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From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
To: Alexei Starovoitov <ast@fb.com>
Cc: Alexei Starovoitov <ast@kernel.org>,
	"David S. Miller" <davem@davemloft.net>,
	Daniel Borkmann <daniel@iogearbox.net>,
	"x86@kernel.org" <x86@kernel.org>,
	Networking <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
	Kernel Team <Kernel-team@fb.com>
Subject: Re: [PATCH v2 bpf-next 10/12] bpf: check types of arguments passed into helpers
Date: Fri, 11 Oct 2019 21:25:27 -0700
Message-ID: <CAEf4BzZ1nPgz-=fHPmKKMBAEt8Yvqp7dsJ4m5FDJUC=0+8qmpw@mail.gmail.com> (raw)
In-Reply-To: <ba51c8c3-94fc-413a-0935-aaa307127666@fb.com>

On Fri, Oct 11, 2019 at 6:39 PM Alexei Starovoitov <ast@fb.com> wrote:
>
> On 10/11/19 12:02 PM, Andrii Nakryiko wrote:
> > On Wed, Oct 9, 2019 at 9:15 PM Alexei Starovoitov <ast@kernel.org> wrote:
> >>
> >>   /* type of values returned from helper functions */
> >> @@ -235,11 +236,17 @@ struct bpf_func_proto {
> >>          bool gpl_only;
> >>          bool pkt_access;
> >>          enum bpf_return_type ret_type;
> >> -       enum bpf_arg_type arg1_type;
> >> -       enum bpf_arg_type arg2_type;
> >> -       enum bpf_arg_type arg3_type;
> >> -       enum bpf_arg_type arg4_type;
> >> -       enum bpf_arg_type arg5_type;
> >> +       union {
> >> +               struct {
> >> +                       enum bpf_arg_type arg1_type;
> >> +                       enum bpf_arg_type arg2_type;
> >> +                       enum bpf_arg_type arg3_type;
> >> +                       enum bpf_arg_type arg4_type;
> >> +                       enum bpf_arg_type arg5_type;
> >> +               };
> >> +               enum bpf_arg_type arg_type[5];
> >> +       };
> >> +       u32 *btf_id; /* BTF ids of arguments */
> >
> > are you trying to save memory with this? otherwise not sure why it's
> > not just `u32 btf_id[5]`? Even in that case it will save at most 12
> > bytes (and I haven't even check alignment padding and stuff). So
> > doesn't seem worth it?
>
> Glad you asked :)
> It cannot be "u32 btf_id[5];".
> Guess why?

/data/users/andriin/linux/kernel/bpf/verifier.c: In function
‘check_helper_call’:
/data/users/andriin/linux/kernel/bpf/verifier.c:4097:19: error:
assignment of read-only location ‘fn->btf_id[i]’
     fn->btf_id[i] = btf_resolve_helper_id(&env->log, fn->func, 0);
                   ^
That answers it :) Yeah, indirection w/ pointer is a clever hack :)

> I think it's a cool trick.
> I was happy when I finally figured out to solve it this way
> after analyzing a bunch of ugly solutions.
>
> >> + *
> >> + *             The value to write, of *size*, is passed through eBPF stack and
> >> + *             pointed by *data*.
> >
> > typo? pointed __to__ by *data*?
>
> I'm not an grammar expert. That was a copy paste from existing comment.
>
> >> + *
> >> + *             *ctx* is a pointer to in-kernel sutrct sk_buff.

randomly spotted "sutrct" here :)

> >> + *
> >> + *             This helper is similar to **bpf_perf_event_output**\ () but
> >> + *             restricted to raw_tracepoint bpf programs.
> >
> > nit: with BTF type tracking enabled?
>
> sure.
>
> >> +       for (i = 0; i < 5; i++) {
> >> +               if (fn->arg_type[i] == ARG_PTR_TO_BTF_ID) {
> >> +                       if (!fn->btf_id[i])
> >> +                               fn->btf_id[i] = btf_resolve_helper_id(&env->log, fn->func, 0);
> >
> > bug: 0 -> i  :)
>
> Nice catch.
> Clearly I don't have a use case yet for 2nd arg being ptr_to_btf.

