Re: [PATCH v2] x86/boot/compressed: Disable jump tables

[Ard Biesheuvel]

On Thu, 23 Jul 2026, at 02:09, Nathan Chancellor 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, and to mirror the other
> x86 startup code in arch/x86/boot/startup, disable jump tables in the
> decompressor.
>
> Cc: stable@xxxxxxxxxxxxxxx
> Closes: https://github.com/ClangBuiltLinux/linux/issues/2165
> Link: 
> https://github.com/llvm/llvm-project/commit/fa02a6ed66b1700c996b49c96c6bc0eb014c9518 
> [1]
> Signed-off-by: Nathan Chancellor <nathan@xxxxxxxxxx>

Acked-by: Ard Biesheuvel <ardb@xxxxxxxxxx>