[PATCH bpf-next 1/2] libbpf: Skip zeroed or null fields if not found in the kernel type.

[Kui-Feng Lee <thinker.li@xxxxxxxxx>]

Accept additional fields of a struct_ops type with all zero values even if
these fields are not in the corresponding type in the kernel. This provides
a way to be backward compatible. User space programs can use the same map
on a machine running an old kernel by clearing fields that do not exist in
the kernel.

Signed-off-by: Kui-Feng Lee <thinker.li@xxxxxxxxx>
---
 tools/lib/bpf/libbpf.c | 30 +++++++++++++++++++++++-------
 1 file changed, 23 insertions(+), 7 deletions(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index efab29b8935b..715879796046 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -1131,11 +1131,33 @@ static int bpf_map__init_kern_struct_ops(struct bpf_map *map)
 		__u32 kern_member_idx;
 		const char *mname;
 
+		mtype = skip_mods_and_typedefs(btf, member->type, &mtype_id);
 		mname = btf__name_by_offset(btf, member->name_off);
+		moff = member->offset / 8;
+		mdata = data + moff;
+		msize = btf__resolve_size(btf, mtype_id);
+		if (msize < 0) {
+			pr_warn("struct_ops init_kern %s: fails to resolve the size of member %s\n",
+				map->name, mname);
+			return msize;
+		}
+
 		kern_member = find_member_by_name(kern_btf, kern_type, mname);
 		if (!kern_member) {
 			pr_warn("struct_ops init_kern %s: Cannot find member %s in kernel BTF\n",
 				map->name, mname);
+
+			/* Skip all zeros or null fields if they are not
+			 * presented in the kernel BTF.
+			 */
+			if (btf_is_ptr(mtype)) {
+				if (!st_ops->progs[i])
+					continue;
+			} else {
+				if (libbpf_is_mem_zeroed(mdata, msize))
+					continue;
+			}
+
 			return -ENOTSUP;
 		}
 
@@ -1147,13 +1169,8 @@ static int bpf_map__init_kern_struct_ops(struct bpf_map *map)
 			return -ENOTSUP;
 		}
 
-		moff = member->offset / 8;
 		kern_moff = kern_member->offset / 8;
-
-		mdata = data + moff;
 		kern_mdata = kern_data + kern_moff;
-
-		mtype = skip_mods_and_typedefs(btf, member->type, &mtype_id);
 		kern_mtype = skip_mods_and_typedefs(kern_btf, kern_member->type,
 						    &kern_mtype_id);
 		if (BTF_INFO_KIND(mtype->info) !=
@@ -1230,9 +1247,8 @@ static int bpf_map__init_kern_struct_ops(struct bpf_map *map)
 			continue;
 		}
 
-		msize = btf__resolve_size(btf, mtype_id);
 		kern_msize = btf__resolve_size(kern_btf, kern_mtype_id);
-		if (msize < 0 || kern_msize < 0 || msize != kern_msize) {
+		if (kern_msize < 0 || msize != kern_msize) {
 			pr_warn("struct_ops init_kern %s: Error in size of member %s: %zd != %zd(kernel)\n",
 				map->name, mname, (ssize_t)msize,
 				(ssize_t)kern_msize);
-- 
2.34.1