[PATCH bpf-next v1 3/4] bpftool: Don't drop a type in the sorted C dump

[Ihor Solodrai <ihor.solodrai@linux.dev>]

The C dump sorts types by default, so that generated headers are
diffable. The sorted dump emits one type fewer than the unsorted dump
of the same BTF.

dump_btf_c() starts its loop at index 1 to skip the void type at BTF
type ID 0. That holds for the unsorted dump, where the array index is
the type ID, but not after qsort(): position 0 is then the lowest ranked
type, and btf_type_rank() ranks an anonymous enum 0 while void takes the
default rank of 10. So the enum is skipped, and void is emitted instead
as a no-op.

Skip by type ID rather than by position.

Fixes: 94133cf24bb3 ("bpftool: Introduce btf c dump sorting")
Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
---
 tools/bpf/bpftool/btf.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/tools/bpf/bpftool/btf.c b/tools/bpf/bpftool/btf.c
index c9589026da8d..345d49a9a22c 100644
--- a/tools/bpf/bpftool/btf.c
+++ b/tools/bpf/bpftool/btf.c
@@ -805,9 +805,13 @@ static int dump_btf_c(const struct btf *btf,
 
 		if (sort_dump)
 			datums = sort_btf_c(btf);
-		for (i = 1; i < cnt; i++) {
+		for (i = 0; i < cnt; i++) {
 			int idx = datums ? datums[i].index : i;
 
+			/* type ID 0 is void, skip it */
+			if (!idx)
+				continue;
+
 			err = btf_dump__dump_type(d, idx);
 			if (err)
 				goto done;
-- 
2.55.0