Re: [PATCH bpf-next 1/2] libbpf: add option to force-anonymize nested structs for BTF dump

[Eduard Zingerman <eddyz87@gmail.com>]

On Tue, 2025-12-16 at 11:00 -0800, Eduard Zingerman wrote:
> On Tue, 2025-12-16 at 17:18 +0000, Alan Maguire wrote:
>
> [...]
>
> > @@ -1460,10 +1466,16 @@ static void btf_dump_emit_type_chain(struct btf_dump *d,
> >  		case BTF_KIND_UNION:
> >  			btf_dump_emit_mods(d, decls);
> >  			/* inline anonymous struct/union */
> > -			if (t->name_off == 0 && !d->skip_anon_defs)
> > +			if (t->name_off == 0 && !d->skip_anon_defs) {
> >  				btf_dump_emit_struct_def(d, id, t, lvl);
> > -			else
> > +			} else if (decls->cnt == 0 && !fname[0] && d->force_anon_struct_members) {
> > +				/* anonymize nested struct and emit it */
> > +				btf_dump_set_anon_type(d, id, true);
> > +				btf_dump_emit_struct_def(d, id, t, lvl);
> > +				btf_dump_set_anon_type(d, id, false);
>
>
> Hi Alan,
>
> I think this is a solid idea.
>
> It seems to me that with current implementation there would be a
> trouble in the following scenario:
>
>   struct foo { struct foo *ptr; };
>   struct bar {
>     struct foo;
>   }
>
> Because state for 'foo' will be anonymize == true at the moment when
> 'ptr' field is printed.
>
> Maybe pass a direct parameter to btf_dump_emit_struct_def()?

Digging a bit more into this, here are a couple of weird examples:

  $ cat ~/tmp/ms-ext-test.c
  struct foo {
    struct foo *ptr;
  };

  struct bar {
    struct foo;
  };

  struct bar root;
  $ gcc -g -c -o ~/tmp/ms-ext-test.o ~/tmp/ms-ext-test.c
  $ pahole -J ~/tmp/ms-ext-test.o
  $ bpftool btf dump file ~/tmp/ms-ext-test.o format c
  #ifndef __VMLINUX_H__
  #define __VMLINUX_H__

  #ifndef BPF_NO_PRESERVE_ACCESS_INDEX
  #pragma clang attribute push (__attribute__((preserve_access_index)), apply_to = record)
  #endif

  #ifndef __ksym
  #define __ksym __attribute__((section(".ksyms")))
  #endif

  #ifndef __weak
  #define __weak __attribute__((weak))
  #endif

  #ifndef __bpf_fastcall
  #if __has_attribute(bpf_fastcall)
  #define __bpf_fastcall __attribute__((bpf_fastcall))
  #else
  #define __bpf_fastcall
  #endif
  #endif

  struct foo {
  	struct foo *ptr;
  };


  /* BPF kfuncs */
  #ifndef BPF_NO_KFUNC_PROTOTYPES
  #endif

  #ifndef BPF_NO_PRESERVE_ACCESS_INDEX
  #pragma clang attribute pop
  #endif

  #endif /* __VMLINUX_H__ */


  $ cat ~/tmp/ms-ext-test.c
  struct foo {
    struct foo *ptr;
  };

  struct bar {
    struct foo;
    int a;
  };

  struct bar root;
  $ cgcc -fms-extensions -g -c -o ~/tmp/ms-ext-test.o ~/tmp/ms-ext-test.c
  $ pahole -J ~/tmp/ms-ext-test.o
  $ tools/bpf/bpftool/bpftool btf dump file ~/tmp/ms-ext-test.o format c
  #ifndef __VMLINUX_H__
  #define __VMLINUX_H__

  #ifndef BPF_NO_PRESERVE_ACCESS_INDEX
  #pragma clang attribute push (__attribute__((preserve_access_index)), apply_to = record)
  #endif

  #ifndef __ksym
  #define __ksym __attribute__((section(".ksyms")))
  #endif

  #ifndef __weak
  #define __weak __attribute__((weak))
  #endif

  #ifndef __bpf_fastcall
  #if __has_attribute(bpf_fastcall)
  #define __bpf_fastcall __attribute__((bpf_fastcall))
  #else
  #define __bpf_fastcall
  #endif
  #endif

  struct foo {
  	struct foo *ptr;
  };

  struct bar {
  	struct  {
  		struct  *ptr;
  	};
  	int a;
  };


  /* BPF kfuncs */
  #ifndef BPF_NO_KFUNC_PROTOTYPES
  #endif

  #ifndef BPF_NO_PRESERVE_ACCESS_INDEX
  #pragma clang attribute pop
  #endif

  #endif /* __VMLINUX_H__ */