Re: [PATCH bpf 1/1] bpf, riscv: Fix stack arguments of struct_ops trampolines

[KaFai Wan <kafai.wan@linux.dev>]

On Thu, 2026-10-01 at 23:40 +0800, KaFai Wan wrote:

I ran the tests yesterday and didn’t notice that Pu Lehui had already sent the same patch. Please
ignore this. Sorry for the noise.

> When a struct_ops trampoline takes more than 8 arguments (up to 12), the
> 9th and beyond are passed on the stack. store_args() copies them into the
> trampoline frame using a hard-coded FP + 16 base:
> 
> 	emit_ld(RV_REG_T1, 16 + (i - RV_MAX_REG_ARGS) * 8, RV_REG_FP, ctx);
> 
> That offset only holds for fentry trampolines, where the traced function's
> T0/FP are saved at FP - 8/FP - 16 and its stack arguments start at FP + 16.
> A struct_ops trampoline is called directly, so its stack arguments start at
> FP + 0 and store_args() reads the wrong words. BPF programs then see garbage
> for arg9 and beyond, which fails the selftest:
> 
>   struct_ops_multi_args/test_trampoline_stack_args:FAIL
> 
> Pass the stack argument base offset to store_args(): 0 for struct_ops
> trampolines, 16 otherwise.
> 
> Fixes: 6801b0aef79d ("riscv, bpf: Add 12-argument support for RV64 bpf trampoline")
> Signed-off-by: KaFai Wan <kafai.wan@linux.dev>
> ---
>  arch/riscv/net/bpf_jit_comp64.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/riscv/net/bpf_jit_comp64.c b/arch/riscv/net/bpf_jit_comp64.c
> index 01fe66774f02..4ae6674f58ed 100644
> --- a/arch/riscv/net/bpf_jit_comp64.c
> +++ b/arch/riscv/net/bpf_jit_comp64.c
> @@ -875,7 +875,7 @@ int bpf_arch_text_poke(void *ip, enum bpf_text_poke_type old_t,
>  	return ret;
>  }
>  
> -static void store_args(int nr_arg_slots, int args_off, struct rv_jit_context *ctx)
> +static void store_args(int nr_arg_slots, int args_off, int stack_base, struct rv_jit_context
> *ctx)
>  {
>  	int i;
>  
> @@ -883,8 +883,7 @@ static void store_args(int nr_arg_slots, int args_off, struct rv_jit_context
> *ct
>  		if (i < RV_MAX_REG_ARGS) {
>  			emit_sd(RV_REG_FP, -args_off, RV_REG_A0 + i, ctx);
>  		} else {
> -			/* skip slots for T0 and FP of traced function */
> -			emit_ld(RV_REG_T1, 16 + (i - RV_MAX_REG_ARGS) * 8, RV_REG_FP, ctx);
> +			emit_ld(RV_REG_T1, stack_base + (i - RV_MAX_REG_ARGS) * 8, RV_REG_FP,
> ctx);
>  			emit_sd(RV_REG_FP, -args_off, RV_REG_T1, ctx);
>  		}
>  		args_off -= 8;
> @@ -1179,7 +1178,8 @@ static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im,
>  	func_meta = nr_arg_slots;
>  	emit_store_stack_imm64(RV_REG_T1, -func_meta_off, func_meta, ctx);
>  
> -	store_args(nr_arg_slots, args_off, ctx);
> +	/* skip slots for T0 and FP of traced function */
> +	store_args(nr_arg_slots, args_off, is_struct_ops ? 0 : 16, ctx);
>  
>  	if (bpf_fsession_cnt(tnodes)) {
>  		/* clear all session cookies' value */

-- 
Thanks,
KaFai

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