btf_dump prints decl tags, so bpftool can annotate types in the C dump
by adding decl tags to the BTF it dumps. Adding them to the input BTF
would make it modifiable, which libbpf refuses for BTF with unknown
header-extension bytes, such as BTF from a newer kernel.

Dump C through an empty split BTF over the input instead, so that the
annotations can go there. btf__new_empty_split() does not inherit the
base pointer size, so set it explicitly.

No functional change.

Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
---
Later in this series bpftool adds decl tags to the split, for record
attributes and type tag spellings.
---
 tools/bpf/bpftool/btf.c | 28 +++++++++++++++++++++-------
 1 file changed, 21 insertions(+), 7 deletions(-)

diff --git a/tools/bpf/bpftool/btf.c b/tools/bpf/bpftool/btf.c
index 385521e6b028..a438b5a03f5a 100644
--- a/tools/bpf/bpftool/btf.c
+++ b/tools/bpf/bpftool/btf.c
@@ -1092,17 +1092,30 @@ static struct sort_datum *sort_btf_c(const struct btf *btf)
 	return datums;
 }
 
-static int dump_btf_c(const struct btf *btf,
+static int dump_btf_c(struct btf *btf,
 		      __u32 *root_type_ids, int root_type_cnt, bool sort_dump)
 {
 	struct sort_datum *datums = NULL;
-	struct btf_dump *d;
+	struct btf_dump *d = NULL;
+	struct btf *split;
+	size_t ptr_sz;
 	int err = 0, i;
 
-	d = btf_dump__new(btf, btf_dump_printf, NULL, NULL);
-	if (!d)
+	split = btf__new_empty_split(btf);
+	if (!split)
 		return -errno;
 
+	/* btf__new_empty_split() does not inherit the base pointer size */
+	ptr_sz = btf__pointer_size(btf);
+	if (ptr_sz)
+		btf__set_pointer_size(split, ptr_sz);
+
+	d = btf_dump__new(split, btf_dump_printf, NULL, NULL);
+	if (!d) {
+		err = -errno;
+		goto done;
+	}
+
 	printf("#ifndef __VMLINUX_H__\n");
 	printf("#define __VMLINUX_H__\n");
 	printf("\n");
@@ -1130,10 +1143,10 @@ static int dump_btf_c(const struct btf *btf,
 				goto done;
 		}
 	} else {
-		int cnt = btf__type_cnt(btf);
+		int cnt = btf__type_cnt(split);
 
 		if (sort_dump)
-			datums = sort_btf_c(btf);
+			datums = sort_btf_c(split);
 		for (i = 0; i < cnt; i++) {
 			int idx = datums ? datums[i].index : i;
 
@@ -1146,7 +1159,7 @@ static int dump_btf_c(const struct btf *btf,
 				goto done;
 		}
 
-		err = dump_btf_kfuncs(d, btf);
+		err = dump_btf_kfuncs(d, split);
 		if (err)
 			goto done;
 	}
@@ -1160,6 +1173,7 @@ static int dump_btf_c(const struct btf *btf,
 done:
 	free(datums);
 	btf_dump__free(d);
+	btf__free(split);
 	return err;
 }
 
-- 
2.56.0