netdev - Re: [PATCH bpf] libbpf: Fix BTF-to-C conversion of noreturn function pointers

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Message-ID: <20200609140504.GA915559@myrica>
Date:   Tue, 9 Jun 2020 16:05:04 +0200
From:   Jean-Philippe Brucker <jean-philippe@...aro.org>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     bpf <bpf@...r.kernel.org>, Andrii Nakryiko <andriin@...com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin Lau <kafai@...com>, Song Liu <songliubraving@...com>,
        Yonghong Song <yhs@...com>,
        john fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Networking <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf] libbpf: Fix BTF-to-C conversion of noreturn function
 pointers

On Mon, Jun 08, 2020 at 04:50:37PM -0700, Andrii Nakryiko wrote:
> On Mon, Jun 8, 2020 at 8:23 AM Jean-Philippe Brucker
> <jean-philippe@...aro.org> wrote:
> >
> > When trying to convert the BTF for a function pointer marked "noreturn"
> > to C code, bpftool currently generates a syntax error. This happens with
> > the exit() pointer in drivers/firmware/efi/libstub/efistub.h, in an
> > arm64 vmlinux. When dealing with this declaration:
> >
> >         efi_status_t __noreturn (__efiapi *exit)(...);
> >
> > bpftool produces the following output:
> >
> >         efi_status_tvolatile  (*exit)(...);
> 
> 
> I'm curious where this volatile is coming from, I don't see it in
> __efiapi. But even if it's there, shouldn't it be inside parens
> instead:
> 
> efi_status_t (volatile *exit)(...);

It's the __noreturn attribute that becomes "volatile", not the __efiapi.
My reproducer is:

  struct my_struct {
          void __attribute__((noreturn)) (*fn)(int);
  };
  struct my_struct a;

When generating DWARF info for this, GCC inserts a DW_TAG_volatile_type.
Clang doesn't add a volatile tag, it just omits the noreturn qualifier.
>From what I could find, it's due to legacy "noreturn" support in GCC [1]:
before version 2.5 the only way to declare a noreturn function was to
declare it volatile.

[1] https://gcc.gnu.org/onlinedocs/gcc-4.7.2/gcc/Function-Attributes.html

Given that not all compilers turn "noreturn" into "volatile", and that I
haven't managed to insert any other modifier (volatile/const/restrict) in
this location (the efistub example above is the only issue on an
allyesconfig kernel), I was considering simply removing this call to
btf_dump_emit_mods(). But I'm not confident enough that it won't ever be
necessary.

> > Fix the error by inserting the space before the function modifier.
> >
> > Fixes: 351131b51c7a ("libbpf: add btf_dump API for BTF-to-C conversion")
> > Signed-off-by: Jean-Philippe Brucker <jean-philippe@...aro.org>
> > ---
> 
> Can you please add tests for this case into selftests (probably
> progs/btf_dump_test_case_syntax.c?) So it's clear what's the input and
> what's the expected output.

Those tests are built with clang, which doesn't emit the "volatile"
modifier. Should I add a separate test for GCC?

Thanks,
Jean