netdev - Re: [PATCH bpf-next v11 02/12] bpf: use the least significant byte for the nr

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Message-ID: <CAEf4BzabUj0APqROb7ibkEovoXVmxbiHq0kNAPWExpEfRmQwiw@mail.gmail.com>
Date: Fri, 23 Jan 2026 14:57:21 -0800
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Menglong Dong <menglong.dong@...ux.dev>
Cc: menglong8.dong@...il.com, andrii@...nel.org, bot+bpf-ci@...nel.org, 
	daniel@...earbox.net, martin.lau@...ux.dev, eddyz87@...il.com, 
	song@...nel.org, yonghong.song@...ux.dev, john.fastabend@...il.com, 
	kpsingh@...nel.org, sdf@...ichev.me, haoluo@...gle.com, jolsa@...nel.org, 
	davem@...emloft.net, dsahern@...nel.org, tglx@...utronix.de, mingo@...hat.com, 
	jiang.biao@...ux.dev, bp@...en8.de, dave.hansen@...ux.intel.com, 
	x86@...nel.org, hpa@...or.com, bpf@...r.kernel.org, netdev@...r.kernel.org, 
	linux-kernel@...r.kernel.org, ast@...nel.org, martin.lau@...nel.org, 
	clm@...a.com, ihor.solodrai@...ux.dev
Subject: Re: [PATCH bpf-next v11 02/12] bpf: use the least significant byte
 for the nr_args in trampoline

On Fri, Jan 23, 2026 at 1:21 AM Menglong Dong <menglong.dong@...ux.dev> wrote:
>
> On 2026/1/23 16:29 Menglong Dong <menglong.dong@...ux.dev> write:
> > On 2026/1/23 16:07 bot+bpf-ci@...nel.org 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.
>

Let's still fix the problem. Just go ahead and resubmit (with the fix
and improved test).

pw-bot: cr


> 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
> > >
> >
> >
> >
> >
> >
> >
>
>
>
>