms | grep -iv '[048c]0 [Tt] ' | wc -l 2057
With the patch applied, the remaining unaligned text labels are a combination of static call trampolines, non-function labels in assembly, and ACPICA functions, which will be dealt with in subsequent patches.
Signed-off-by: Mark Rutland <mark.rutland@arm.com> Cc: Catalin Marinas <catalin.marinas@arm.com> Cc: Florent Revest <revest@chromium.org> Cc: Masami Hiramatsu <mhiramat@kernel.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Steven Rostedt <rostedt@goodmis.org> Cc: Will Deacon <will@kernel.org> Cc: Miguel Ojeda <ojeda@kernel.org> Cc: Nick Desaulniers <ndesaulniers@google.com> --- include/linux/compiler_attributes.h | 23 ++++++++++++++++++++++- 1 file changed, 22 insertions(+), 1 deletion(-)
diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h index 898b3458b24a0..dcb7ac67b764f 100644 --- a/include/linux/compiler_attributes.h +++ b/include/linux/compiler_attributes.h @@ -33,6 +33,17 @@ #define __aligned(x) __attribute__((__aligned__(x))) #define __aligned_largest __attribute__((__aligned__)) +/* + * Contemporary versions of GCC (e.g. 12.2.0) don't always respect + * '-falign-functions=N', and require alignment to be specificed via a function + * attribute in some cases. + */ +#if CONFIG_FUNCTION_ALIGNMENT > 0 +#define __function_aligned __aligned(CONFIG_FUNCTION_ALIGNMENT) +#else +#define __function_aligned +#endif + /* * Note: do not use this directly. Instead, use __alloc_size() since it is conditionally * available and includes other attributes. For GCC < 9.1, __alloc_size__ gets undefined @@ -78,8 +89,15 @@ /* * gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-cold-function-attribute * gcc: https://gcc.gnu.org/onlinedocs/gcc/Label-Attributes.html#index-cold-label-attribute + * + * GCC drops function alignment when the __cold__ attribute is used. Avoid the + * __cold__ attribute if function alignment is required. */ +#if !defined(CONFIG_CC_IS_GCC) || (CONFIG_FUNCTION_ALIGNMENT == 0) #define __cold __attribute__((__cold__)) +#else +#define __cold +#endif /* * Note the long name. @@ -369,8 +387,11 @@ /* * gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-weak-function-attribute * gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html#index-weak-variable-attribute + * + * GCC drops function alignment when the __weak__ attribute is used. This can + * be restored with function attributes. */ -#define __weak __attribute__((__weak__)) +#define __weak __attribute__((__weak__)) __function_aligned /* * Used by functions that use '__builtin_return_address'. These function -- 2.30.2
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