This actually brings an interesting question. There are a bunch of
helpers that track stuff like iphdr and so on. You could use that to
test, except you can't, because their args are not marked as
ARG_PTR_TO_BTF_ID. But marking it as such would break usual program
types that don't track BTF. I wonder if it's possible to have some
arrangement, that make the same helper be, sort of "BTF-enabled" for
BTF-enabled type of programs (so far just raw tracepoint with
attach_btf_id set), but for other programs where BTF types are not
tracked it still allows normal semantics. Thoughts?

  reply index

Thread overview: 33+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-10-10  4:14 [PATCH v2 bpf-next 00/12] bpf: revolutionize bpf tracing Alexei Starovoitov
2019-10-10  4:14 ` [PATCH v2 bpf-next 01/12] bpf: add typecast to raw_tracepoints to help BTF generation Alexei Starovoitov
2019-10-10  4:14 ` [PATCH v2 bpf-next 02/12] bpf: add typecast to bpf helpers " Alexei Starovoitov
2019-10-10  4:14 ` [PATCH v2 bpf-next 03/12] bpf: process in-kernel BTF Alexei Starovoitov
2019-10-11 17:56   ` Andrii Nakryiko
2019-10-10  4:14 ` [PATCH v2 bpf-next 04/12] bpf: add attach_btf_id attribute to program load Alexei Starovoitov
2019-10-11 17:58   ` Andrii Nakryiko
2019-10-10  4:14 ` [PATCH v2 bpf-next 05/12] libbpf: auto-detect btf_id of raw_tracepoint Alexei Starovoitov
2019-10-11 18:02   ` Andrii Nakryiko
2019-10-11 18:07   ` Andrii Nakryiko
2019-10-12  0:40     ` Alexei Starovoitov
2019-10-12  1:29       ` Alexei Starovoitov
2019-10-12  4:38         ` Andrii Nakryiko
2019-10-12  4:53           ` Alexei Starovoitov
2019-10-12  4:39       ` Andrii Nakryiko
2019-10-10  4:14 ` [PATCH v2 bpf-next 06/12] bpf: implement accurate raw_tp context access via BTF Alexei Starovoitov
2019-10-11 18:31   ` Andrii Nakryiko
2019-10-11 23:13     ` Andrii Nakryiko
2019-10-10  4:14 ` [PATCH v2 bpf-next 07/12] bpf: attach raw_tp program with BTF via type name Alexei Starovoitov
2019-10-11 18:44   ` Andrii Nakryiko
2019-10-10  4:14 ` [PATCH v2 bpf-next 08/12] bpf: add support for BTF pointers to interpreter Alexei Starovoitov
2019-10-10  4:15 ` [PATCH v2 bpf-next 09/12] bpf: add support for BTF pointers to x86 JIT Alexei Starovoitov
2019-10-11 18:48   ` Andrii Nakryiko
2019-10-10  4:15 ` [PATCH v2 bpf-next 10/12] bpf: check types of arguments passed into helpers Alexei Starovoitov
2019-10-11 19:02   ` Andrii Nakryiko
2019-10-12  1:39     ` Alexei Starovoitov
2019-10-12  4:25       ` Andrii Nakryiko [this message]
2019-10-10  4:15 ` [PATCH v2 bpf-next 11/12] bpf: disallow bpf_probe_read[_str] helpers Alexei Starovoitov
2019-10-11 19:03   ` Andrii Nakryiko
2019-10-10  4:15 ` [PATCH v2 bpf-next 12/12] selftests/bpf: add kfree_skb raw_tp test Alexei Starovoitov
2019-10-10 11:07   ` Ido Schimmel
2019-10-10 19:07     ` Alexei Starovoitov
2019-10-11 19:05   ` Andrii Nakryiko

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