Re: [PATCH bpf-next v9 02/11] bpf: use last 8-bits for the nr_args in trampoline

[Andrii Nakryiko]

On Sat, Jan 10, 2026 at 6:11 AM Menglong Dong <menglong8.dong@xxxxxxxxx> wrote:
>
> For now, ctx[-1] is used to store the nr_args in the trampoline. However,
> 1-byte is enough to store such information. Therefore, we use only the
> last byte of ctx[-1] to store the nr_args, and reserve the rest for other

Looking at the assembly below I think you are extracting the least
significant byte, right? I'd definitely not call it "last" byte...
Let's be precise and unambiguous here.

> usages.
>
> Signed-off-by: Menglong Dong <dongml2@xxxxxxxxxxxxxxx>
> ---
> v8:
> - fix the missed get_func_arg_cnt
> ---
>  kernel/bpf/verifier.c    | 35 +++++++++++++++++++----------------
>  kernel/trace/bpf_trace.c |  6 +++---
>  2 files changed, 22 insertions(+), 19 deletions(-)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 774c9b0aafa3..bfff3f84fd91 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -23277,15 +23277,16 @@ static int do_misc_fixups(struct bpf_verifier_env *env)
>                     insn->imm == BPF_FUNC_get_func_arg) {
>                         /* Load nr_args from ctx - 8 */
>                         insn_buf[0] = BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_1, -8);
> -                       insn_buf[1] = BPF_JMP32_REG(BPF_JGE, BPF_REG_2, BPF_REG_0, 6);
> -                       insn_buf[2] = BPF_ALU64_IMM(BPF_LSH, BPF_REG_2, 3);
> -                       insn_buf[3] = BPF_ALU64_REG(BPF_ADD, BPF_REG_2, BPF_REG_1);
> -                       insn_buf[4] = BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_2, 0);
> -                       insn_buf[5] = BPF_STX_MEM(BPF_DW, BPF_REG_3, BPF_REG_0, 0);
> -                       insn_buf[6] = BPF_MOV64_IMM(BPF_REG_0, 0);
> -                       insn_buf[7] = BPF_JMP_A(1);
> -                       insn_buf[8] = BPF_MOV64_IMM(BPF_REG_0, -EINVAL);
> -                       cnt = 9;
> +                       insn_buf[1] = BPF_ALU64_IMM(BPF_AND, BPF_REG_0, 0xFF);
> +                       insn_buf[2] = BPF_JMP32_REG(BPF_JGE, BPF_REG_2, BPF_REG_0, 6);
> +                       insn_buf[3] = BPF_ALU64_IMM(BPF_LSH, BPF_REG_2, 3);
> +                       insn_buf[4] = BPF_ALU64_REG(BPF_ADD, BPF_REG_2, BPF_REG_1);
> +                       insn_buf[5] = BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_2, 0);
> +                       insn_buf[6] = BPF_STX_MEM(BPF_DW, BPF_REG_3, BPF_REG_0, 0);
> +                       insn_buf[7] = BPF_MOV64_IMM(BPF_REG_0, 0);
> +                       insn_buf[8] = BPF_JMP_A(1);
> +                       insn_buf[9] = BPF_MOV64_IMM(BPF_REG_0, -EINVAL);
> +                       cnt = 10;
>

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