Subject: Re: [PATCH bpf-next v11 02/12] bpf: use the least significant byte for the nr_args in trampoline

From: Menglong Dong <[email-masked]>
Date: Fri, 23 Jan 2026 16:29:01 +0800

On 2026/1/23 16:07 [email-masked] write:
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 41bbed6418b5..b3f912ad18ba 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> 
> [ ... ]
> 
> > @@ -23779,12 +23781,13 @@ static int do_misc_fixups(struct bpf_verifier_env *env)
> >  			    eatype == BPF_MODIFY_RETURN) {
> >  				/* 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_ALU64_IMM(BPF_LSH, BPF_REG_0, 3);
> > +				insn_buf[1] = BPF_ALU64_IMM(BPF_AND, BPF_REG_0, 0xFF);
> > +				insn_buf[2] = BPF_ALU64_IMM(BPF_LSH, BPF_REG_0, 3);
> 
> [ ... ]
> 
> > -				cnt = 6;
> > +				cnt = 7;
> >  			} else {
> >  				insn_buf[0] = BPF_MOV64_IMM(BPF_REG_0, -EOPNOTSUPP);
> >  				cnt = 1;
> >  			}
> >
> >  			new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, cnt);
> 
> [ ... ]
> 
> > 			delta    += cnt - 1;
> 
> Note that the bpf_get_func_ret block above correctly updates delta after
> patching.
> 
> > @@ -23808,12 +23811,15 @@ static int do_misc_fixups(struct bpf_verifier_env *env)
> >
> >  				/* skip 'void *__data' in btf_trace_##name() and save to reg0 */
> >  				insn_buf[0] = BPF_MOV64_IMM(BPF_REG_0, nr_args - 1);
> > +				cnt = 1;
> >  			} else {
> >  				/* 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_ALU64_IMM(BPF_AND, BPF_REG_0, 0xFF);
> > +				cnt = 2;
> >  			}
> >
> > -			new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, 1);
> > +			new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, cnt);
> >  			if (!new_prog)
> >  				return -ENOMEM;
> >
> >  			env->prog = prog = new_prog;
> >  			insn      = new_prog->insnsi + i + delta;
> >  			goto next_insn;
> >  		}
> 
> Should there be a "delta += cnt - 1;" before assigning env->prog in this
> bpf_get_func_arg_cnt block? When cnt is 2 (non-RAW_TP path), the program
> grows by one instruction, but delta is not updated. This could cause
> subsequent bpf_patch_insn_data() calls to use incorrect offsets.
> 
> The bpf_get_func_arg block at the beginning of this patch has:
> 
>     delta    += cnt - 1;
>     env->prog = prog = new_prog;
> 
> And the bpf_get_func_ret block similarly has delta updated. But this
> bpf_get_func_arg_cnt block goes directly to env->prog assignment without
> updating delta.

Ah, good point, I think this is a valid problem. The selftests didn't cover
this case, and I think I'd better to use bpf_get_func_arg() and bpf_get_func_ret()
in the exit path of fsession to cover it.

Will fix it in the next version.

Thanks!
Menglong Dong

> 
> 
> ---
> AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
> See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md
> 
> CI run summary: https://github.com/kernel-patches/bpf/actions/runs/21278745581
>