Re: [PATCH] x86: assemble with -Wa,--noexecstack to avoid BFD 2.39 warning

[Nathan Chancellor <nathan@kernel.org>]

On Mon, Aug 08, 2022 at 12:23:05PM -0700, Nick Desaulniers wrote:
> Users of GNU ld (BFD) from binutils 2.39+ will observe multiple instance
> of a new warning when linking kernels in the form:
> 
>   ld: warning: arch/x86/realmode/rm/bioscall.o: missing .note.GNU-stack
>   section implies executable stack
>   ld: NOTE: This behaviour is deprecated and will be removed in a future
>   version of the linker
> 
> The object files producing these all happen to be out of line assembler
> sources (*.S files).
> 
> Generally, we would like to avoid the stack being executable. Because
> there could be a need for the stack to be executable, assembler sources
> have to opt-in to this security feature via explicit creation of the
> .note.GNU-stack feature (which compilers create by default) or command
> line flag --noexecstack.
> 
> Boot tested defconfig and i386_defconfig in QEMU. If any assembler
> sources do require executable stack, they can be built with
> -Wa,--execstack, though the linker warning would have to be disabled. We
> might need to extend this more generally to the top level Makefile for
> all architectures, but I'm not equipped to test the result of such a
> change.
> 
> LLVM's LLD linker defaults to -z noexecstack, so this flag isn't
> strictly necessary when linking with LLD, only BFD, but it doesn't hurt
> to be explicit here for all linkers IMO.
> 
> Link: https://lore.kernel.org/linux-block/3af4127a-f453-4cf7-f133-a181cce06f73@kernel.dk/
> Link: https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=ba951afb99912da01a6e8434126b8fac7aa75107
> Link: https://github.com/llvm/llvm-project/issues/57009
> Reported-by: Jens Axboe <axboe@kernel.dk>
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>

FWIW, this warning appears for other architectures, as I have seen it
with powerpc and s390 and there is also a patch for ARM:

https://lore.kernel.org/20220721204709.445216-1-ben-linux@fluff.org/

Perhaps this should be done for all architectures in the main Makefile?

> ---
>  arch/x86/Makefile                 | 2 ++
>  arch/x86/boot/Makefile            | 2 +-
>  arch/x86/boot/compressed/Makefile | 2 +-
>  arch/x86/realmode/rm/Makefile     | 2 +-
>  4 files changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> index 7854685c5f25..571546775725 100644
> --- a/arch/x86/Makefile
> +++ b/arch/x86/Makefile
> @@ -159,6 +159,8 @@ else
>          KBUILD_CFLAGS += -mcmodel=kernel
>  endif
>  
> +KBUILD_AFLAGS	+= -Wa,--noexecstack
> +
>  #
>  # If the function graph tracer is used with mcount instead of fentry,
>  # '-maccumulate-outgoing-args' is needed to prevent a GCC bug
> diff --git a/arch/x86/boot/Makefile b/arch/x86/boot/Makefile
> index b5aecb524a8a..d7f2130f2277 100644
> --- a/arch/x86/boot/Makefile
> +++ b/arch/x86/boot/Makefile
> @@ -67,7 +67,7 @@ targets += cpustr.h
>  # ---------------------------------------------------------------------------
>  
>  KBUILD_CFLAGS	:= $(REALMODE_CFLAGS) -D_SETUP
> -KBUILD_AFLAGS	:= $(KBUILD_CFLAGS) -D__ASSEMBLY__
> +KBUILD_AFLAGS	:= $(KBUILD_CFLAGS) -D__ASSEMBLY__ -Wa,--noexecstack
>  KBUILD_CFLAGS	+= $(call cc-option,-fmacro-prefix-map=$(srctree)/=)
>  KBUILD_CFLAGS	+= -fno-asynchronous-unwind-tables
>  GCOV_PROFILE := n
> diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
> index 19e1905dcbf6..1587a21a132d 100644
> --- a/arch/x86/boot/compressed/Makefile
> +++ b/arch/x86/boot/compressed/Makefile
> @@ -57,7 +57,7 @@ KBUILD_CFLAGS += -include $(srctree)/include/linux/hidden.h
>  # that the compiler finds it even with out-of-tree builds (make O=/some/path).
>  CFLAGS_sev.o += -I$(objtree)/arch/x86/lib/
>  
> -KBUILD_AFLAGS  := $(KBUILD_CFLAGS) -D__ASSEMBLY__
> +KBUILD_AFLAGS	:= $(KBUILD_CFLAGS) -D__ASSEMBLY__ -Wa,--noexecstack
>  GCOV_PROFILE := n
>  UBSAN_SANITIZE :=n
>  
> diff --git a/arch/x86/realmode/rm/Makefile b/arch/x86/realmode/rm/Makefile
> index 83f1b6a56449..5f2fdafaa034 100644
> --- a/arch/x86/realmode/rm/Makefile
> +++ b/arch/x86/realmode/rm/Makefile
> @@ -73,7 +73,7 @@ $(obj)/realmode.relocs: $(obj)/realmode.elf FORCE
>  
>  KBUILD_CFLAGS	:= $(REALMODE_CFLAGS) -D_SETUP -D_WAKEUP \
>  		   -I$(srctree)/arch/x86/boot
> -KBUILD_AFLAGS	:= $(KBUILD_CFLAGS) -D__ASSEMBLY__
> +KBUILD_AFLAGS	:= $(KBUILD_CFLAGS) -D__ASSEMBLY__ -Wa,--noexecstack
>  KBUILD_CFLAGS	+= -fno-asynchronous-unwind-tables
>  GCOV_PROFILE := n
>  UBSAN_SANITIZE := n
> -- 
> 2.37.1.559.g78731f0fdb-goog
>