Re: [PATCH bpf-next] bpf, arm64: Fix compile error in dummy_tramp()

[Jean-Philippe Brucker]

On Thu, Jul 21, 2022 at 08:13:19AM -0400, Xu Kuohai wrote:
> From: Xu Kuohai <xukuohai@xxxxxxxxxx>
> 
> dummy_tramp() uses "lr" to refer to the x30 register, but some assembler
> does not recognize "lr" and reports a build failure:
> 
> /tmp/cc52xO0c.s: Assembler messages:
> /tmp/cc52xO0c.s:8: Error: operand 1 should be an integer register -- `mov lr,x9'
> /tmp/cc52xO0c.s:7: Error: undefined symbol lr used as an immediate value
> make[2]: *** [scripts/Makefile.build:250: arch/arm64/net/bpf_jit_comp.o] Error 1
> make[1]: *** [scripts/Makefile.build:525: arch/arm64/net] Error 2
> 
> So replace "lr" with "x30" to fix it.
> 
> Fixes: b2ad54e1533e ("bpf, arm64: Implement bpf_arch_text_poke() for arm64")
> Reported-by: Jon Hunter <jonathanh@xxxxxxxxxx>
> Tested-by: Jon Hunter <jonathanh@xxxxxxxxxx>
> Signed-off-by: Xu Kuohai <xukuohai@xxxxxxxxxx>

Right, it looks like the lr alias was only added to binutils 2.30, and
Linux supports 2.23+

Reviewed-by: Jean-Philippe Brucker <jean-philippe@xxxxxxxxxx>

> ---
>  arch/arm64/net/bpf_jit_comp.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
> index dcc572b7d4da..7ca8779ae34f 100644
> --- a/arch/arm64/net/bpf_jit_comp.c
> +++ b/arch/arm64/net/bpf_jit_comp.c
> @@ -610,8 +610,8 @@ asm (
>  #if IS_ENABLED(CONFIG_ARM64_BTI_KERNEL)
>  "	bti j\n" /* dummy_tramp is called via "br x10" */
>  #endif
> -"	mov x10, lr\n"
> -"	mov lr, x9\n"
> +"	mov x10, x30\n"
> +"	mov x30, x9\n"
>  "	ret x10\n"
>  "	.size dummy_tramp, .-dummy_tramp\n"
>  "	.popsection\n"
> -- 
> 2.30.2
>