[PATCH pahole] btf_encoder: Fix void handling in FUNC_PROTO.

[Andrii Nakryiko <andriin@xxxxxx>]

When dealing with BTF encoding of multiple CUs, special `void` type
should be handled explicitly. This is already handled for all BTF
types except recently added FUNC_PROTO. Without this fix, any `void`
type directly referenced from FUNC_PROTO will turn into last type
from previous CU (see example below, for FUNC_PROTO [4]).

  $ cat test1.c
  void (*foo)(void);

  int main() {
      return 0;
  }

  $ cat test2.c
  void (*bar)(void);

  $ cc -g test1.c test2.c -o test

Without fix:

  $ LLVM_OBJCOPY=objcopy ~/local/pahole/build/pahole -JV /tmp/test
  File /tmp/test:
  [1] INT int size=4 bit_offset=0 nr_bits=32 encoding=SIGNED
  [2] FUNC_PROTO (anon) return=0 args=(void)
  [3] PTR (anon) type_id=2
  [4] FUNC_PROTO (anon) return=3 args=(void)
  [5] PTR (anon) type_id=4

With fix:

  $ LLVM_OBJCOPY=objcopy ~/local/pahole/build/pahole -JV /tmp/test
  File /tmp/test:
  [1] INT int size=4 bit_offset=0 nr_bits=32 encoding=SIGNED
  [2] FUNC_PROTO (anon) return=0 args=(void)
  [3] PTR (anon) type_id=2
  [4] FUNC_PROTO (anon) return=0 args=(void)
  [5] PTR (anon) type_id=4

Signed-off-by: Andrii Nakryiko <andriin@xxxxxx>
---
 libbtf.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/libbtf.c b/libbtf.c
index 478393b..c988d2a 100644
--- a/libbtf.c
+++ b/libbtf.c
@@ -535,7 +535,7 @@ int32_t btf__add_func_proto(struct btf *btf, struct ftype *ftype,
 
  t.name_off = 0;
  t.info = BTF_INFO_ENCODE(BTF_KIND_FUNC_PROTO, 0, nr_params);
- t.type = type_id_off + ftype->tag.type;
+ t.type = ftype->tag.type == 0 ? 0 : type_id_off + ftype->tag.type;
 
  ++btf->type_index;
  if (gobuffer__add(&btf->types, &t, sizeof(t)) >= 0) {
@@ -552,9 +552,12 @@ int32_t btf__add_func_proto(struct btf *btf, struct ftype *ftype,
  /* add parameters */
  param_idx = 0;
  ftype__for_each_parameter(ftype, param) {
+   uint32_t param_type_id = param->tag.type == 0
+     ? 0
+     : type_id_off + param->tag.type;
    ++param_idx;
    if (btf__add_func_proto_param(btf, param->name,
-               type_id_off + param->tag.type,
+               param_type_id,
                param_idx == nr_params))
      return -1;
  }
-- 
2.17.1