linux-kernel - Re: [PATCH bpf-next] bpf: limit bpf_core_types_are

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Date:   Fri, 28 Jan 2022 12:08:49 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Matteo Croce <mcroce@...ux.microsoft.com>
Cc:     Yonghong Song <yhs@...com>, bpf <bpf@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next] bpf: limit bpf_core_types_are_compat() recursion

On Fri, Jan 28, 2022 at 10:51 AM Matteo Croce
<mcroce@...ux.microsoft.com> wrote:
>
> On Fri, Jan 28, 2022 at 6:31 AM Alexei Starovoitov
> <alexei.starovoitov@...il.com> wrote:
> >
> > On Mon, Dec 20, 2021 at 10:34 PM Yonghong Song <yhs@...com> wrote:
> > >
> > >
> > > https://reviews.llvm.org/D116063 improved the error message as below
> > > to make it a little bit more evident what is the problem:
> > >
> > > $ clang -target bpf -O2 -g -c bug.c
> > >
> > > fatal error: error in backend: SubroutineType not supported for
> > > BTF_TYPE_ID_REMOTE reloc
> >
> > Hi Matteo,
> >
> > Are you still working on a test?
> > What's a timeline to repost the patch set?
> >
> > Thanks!
>
> Hi Alexei,
>
> The change itself is ready, I'm just stuck at writing a test which
> will effectively calls __bpf_core_types_are_compat() with some
> recursion.
> I guess that I have to generate a BTF_KIND_FUNC_PROTO type somehow, so
> __bpf_core_types_are_compat() is called again to check the prototipe
> arguments type.
> I tried with these two, with no luck:
>
> // 1
> typedef int (*func_proto_typedef)(struct sk_buff *);
> bpf_core_type_exists(func_proto_typedef);
>
> // 2
> void func_proto(int, unsigned int);
> bpf_core_type_id_kernel(func_proto);
>
> Which is a simple way to generate a BTF_KIND_FUNC_PROTO BTF field?

What do you mean 'no luck'?
Have you tried what progs/test_core_reloc_type_id.c is doing?
typedef int (*func_proto_typedef)(long);
bpf_core_type_id_kernel(func_proto_typedef);

Without macros:
typedef int (*func_proto_typedef)(long);

int test() {
   return __builtin_btf_type_id(*(typeof(func_proto_typedef) *)0, 1);
}
int test2() {
   return __builtin_preserve_type_info(*(typeof(func_proto_typedef) *)0, 0);
}


compiles fine and generates relos.