The following commit has been merged into the x86/build branch of tip: Commit-ID: 9b2687f29bc1a050ffd63b425129aa9db987e4f3 Gitweb: https://git.kernel.org/tip/9b2687f29bc1a050ffd63b425129aa9db987e4f3 Author: Nick Desaulniers <ndesaulniers@xxxxxxxxxx> AuthorDate: Thu, 03 Feb 2022 12:40:25 -08:00 Committer: Borislav Petkov <bp@xxxxxxx> CommitterDate: Thu, 07 Apr 2022 11:55:42 +02:00 x86/build: Don't build CONFIG_X86_32 as -ffreestanding -ffreestanding typically inhibits "libcall optimizations" where calls to certain library functions can be replaced by the compiler in certain cases to calls to other library functions that may be more efficient. This can be problematic for embedded targets that don't provide full libc implementations. -ffreestanding inhibits all such optimizations, which is the safe choice, but generally we want the optimizations that are performed. The Linux kernel does implement a fair amount of libc routines. Instead of -ffreestanding (which makes more sense in smaller images like kexec's purgatory image), prefer -fno-builtin-* flags to disable the compiler from emitting calls to functions which may not be defined. If you see a linkage failure due to a missing symbol that's typically defined in a libc, and not explicitly called from the source code, then the compiler may have done such a transform. You can either implement such a function (i.e. in lib/string.c) or disable the transform outright via -fno-builtin-* flag (where * is the name of the library routine, i.e. -fno-builtin-bcmp). i386_defconfig build+boot tested with GCC and Clang. Removes a pretty old TODO from the code base. [kees: These libcall optimizations are specifically needed to allow Clang to correctly optimize the string functions under CONFIG_FORTIFY_SOURCE.] Fixes: 6edfba1b33c7 ("[PATCH] x86_64: Don't define string functions to builtin") Suggested-by: Arvind Sankar <nivedita@xxxxxxxxxxxx> Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx> Signed-off-by: Borislav Petkov <bp@xxxxxxx> Reviewed-by: Fangrui Song <maskray@xxxxxxxxxx> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx> Link: https://lore.kernel.org/r/20200817220212.338670-5-ndesaulniers@xxxxxxxxxx Link: https://lore.kernel.org/r/20220203204025.1153397-1-keescook@xxxxxxxxxxxx --- arch/x86/Makefile | 3 --- 1 file changed, 3 deletions(-) diff --git a/arch/x86/Makefile b/arch/x86/Makefile index 1abd7cc..670fe40 100644 --- a/arch/x86/Makefile +++ b/arch/x86/Makefile @@ -100,9 +100,6 @@ ifeq ($(CONFIG_X86_32),y) include $(srctree)/arch/x86/Makefile_32.cpu KBUILD_CFLAGS += $(cflags-y) - # temporary until string.h is fixed - KBUILD_CFLAGS += -ffreestanding - ifeq ($(CONFIG_STACKPROTECTOR),y) ifeq ($(CONFIG_SMP),y) KBUILD_CFLAGS += -mstack-protector-guard-reg=fs -mstack-protector-guard-symbol=__stack_chk_guard
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