Re: [PATCH] arm64: alternatives: Move length validation in alternative_{insn,endif}

[Nick Desaulniers]

On Tue, Apr 13, 2021 at 5:09 PM Nathan Chancellor <nathan@xxxxxxxxxx> wrote:
>
> After commit 2decad92f473 ("arm64: mte: Ensure TIF_MTE_ASYNC_FAULT is
> set atomically"), LLVM's integrated assembler fails to build entry.S:
>
> <instantiation>:5:7: error: expected assembly-time absolute expression
>  .org . - (664b-663b) + (662b-661b)
>       ^
> <instantiation>:6:7: error: expected assembly-time absolute expression
>  .org . - (662b-661b) + (664b-663b)
>       ^
>
> The root cause is LLVM's assembler has a one-pass design, meaning it
> cannot figure out these instruction lengths when the .org directive is
> outside of the subsection that they are in, which was changed by the
> .arch_extension directive added in the above commit.
>
> Apply the same fix from commit 966a0acce2fc ("arm64/alternatives: move
> length validation inside the subsection") to the alternative_endif
> macro, shuffling the .org directives so that the length validation
> happen will always happen in the same subsections. alternative_insn has
> not shown any issue yet but it appears that it could have the same issue
> in the future so just preemptively change it.

Thanks Nathan.
Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Tested-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>

I did some additional disassembly comparison.  In case we ever need it
again, I'll copy it below for posterity.

$ ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu make LLVM=1 LLVM_IAS=1
-j72 O=/tmp/a defconfig all
$ b4 am https://lore.kernel.org/lkml/20210414000803.662534-1-nathan@xxxxxxxxxx/
-o - | git am -3
$ ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu make LLVM=1 LLVM_IAS=1
-j72 O=/tmp/b defconfig all
$ for f in $(find /tmp/a/arch/arm64 -name \*.o); do llvm-objdump -dr
$f > $f.txt; done
$ for f in $(find /tmp/b/arch/arm64 -name \*.o); do llvm-objdump -dr
$f > $f.txt; done
$ for f in $(find /tmp/a/arch/arm64 -name \*.o); do diff -u $f.txt
$(echo $f.txt|sed 's/a/b/'); done | less

For no difference.  You can check more sections than .text by changing
`-d` to `-D` for llvm-objdump, though you're going to get a lot of
noise related to changes in .strtab and relocations referring to debug
info (.debug_str).  But if I drop your patch, rebuild, and recompare,
I see the same differences.

>
> Cc: stable@xxxxxxxxxxxxxxx
> Fixes: f7b93d42945c ("arm64/alternatives: use subsections for replacement sequences")
> Link: https://github.com/ClangBuiltLinux/linux/issues/1347
> Signed-off-by: Nathan Chancellor <nathan@xxxxxxxxxx>
> ---
>
> Apologies if my explanation or terminology is off, I am only now getting
> more familiar with assembly.
>
>  arch/arm64/include/asm/alternative-macros.h | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/arch/arm64/include/asm/alternative-macros.h b/arch/arm64/include/asm/alternative-macros.h
> index 5df500dcc627..8a078fc662ac 100644
> --- a/arch/arm64/include/asm/alternative-macros.h
> +++ b/arch/arm64/include/asm/alternative-macros.h
> @@ -97,9 +97,9 @@
>         .popsection
>         .subsection 1
>  663:   \insn2
> -664:   .previous
> -       .org    . - (664b-663b) + (662b-661b)
> +664:   .org    . - (664b-663b) + (662b-661b)
>         .org    . - (662b-661b) + (664b-663b)
> +       .previous
>         .endif
>  .endm
>
> @@ -169,11 +169,11 @@
>   */
>  .macro alternative_endif
>  664:
> +       .org    . - (664b-663b) + (662b-661b)
> +       .org    . - (662b-661b) + (664b-663b)
>         .if .Lasm_alt_mode==0
>         .previous
>         .endif
> -       .org    . - (664b-663b) + (662b-661b)
> -       .org    . - (662b-661b) + (664b-663b)
>  .endm
>
>  /*
>
> base-commit: 738fa58ee1328481d1d7889e7c430b3401c571b9
> --
> 2.31.1.272.g89b43f80a5
>


-- 
Thanks,
~Nick Desaulniers