Re: [PATCH] x86/boot/compressed: Disable jump tables for clang

[Ingo Molnar <mingo@kernel.org>]

* Nathan Chancellor <nathan@kernel.org> wrote:

> After a recent upstream LLVM change to start generating jump and lookup
> tables in switch statements in more instances [1], linking the
> compressed x86 boot image when CONFIG_KERNEL_ZSTD is enabled fails with:
> 
>   ld.lld: error: Unexpected run-time relocations (.rela) detected!
> 
> Dumping the relocations in misc.o, which is the only file influenced by
> CONFIG_KERNEL_ZSTD in the decompressor, shows dynamic relocations to
> some string constants, which correspond to the string literals in the
> switch statement in handle_zstd_error():
> 
>   Relocation section '.rela.data.rel.ro' at offset 0x277b0 contains 31 entries:
>       Offset             Info             Type               Symbol's Value  Symbol's Name + Addend
>   0000000000000000  0000006600000001 R_X86_64_64            0000000000000000 .rodata.str1.1 + 73a
>   0000000000000008  0000006600000001 R_X86_64_64            0000000000000000 .rodata.str1.1 + 78e
>   0000000000000010  0000006600000001 R_X86_64_64            0000000000000000 .rodata.str1.1 + 78e
>   0000000000000018  0000006600000001 R_X86_64_64            0000000000000000 .rodata.str1.1 + 78e
>   ...
> 
> This optimization is problematic for the decompressor environment, as it
> is built as -fPIE without any explicit absolute references (as described
> at the top of misc.c) while not applying any dynamic relocations, hence
> the linker assertion. To opt out of this optimization, which is of
> little value in this special early boot code, disable jump tables in the
> decompressor when building with clang. This mirrors the other x86
> startup code in arch/x86/boot/startup.
> 
> Cc: stable@vger.kernel.org
> Closes: https://github.com/ClangBuiltLinux/linux/issues/2165
> Link: https://github.com/llvm/llvm-project/commit/fa02a6ed66b1700c996b49c96c6bc0eb014c9518 [1]
> Signed-off-by: Nathan Chancellor <nathan@kernel.org>
> ---
>  arch/x86/boot/compressed/Makefile | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
> index 07e0e64b9a98..1c0d29e3eeba 100644
> --- a/arch/x86/boot/compressed/Makefile
> +++ b/arch/x86/boot/compressed/Makefile
> @@ -31,6 +31,7 @@ KBUILD_CFLAGS += -Wundef
>  KBUILD_CFLAGS += -DDISABLE_BRANCH_PROFILING
>  cflags-$(CONFIG_X86_32) := -march=i386
>  cflags-$(CONFIG_X86_64) := -mcmodel=small -mno-red-zone
> +cflags-$(CONFIG_CC_IS_CLANG) += -fno-jump-tables

So, shouldn't we just use -fno-jump-tables for *all* compilers,
like we do in arch/x86/boot/startup/Makefile?

The point wouldn't be to just work around any Clang
jump-table optimization complications alone, but also
to synchronize the build options of very early code and such.

Thanks,

	Ingo