btf__add_btf() currently rejects split BTF sources with -ENOTSUP.
This prevents merging types from multiple kernel module BTFs that
are all split against the same vmlinux base.

Extend btf__add_btf() to handle split BTF sources by:

- Replacing the blanket -ENOTSUP with a validation that src and dst
  share the same base BTF pointer when both are split, returning
  -EOPNOTSUPP on mismatch.

- Computing src_start_id from the source's base to distinguish base
  type ID references (which must remain unchanged) from split type
  IDs (which must be remapped to new positions in the destination).

- Using src_btf->nr_types instead of btf__type_cnt()-1 for the type
  count, which is correct for both split and non-split sources.

- Skipping base string offsets (< start_str_off) during the string
  rewrite loop, mirroring the type ID skip pattern.  Since src and
  dst share the same base BTF, base string offsets are already valid
  and need no remapping.

For non-split sources the behavior is identical: src_start_id is 1,
the type_id < 1 guard is never true (VOID is already skipped), and
the remapping formula reduces to the original.  start_str_off is 0
so no string offsets are skipped.

Assisted-by: Claude:claude-opus-4-6
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
 tools/lib/bpf/btf.c | 37 +++++++++++++++++++++++++++++--------
 1 file changed, 29 insertions(+), 8 deletions(-)

diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
index 83fe79ffcb8f..be8cc4ee83fe 100644
--- a/tools/lib/bpf/btf.c
+++ b/tools/lib/bpf/btf.c
@@ -2004,12 +2004,18 @@ int btf__add_btf(struct btf *btf, const struct btf *src_btf)
 {
 	struct btf_pipe p = { .src = src_btf, .dst = btf };
 	int data_sz, sz, cnt, i, err, old_strs_len;
+	__u32 src_start_id;
 	__u32 *off;
 	void *t;
 
-	/* appending split BTF isn't supported yet */
-	if (src_btf->base_btf)
-		return libbpf_err(-ENOTSUP);
+	/* When appending split BTF, the destination must share the same base
+	 * BTF so that base type ID references remain valid.
+	 */
+	if (src_btf->base_btf &&
+	    src_btf->base_btf != btf->base_btf)
+		return libbpf_err(-EOPNOTSUPP);
+
+	src_start_id = src_btf->base_btf ? btf__type_cnt(src_btf->base_btf) : 1;
 
 	/* deconstruct BTF, if necessary, and invalidate raw_data */
 	if (btf_ensure_modifiable(btf))
@@ -2021,7 +2027,7 @@ int btf__add_btf(struct btf *btf, const struct btf *src_btf)
 	old_strs_len = btf->hdr->str_len;
 
 	data_sz = src_btf->hdr->type_len;
-	cnt = btf__type_cnt(src_btf) - 1;
+	cnt = src_btf->nr_types;
 
 	/* pre-allocate enough memory for new types */
 	t = btf_add_type_mem(btf, data_sz);
@@ -2060,6 +2066,16 @@ int btf__add_btf(struct btf *btf, const struct btf *src_btf)
 		if (err)
 			goto err_out;
 		while ((str_off = btf_field_iter_next(&it))) {
+			/* For split BTF sources, string offsets referencing
+			 * the base BTF (< start_str_off) remain unchanged
+			 * since dst shares the same base.  Only remap
+			 * strings in the split portion.  For non-split
+			 * sources start_str_off is 0 so all offsets except
+			 * the empty string (handled inside btf_rewrite_str)
+			 * are remapped as before.
+			 */
+			if (*str_off < src_btf->start_str_off)
+				continue;
 			err = btf_rewrite_str(&p, str_off);
 			if (err)
 				goto err_out;
@@ -2074,11 +2090,16 @@ int btf__add_btf(struct btf *btf, const struct btf *src_btf)
 			if (!*type_id) /* nothing to do for VOID references */
 				continue;
 
-			/* we haven't updated btf's type count yet, so
-			 * btf->start_id + btf->nr_types - 1 is the type ID offset we should
-			 * add to all newly added BTF types
+			/* For split BTF sources, type IDs referencing the
+			 * base BTF (< src_start_id) remain unchanged since
+			 * dst shares the same base.  Only remap IDs in the
+			 * split portion.  For non-split sources src_start_id
+			 * is 1 so all IDs are remapped as before.
 			 */
-			*type_id += btf->start_id + btf->nr_types - 1;
+			if (*type_id < src_start_id)
+				continue;
+
+			*type_id += btf->start_id + btf->nr_types - src_start_id;
 		}
 
 		/* go to next type data and type offset index entry */
-- 
2.53.0