| Message ID | 20200817220212.338670-5-ndesaulniers@google.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | -ffreestanding/-fno-builtin-* patches | expand |
On Mon, Aug 17, 2020 at 03:02:12PM -0700, Nick Desaulniers wrote: > -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 (ie. in lib/string.c) or disable the transform outright > via -fno-builtin-* flag (where * is the name of the library routine, ie. > -fno-builtin-bcmp). > > i386_defconfig build+boot tested with GCC and Clang. Removes a pretty > old TODO from the codebase. > > Fixes: 6edfba1b33c7 ("x86_64: Don't define string functions to builtin") > Suggested-by: Arvind Sankar <nivedita@alum.mit.edu> > Signed-off-by: Nick Desaulniers <ndesaulniers@google.com> Reviewed-by: Kees Cook <keescook@chromium.org>
On 2020-08-17, Nick Desaulniers wrote: >-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 (ie. in lib/string.c) or disable the transform outright >via -fno-builtin-* flag (where * is the name of the library routine, ie. >-fno-builtin-bcmp). > >i386_defconfig build+boot tested with GCC and Clang. Removes a pretty >old TODO from the codebase. > >Fixes: 6edfba1b33c7 ("x86_64: Don't define string functions to builtin") >Suggested-by: Arvind Sankar <nivedita@alum.mit.edu> >Signed-off-by: Nick Desaulniers <ndesaulniers@google.com> >Reviewed-by: Kees Cook <keescook@chromium.org> >--- > arch/x86/Makefile | 3 --- > 1 file changed, 3 deletions(-) > >diff --git a/arch/x86/Makefile b/arch/x86/Makefile >index 4346ffb2e39f..2383a96cf4fd 100644 >--- a/arch/x86/Makefile >+++ b/arch/x86/Makefile >@@ -80,9 +80,6 @@ ifeq ($(CONFIG_X86_32),y) > # CPU-specific tuning. Anything which can be shared with UML should go here. > include arch/x86/Makefile_32.cpu > KBUILD_CFLAGS += $(cflags-y) >- >- # temporary until string.h is fixed >- KBUILD_CFLAGS += -ffreestanding > else > BITS := 64 > UTS_MACHINE := x86_64 >-- >2.28.0.220.ged08abb693-goog Reviewed-by: Fangrui Song <maskray@google.com> But dropping -ffreestanding causes compiler produced declarations which require https://lore.kernel.org/lkml/20210107001739.1321725-1-maskray@google.com/ "x86: Treat R_386_PLT32 as R_386_PC32" as a prerequisite to build with trunk Clang https://github.com/ClangBuiltLinux/linux/issues/1210 Since there have been more than 4 months, it seems that something else regressed the non -ffreestanding build. Maybe another -fno-builtin-* is needed somewhere.
diff --git a/arch/x86/Makefile b/arch/x86/Makefile index 4346ffb2e39f..2383a96cf4fd 100644 --- a/arch/x86/Makefile +++ b/arch/x86/Makefile @@ -80,9 +80,6 @@ ifeq ($(CONFIG_X86_32),y) # CPU-specific tuning. Anything which can be shared with UML should go here. include arch/x86/Makefile_32.cpu KBUILD_CFLAGS += $(cflags-y) - - # temporary until string.h is fixed - KBUILD_CFLAGS += -ffreestanding else BITS := 64 UTS_MACHINE := x86_64
-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 (ie. in lib/string.c) or disable the transform outright via -fno-builtin-* flag (where * is the name of the library routine, ie. -fno-builtin-bcmp). i386_defconfig build+boot tested with GCC and Clang. Removes a pretty old TODO from the codebase. Fixes: 6edfba1b33c7 ("x86_64: Don't define string functions to builtin") Suggested-by: Arvind Sankar <nivedita@alum.mit.edu> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com> --- arch/x86/Makefile | 3 --- 1 file changed, 3 deletions(-)