linux-kernel - Re: [PATCH v2] bpf: Fix bpf_get_smp_processor_id() on !CONFIG

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Message-ID: <Z2HWuv6o86W5yY7j@gpd3>
Date: Tue, 17 Dec 2024 20:53:30 +0100
From: Andrea Righi <arighi@...dia.com>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc: Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	John Fastabend <john.fastabend@...il.com>,
	Andrii Nakryiko <andrii@...nel.org>,
	Martin KaFai Lau <martin.lau@...ux.dev>,
	Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>,
	Yonghong Song <yonghong.song@...ux.dev>,
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>,
	Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
	bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] bpf: Fix bpf_get_smp_processor_id() on !CONFIG_SMP

On Tue, Dec 17, 2024 at 10:38:04AM -0800, Andrii Nakryiko wrote:
...
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index f7f892a52a37..761c70899754 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -21281,11 +21281,15 @@ static int do_misc_fixups(struct bpf_verifier_env *env)
> >                          * changed in some incompatible and hard to support
> >                          * way, it's fine to back out this inlining logic
> >                          */
> > +#ifdef CONFIG_SMP
> >                         insn_buf[0] = BPF_MOV32_IMM(BPF_REG_0, (u32)(unsigned long)&pcpu_hot.cpu_number);
> >                         insn_buf[1] = BPF_MOV64_PERCPU_REG(BPF_REG_0, BPF_REG_0);
> >                         insn_buf[2] = BPF_LDX_MEM(BPF_W, BPF_REG_0, BPF_REG_0, 0);
> >                         cnt = 3;
> > -
> > +#else
> > +                       BPF_ALU32_REG(BPF_XOR, BPF_REG_0, BPF_REG_0),
> 
> um... shouldn't this be `insns_buf[0] = ` assignment? And that comma
> instead of semicolon at the end?

Yeah.. my bad, I tested it with the wrong .config that has CONFIG_SMP
enabled.

I'll send a v3 with the proper code, sorry for the noise.

-Andrea

> 
> pw-bot: cr
> 
> > +                       cnt = 1;
> > +#endif
> >                         new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, cnt);
> >                         if (!new_prog)
> >                                 return -ENOMEM;
> > --
> > 2.47.1
> >