Re: [PATCH bpf] libbpf: handle GCC noreturn-turned-volatile quirk

On Tue, Jun 09, 2020 at 10:23:35PM -0700, Andrii Nakryiko wrote:

Handle a GCC quirk of emitting extra volatile modifier in DWARF (and
subsequently preserved in BTF by pahole) for function pointers marked as
__attribute__((noreturn)). This was the way to mark such functions before GCC
2.5 added noreturn attribute. Drop such func_proto modifiers, similarly to how
it's done for array (also to handle GCC quirk/bug).

Such volatile attribute is emitted by GCC only, so existing selftests can't
express such test. Simple repro is like this (compiled with GCC + BTF
generated by pahole):

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

Without this fix, output will be:

struct my_struct {
    voidvolatile  (*fn)(int);
};

With the fix:

struct my_struct {
    void (*fn)(int);
};

Reported-by: Jean-Philippe Brucker <redacted>
Fixes: 351131b51c7a ("libbpf: add btf_dump API for BTF-to-C conversion")
Signed-off-by: Andrii Nakryiko <redacted>

Thanks, this fixes the issue I was seeing with the arm64 vmlinux

Tested-by: Jean-Philippe Brucker <redacted>