Re: [PATCH] x86/csum: Relax the len and proto constraints in 32-bit csum_ipv6_magic()

[Nick Desaulniers]

On Mon, Oct 19, 2026 at 5:32 PM Sasha Levin <sashal@xxxxxxxxxx> wrote:
>
> Building an i386 allmodconfig kernel with clang fails since allmodconfig
> started building netfilter with GCOV profiling:
>
>   In file included from net/netfilter/nf_synproxy_core.c:7:
>   In file included from ./include/linux/skbuff.h:26:
>   In file included from ./include/net/checksum.h:21:
>   In file included from ./arch/x86/include/asm/checksum.h:9:
>   ./arch/x86/include/asm/checksum_32.h:149:6: error: inline assembly requires more registers than available
>
> The csum_ipv6_magic() asm needs five registers at once: the early-clobber
> sum plus register-only inputs for saddr, daddr, htonl(len) and
> htonl(proto). With frame pointers enabled, six general purpose registers
> are left to choose from.
>
> In synproxy_send_tcp_ipv6(), the GCOV instrumentation lets clang's
> machine scheduler move the copies of two incoming register arguments,
> EDX and ECX, below the asm. Both registers then stay live across the
> asm, which leaves four registers for its five operands. This is LLVM
> issue #206307. The error goes away without either GCOV or frame
> pointers, and it reproduces with clang 19, 21 and 22.
>
> Neither len nor proto has to be in a register: adcl also takes a 32-bit
> memory or immediate source operand. Use "g" for both, so the asm needs
> at most three registers whatever the caller looks like.
>
> ASM_INPUT_G, which normally replaces "g" to avoid clang's poor handling
> of it, is not enough here. It expands to "ir" for clang, so callers that
> pass a variable protocol, like nf_ip6_checksum(), would still need five
> registers. With "g", clang passes a non-constant len or proto through
> the stack and still turns constants into immediates.
>
> Fixes: 403cec8ab6d0 ("netfilter: add option for GCOV profiling")
> Reported-by: kernelci.org bot <bot@xxxxxxxxxxxx>
> Closes: https://dashboard.kernelci.org/build/maestro:6ad55fe255fda507603d2651
> Reported-by: Jeff Johnson <jeff.johnson@xxxxxxxxxxxxxxxx>
> Closes: https://lore.kernel.org/a28df759-d0e9-41c0-97c7-5f89a18d54e2@xxxxxxxxxxxxxxxx
> Assisted-by: LLM
> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Link: https://github.com/llvm/llvm-project/issues/206307\

Acked-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>

> ---
> KernelCI's kbuild-clang-21-i386-allmodconfig job has failed on mainline
> with this error since the 7.2 merge window, v7.3 included.
>
> Tested on i386 with KernelCI's allmodconfig-based .config: with clang
> 19.1.7, 21.1.8 and 22.1.8, nf_synproxy_core.o fails before this change
> and builds after it, and net/{netfilter,ipv4,ipv6,bridge}/ and
> arch/x86/lib/ build without errors or warnings. The same directories
> also build cleanly with GCC 16.2 (i386 allmodconfig). A 32-bit userspace
> harness comparing the asm against a C reference over 2M random and
> carry-edge inputs shows no mismatches with either compiler. No full
> allmodconfig build or boot test.
>
>  arch/x86/include/asm/checksum_32.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/include/asm/checksum_32.h b/arch/x86/include/asm/checksum_32.h
> index 17da953879976..f74873a5e3510 100644
> --- a/arch/x86/include/asm/checksum_32.h
> +++ b/arch/x86/include/asm/checksum_32.h
> @@ -159,7 +159,7 @@ static inline __sum16 csum_ipv6_magic(const struct in6_addr *saddr,
>             "adcl $0, %0        ;\n"
>             : "=&r" (sum)
>             : "r" (saddr), "r" (daddr),
> -             "r" (htonl(len)), "r" (htonl(proto)), "0" (sum)
> +             "g" (htonl(len)), "g" (htonl(proto)), "0" (sum)
>             : "memory");
>
>         return csum_fold(sum);
> --
> 2.53.0
>


-- 
Thanks,
~Nick Desaulniers