On Thu, Feb 15, 2024 at 04:16:42PM +0100, Petr Pavlu wrote: > 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@xxxxxxxx> I don't have a GCC 14 build to play with, but this looks sound to me. Petr, are you able to test an arm64 kernel with this and DYNAMIC_FTRACE enabled? i.e. build that, and check that function symbols are all aligned to 8 bytes using objdump or similar? That way we could be pretty sure there's no other latent issue in this area. FWIW, given the structure looks sound: Acked-by: Mark Rutland <mark.rutland@xxxxxxx> Mark. > --- > > Changes since v1 [1]: > - Check the availability of -fmin-function-alignment only in one place. > > [1] https://lore.kernel.org/linux-kbuild/20240212145355.1050-1-petr.pavlu@xxxxxxxx/ > > Makefile | 7 +++++++ > arch/Kconfig | 12 ++++++++++++ > include/linux/compiler_types.h | 10 +++++----- > kernel/exit.c | 5 ++++- > 4 files changed, 28 insertions(+), 6 deletions(-) > > diff --git a/Makefile b/Makefile > index 7e0b2ad98905..6f20ab5e2e44 100644 > --- a/Makefile > +++ b/Makefile > @@ -974,8 +974,15 @@ export CC_FLAGS_CFI > endif > > ifneq ($(CONFIG_FUNCTION_ALIGNMENT),0) > +# Set the minimal function alignment. Use the newer GCC option > +# -fmin-function-alignment if it is available, or fall back to -falign-funtions. > +# See also CONFIG_CC_HAS_SANE_FUNCTION_ALIGNMENT. > +ifdef CONFIG_CC_HAS_MIN_FUNCTION_ALIGNMENT > +KBUILD_CFLAGS += -fmin-function-alignment=$(CONFIG_FUNCTION_ALIGNMENT) > +else > KBUILD_CFLAGS += -falign-functions=$(CONFIG_FUNCTION_ALIGNMENT) > endif > +endif > > # arch Makefile may override CC so keep this after arch Makefile is included > NOSTDINC_FLAGS += -nostdinc > diff --git a/arch/Kconfig b/arch/Kconfig > index a5af0edd3eb8..bd6c6335efac 100644 > --- a/arch/Kconfig > +++ b/arch/Kconfig > @@ -1507,4 +1507,16 @@ config FUNCTION_ALIGNMENT > default 4 if FUNCTION_ALIGNMENT_4B > default 0 > > +config CC_HAS_MIN_FUNCTION_ALIGNMENT > + # Detect availability of the GCC option -fmin-function-alignment which > + # guarantees minimal alignment for all functions, unlike > + # -falign-functions which the compiler ignores for cold functions. > + def_bool $(cc-option, -fmin-function-alignment=8) > + > +config CC_HAS_SANE_FUNCTION_ALIGNMENT > + # Set if the guaranteed alignment with -fmin-function-alignment is > + # available or extra care is required in the kernel. Clang provides > + # strict alignment always, even with -falign-functions. > + def_bool CC_HAS_MIN_FUNCTION_ALIGNMENT || 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 >
