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 17:21:06 +0800

On 2026/1/23 16:29 Menglong Dong <[email-masked]> write:
> 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.

Oh, the problem doesn't have much impact. The only impact
is that the verifier will check the "r0 &= 0xFF" instruction redundantly.

I'll see if there is more comment before I send next version.

Thanks!
Menglong Dong

> 
> 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
> > 
> 
> 
> 
> 
> 
>