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@xxxxxxxxxxxx>
Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
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@xxxxxxxxxx>
But dropping -ffreestanding causes compiler produced declarations which
require
https://lore.kernel.org/lkml/20210107001739.1321725-1-maskray@xxxxxxxxxx/
"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.
