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From:   Matteo Croce <mcroce@linux.microsoft.com>
Date:   Fri, 17 Dec 2021 20:31:18 +0100
Message-ID: <CAFnufp35YbxhbQR7stq39WOhAZm4LYHu6FfYBeHJ8-xRSo7TnQ@mail.gmail.com>
Subject: Re: [PATCH bpf-next] bpf: limit bpf_core_types_are_compat() recursion
To:     Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc:     bpf <bpf@vger.kernel.org>, Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
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 On Wed, Dec 15, 2021 at 7:21 PM Matteo Croce
<mcroce@linux.microsoft.com> wrote:
>
> On Wed, Dec 15, 2021 at 6:29 PM Alexei Starovoitov
> <alexei.starovoitov@gmail.com> wrote:
> >
> > On Wed, Dec 15, 2021 at 6:54 AM Matteo Croce <mcroce@linux.microsoft.com> wrote:
> > > >
> > > > Maybe do a level check here?
> > > > Since calling it and immediately returning doesn't conserve
> > > > the stack.
> > > > If it gets called it can finish fine, but
> > > > calling it again would be too much.
> > > > In other words checking the level here gives us
> > > > room for one more frame.
> > > >
> > >
> > > I thought that the compiler was smart enough to return before
> > > allocating most of the frame.
> > > I tried and this is true only with gcc, not with clang.
> >
> > Interesting. That's a surprise.
> > Could you share the asm that gcc generates?
> >
>
> Sure,
>
> This is the gcc x86_64 asm on a stripped down
> bpf_core_types_are_compat() with a 1k struct on the stack:
>
> bpf_core_types_are_compat:
> test esi, esi
> jle .L69
> push r15
> push r14
> push r13
> push r12
> push rbp
> mov rbp, rdi
> push rbx
> mov ebx, esi
> sub rsp, 9112
> [...]
> .L69:
> or eax, -1
> ret
>
> This latest clang:
>
> bpf_core_types_are_compat: # @bpf_core_types_are_compat
> push rbp
> push r15
> push r14
> push rbx
> sub rsp, 1000
> mov r14d, -1
> test esi, esi
> jle .LBB0_7
> [...]
> .LBB0_7:
> mov eax, r14d
> add rsp, 1000
> pop rbx
> pop r14
> pop r15
> pop rbp
> ret
>
> > > > > +                       err = __bpf_core_types_are_compat(local_btf, local_id,
> > > > > +                                                         targ_btf, targ_id,
> > > > > +                                                         level - 1);
> > > > > +                       if (err <= 0)
> > > > > +                               return err;
> > > > > +               }
> > > > > +
> > > > > +               /* tail recurse for return type check */
> > > > > +               btf_type_skip_modifiers(local_btf, local_type->type, &local_id);
> > > > > +               btf_type_skip_modifiers(targ_btf, targ_type->type, &targ_id);
> > > > > +               goto recur;
> > > > > +       }
> > > > > +       default:
> > > > > +               pr_warn("unexpected kind %s relocated, local [%d], target [%d]\n",
> > > > > +                       btf_type_str(local_type), local_id, targ_id);
> > > >
> > > > That should be bpf_log() instead.
> > > >
> > >
> > > To do that I need a struct bpf_verifier_log, which is not present
> > > there, neither in bpf_core_spec_match() or bpf_core_apply_relo_insn().
> >
> > It is there. See:
> >         err = bpf_core_apply_relo_insn((void *)ctx->log, insn, ...
> >
> > > Should we drop the message at all?
> >
> > Passing it into bpf_core_spec_match() and further into
> > bpf_core_types_are_compat() is probably unnecessary.
> > All callers have an error check with a log right after.
> > So I think we won't lose anything if we drop this log.
> >
>
> Nice.
>
> > >
> > > > > +               return 0;
> > > > > +       }
> > > > > +}
> > > >
> > > > Please add tests that exercise this logic by enabling
> > > > additional lskels and a new test that hits the recursion limit.
> > > > I suspect we don't have such case in selftests.
> > > >
> > > > Thanks!
> > >
> > > Will do!
> >
> > Thanks!
>

Hi,

I'm writing a test which exercise that function.
I can succesfully trigger a call to __bpf_core_types_are_compat() with
these  calls:

bpf_core_type_id_kernel(struct sk_buff);
bpf_core_type_exists(struct sk_buff);
bpf_core_type_size(struct sk_buff);

but the kind will obviously be BTF_KIND_STRUCT.
I'm trying to do the same with kind BTF_KIND_FUNC_PROTO instead with:

void func_proto(int, unsigned int);
bpf_core_type_id_kernel(func_proto);

but I get a clang crash[1], while just checking the existence with:

typedef int (*func_proto_typedef)(struct sk_buff *);
bpf_core_type_exists(func_proto_typedef);

I don't reach even bpf_core_spec_match().

Which is a simple way to generate a BTF_KIND_FUNC_PROTO BTF field?

[1] https://github.com/llvm/llvm-project/issues/52779

Regards,
-- 
per aspera ad upstream