linux-kernel - Re: [PATCH] bpf: verifier: allow arrays of progs to be used in sleepable context

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Message-ID: <CAADnVQJaG8kDaJr5LV29ces+gVpgARLAWiUvE9Ee5huuiW5X=Q@mail.gmail.com>
Date: Wed, 24 Apr 2024 12:45:03 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Benjamin Tissoires <bentiss@...nel.org>
Cc: Daniel Borkmann <daniel@...earbox.net>, Alexei Starovoitov <ast@...nel.org>, 
	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@...gle.com>, Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>, 
	bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] bpf: verifier: allow arrays of progs to be used in
 sleepable context

On Wed, Apr 24, 2024 at 7:17 AM Benjamin Tissoires <bentiss@...nel.org> wrote:
>
> On Apr 22 2024, Benjamin Tissoires wrote:
> > On Apr 22 2024, Daniel Borkmann wrote:
> > > On 4/22/24 9:16 AM, Benjamin Tissoires wrote:
> > > > Arrays of progs are underlying using regular arrays, but they can only
> > > > be updated from a syscall.
> > > > Therefore, they should be safe to use while in a sleepable context.
> > > >
> > > > This is required to be able to call bpf_tail_call() from a sleepable
> > > > tracing bpf program.
> > > >
> > > > Signed-off-by: Benjamin Tissoires <bentiss@...nel.org>
> > > > ---
> > > > Hi,
> > > >
> > > > a small patch to allow to have:
> > > >
> > > > ```
> > > > SEC("fmod_ret.s/__hid_bpf_tail_call_sleepable")
> > > > int BPF_PROG(hid_tail_call_sleepable, struct hid_bpf_ctx *hctx)
> > > > {
> > > >   bpf_tail_call(ctx, &hid_jmp_table, hctx->index);
> > > >
> > > >   return 0;
> > > > }
> > > > ```
> > > >
> > > > This should allow me to add bpf hooks to functions that communicate with
> > > > the hardware.
> > >
> > > Could you also add selftests to it? In particular, I'm thinking that this is not
> > > sufficient given also bpf_prog_map_compatible() needs to be extended to check on
> > > prog->sleepable. For example we would need to disallow calling sleepable programs
> > > in that map from non-sleepable context.
> >
> > Just to be sure, if I have to change bpf_prog_map_compatible(), that
> > means that a prog array map can only have sleepable or non-sleepable
> > programs, but not both at the same time?
> >
> > FWIW, indeed, I just tested and the BPF verifier/core is happy with this
> > patch only if the bpf_tail_call is issued from a non-sleepable context
> > (and crashes as expected).
> >
> > But that seems to be a different issue TBH: I can store a sleepable BPF
> > program in a prog array and run it from a non sleepable context. I don't
> > need the patch at all as bpf_tail_call() is normally declared. I assume
> > your suggestion to change bpf_prog_map_compatible() will fix that part.
> >
> > I'll digg some more tomorrow.
> >
>
> Quick update:
> forcing the prog array to only contain sleepable programs or not seems
> to do the trick, but I'm down a rabbit hole as when I return from my
> trampoline, I get an invalid page fault, trying to execute NX-protected
> page.
>
> I'll report if it's because of HID-BPF or if there are more work to be
> doing for bpf_tail_call (which I suspect).

bpf_tail_call is an old mechanism.
Instead of making it work for sleepable (which is ok to do)
have you considered using "freplace" logic to "add bpf hooks to functions" ?
You can have a global noinline function and replace it at run-time
with another bpf program.
Like:
__attribute__ ((noinline))
int get_constant(long val)
{
        return val - 122;
}

in progs/test_pkt_access.c

is replaced with progs/freplace_get_constant.c

With freplace you can pass normal arguments, do the call and get
return value, while with bpf_tail_call it's ctx only and no return.