linux-kernel - Re: [RFC PATCH bpf-next 2/2] bpf: Get fentry func addr from user when BTF info invalid

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Message-ID: <CAADnVQKPLH7q2KcJM_Nkgc1z=OZmOPZes-0c8A-5gty1xEOKzA@mail.gmail.com>
Date: Wed, 30 Apr 2025 15:23:13 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Alan Maguire <alan.maguire@...cle.com>
Cc: Tao Chen <chen.dylane@...ux.dev>, 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 <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 <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH bpf-next 2/2] bpf: Get fentry func addr from user when
 BTF info invalid

On Wed, Apr 30, 2025 at 10:57 AM Alan Maguire <alan.maguire@...cle.com> wrote:
> > +
> > +                     if (!addr && (prog->expected_attach_type == BPF_TRACE_FENTRY ||
> > +                                     prog->expected_attach_type == BPF_TRACE_FEXIT)) {
> > +                             fname = kallsyms_lookup((unsigned long)prog->aux->fentry_func,
> > +                                                     NULL, NULL, NULL, trace_symbol);
> > +                             if (fname)
> > +                                     addr = (long)prog->aux->fentry_func;
>
>
> We should do some validation that the fname we get back matches the BTF
> func name prefix (fname "foo.isra.0" matches "foo") I think?

I don't think that will be enough.
User space should not be able to pass a random kernel address
and convince the kernel that it matches a particular btf_id.
As discussed in the other thread matching based on name is
breaking apart.
pahole does all the safety check to make sure name/addr/btf_id
are consistent.
We shouldn't be adding workarounds like this because
pahole/btf/kernel build is not smart enough.

pw-bot: cr