Re: [PATCH] s390/bpf: Zero-extend bpf prog return values and kfunc arguments

[Ihor Solodrai <ihor.solodrai@linux.dev>]

On 3/13/26 10:46 AM, Ilya Leoshkevich wrote:
> s390x ABI requires callers to zero-extend unsigned arguments and
> sign-extend signed arguments, and callees to zero-extend unsigned
> return values and sign-extend signed return values.
> 
> s390 BPF JIT currently implements only sign extension. Fix this
> omission and implement zero extension too.
> 
> Fixes: 528eb2cb87bc ("s390/bpf: Implement arch_prepare_bpf_trampoline()")
> Reported-by: Hari Bathini <hbathini@linux.ibm.com>
> Closes: https://lore.kernel.org/bpf/20260312080113.843408-1-hbathini@linux.ibm.com/
> Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
> ---
>  arch/s390/net/bpf_jit_comp.c | 39 ++++++++++++++++++++++--------------
>  1 file changed, 24 insertions(+), 15 deletions(-)

Hi Ilya,

Tested-by: Ihor Solodrai <ihor.solodrai@linux.dev>

This bug made the BPF CI red. I applied as temporary CI diff.

Interestingly, an AI bot investigated the test failure and came up
with a similar patch [1]. Gotta teach it to scan upstream for existing
fixes first :)

Thank you!

[1] https://github.com/kernel-patches/vmtest/issues/459


> 
> diff --git a/arch/s390/net/bpf_jit_comp.c b/arch/s390/net/bpf_jit_comp.c
> index 1f9a6b728beb..d08d159b6319 100644
> --- a/arch/s390/net/bpf_jit_comp.c
> +++ b/arch/s390/net/bpf_jit_comp.c
> @@ -830,25 +830,34 @@ static int bpf_jit_probe_post(struct bpf_jit *jit, struct bpf_prog *fp,
>  }
>  
>  /*
> - * Sign-extend the register if necessary
> + * Sign- or zero-extend the register if necessary
>   */
> -static int sign_extend(struct bpf_jit *jit, int r, u8 size, u8 flags)
> +static int sign_zero_extend(struct bpf_jit *jit, int r, u8 size, u8 flags)
>  {
> -	if (!(flags & BTF_FMODEL_SIGNED_ARG))
> -		return 0;
> -
>  	switch (size) {
>  	case 1:
> -		/* lgbr %r,%r */
> -		EMIT4(0xb9060000, r, r);
> +		if (flags & BTF_FMODEL_SIGNED_ARG)
> +			/* lgbr %r,%r */
> +			EMIT4(0xb9060000, r, r);
> +		else
> +			/* llgcr %r,%r */
> +			EMIT4(0xb9840000, r, r);
>  		return 0;
>  	case 2:
> -		/* lghr %r,%r */
> -		EMIT4(0xb9070000, r, r);
> +		if (flags & BTF_FMODEL_SIGNED_ARG)
> +			/* lghr %r,%r */
> +			EMIT4(0xb9070000, r, r);
> +		else
> +			/* llghr %r,%r */
> +			EMIT4(0xb9850000, r, r);
>  		return 0;
>  	case 4:
> -		/* lgfr %r,%r */
> -		EMIT4(0xb9140000, r, r);
> +		if (flags & BTF_FMODEL_SIGNED_ARG)
> +			/* lgfr %r,%r */
> +			EMIT4(0xb9140000, r, r);
> +		else
> +			/* llgfr %r,%r */
> +			EMIT4(0xb9160000, r, r);
>  		return 0;
>  	case 8:
>  		return 0;
> @@ -1798,9 +1807,9 @@ static noinline int bpf_jit_insn(struct bpf_jit *jit, struct bpf_prog *fp,
>  				return -1;
>  
>  			for (j = 0; j < m->nr_args; j++) {
> -				if (sign_extend(jit, BPF_REG_1 + j,
> -						m->arg_size[j],
> -						m->arg_flags[j]))
> +				if (sign_zero_extend(jit, BPF_REG_1 + j,
> +						     m->arg_size[j],
> +						     m->arg_flags[j]))
>  					return -1;
>  			}
>  		}
> @@ -2566,7 +2575,7 @@ static int invoke_bpf_prog(struct bpf_tramp_jit *tjit,
>  	EMIT6_PCREL_RILB_PTR(0xc0050000, REG_14, p->bpf_func);
>  	/* stg %r2,retval_off(%r15) */
>  	if (save_ret) {
> -		if (sign_extend(jit, REG_2, m->ret_size, m->ret_flags))
> +		if (sign_zero_extend(jit, REG_2, m->ret_size, m->ret_flags))
>  			return -1;
>  		EMIT6_DISP_LH(0xe3000000, 0x0024, REG_2, REG_0, REG_15,
>  			      tjit->retval_off);