linux-kernel - Re: [PATCH RFC bpf-next v3 08/16] bpf/verifier: do_misc_fixups for is_bpf_timer_set_sleepable_cb

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Message-ID: <CAO-hwJ+TGiLrc4De7htvKaSsMfQnZahK-zONAMNgUMYHEQb-7g@mail.gmail.com>
Date: Tue, 27 Feb 2024 17:51:39 +0100
From: Benjamin Tissoires <benjamin.tissoires@...hat.com>
To: Eduard Zingerman <eddyz87@...il.com>
Cc: Benjamin Tissoires <bentiss@...nel.org>, 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>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, KP Singh <kpsingh@...nel.org>, 
	Stanislav Fomichev <sdf@...gle.com>, Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>, 
	Jiri Kosina <jikos@...nel.org>, Jonathan Corbet <corbet@....net>, Shuah Khan <shuah@...nel.org>, 
	bpf@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-input@...r.kernel.org, linux-doc@...r.kernel.org, 
	linux-kselftest@...r.kernel.org
Subject: Re: [PATCH RFC bpf-next v3 08/16] bpf/verifier: do_misc_fixups for is_bpf_timer_set_sleepable_cb_kfunc

On Tue, Feb 27, 2024 at 5:36 PM Eduard Zingerman <eddyz87@...il.com> wrote:
>
> On Tue, 2024-02-27 at 17:18 +0100, Benjamin Tissoires wrote:
> [...]
>
> > Hmm, I must still be missing a piece of the puzzle:
> > if I declare bpf_timer_set_sleepable_cb() to take a third "aux"
> > argument, given that it is declared as kfunc, I also must declare it in
> > my bpf program, or I get the following:
> >
> > # libbpf: extern (func ksym) 'bpf_timer_set_sleepable_cb': func_proto [264] incompatible with vmlinux [18151]
> >
> > And if I declare it, then I don't know what to pass, given that this is
> > purely added by the verifier:
> >
> > 43: (85) call bpf_timer_set_sleepable_cb#18152
> > arg#2 pointer type STRUCT bpf_prog_aux must point to scalar, or struct with scalar
>
> Right, something has to be done about number of arguments and we don't
> have a convenient mechanism for this afaik.
>
> The simplest way would be to have two kfuncs:
> - one with 2 arguments, used form bpf program;
> - another with 3 arguments, used at runtime;
> - replace former by latter during rewrite.

It's hacky but seems interesting enough to be tested :)

>
> > Maybe I should teach the verifier that this kfunc only takes 2
> > arguments, and the third one is virtual, but that also means that when
> > the kfunc definitions are to be included in vmlinux.h, they would also
> > have this special case.
>
> It might be a somewhat generic mechanism, e.g. btf_decl_tag("hidden")
> for kfunc parameter.

We also could use the suffix (like __uninit, __k, etc...), but it
might introduce more headaches than the 2 kfuncs you are proposing.

>
> imho, having two kfuncs is less hacky.
>
> > (I just tried with a blank u64 instead of the struct bpf_prog_aux*, but
> >  it crashes with KASAN complaining).
>
> For my understanding:
> - you added a 3rd param (void *) to kfunc;

it was struct bpf_prog_aux *, but yes

> - passed it as zero in BPF program;
> - applied the above rewrite, so that r3 equals to prog->aux;
> - and now KASAN complains, right?

yep, but see below

>
> Could you please provide more details on what exactly it complains about?
>

Well, there is a simple reason: that code is never reached because, in
that function, there is a `if (insn->src_reg ==
BPF_PSEUDO_KFUNC_CALL)` above that unconditionally terminates with a
`continue`. So basically this part of the code is never hit.

I'll include that new third argument and the dual kfunc call in
fixup_kfunc_call() and report if it works from here.

Cheers,
Benjamin