When -fms-extension is used, it becomes possible to declare a
non-anonymous nested struct without a member name; for example

struct bar {
	int baz;
};

struct foo {
	struct bar;
};

These are used in the kernel now, and hence make it into BTF.
For example:

struct ns_tree {
        u64 ns_id;
        atomic_t __ns_ref_active;
        struct ns_tree_node ns_unified_node;
        struct ns_tree_node ns_tree_node;
        struct ns_tree_node ns_owner_node;
        struct ns_tree_root ns_owner_root;
};

struct proc_ns_operations;

struct ns_common {
        u32 ns_type;
        struct dentry *stashed;
        const struct proc_ns_operations *ops;
        unsigned int inum;
        refcount_t __ns_ref;
        union {
>>>>            struct ns_tree; <<<<
		struct callback_head ns_rcu;
	};
};

In order to generate a vmlinux.h from such BTF that does
not force support of -fms-extension, provide an option to
btf_dump__new() to replace structs that have names but no
referring member name with an equivalent anonymous struct.
bpftool can then make use of this feature.

With this approach, the above would become:

truct ns_common {
        u32 ns_type;
        struct dentry *stashed;
        const struct proc_ns_operations *ops;
        unsigned int inum;
        refcount_t __ns_ref;
        union {
                struct  {
                        u64 ns_id;
                        atomic_t __ns_ref_active;
                        struct ns_tree_node ns_unified_node;
                        struct ns_tree_node ns_tree_node;
                        struct ns_tree_node ns_owner_node;
                        struct ns_tree_root ns_owner_root;
                };
                struct callback_head ns_rcu;
        };
};

Fields are still accessible, size is the same but we have
included an anonymous equivalent of the struct instead since
this is more compatible.

Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
---
 tools/lib/bpf/btf.h      | 25 ++++++++++++++++++++++++-
 tools/lib/bpf/btf_dump.c | 25 ++++++++++++++++++++++---
 2 files changed, 46 insertions(+), 4 deletions(-)

diff --git a/tools/lib/bpf/btf.h b/tools/lib/bpf/btf.h
index cc01494d6210..fc32e488c05c 100644
--- a/tools/lib/bpf/btf.h
+++ b/tools/lib/bpf/btf.h
@@ -285,8 +285,31 @@ struct btf_dump;
 
 struct btf_dump_opts {
 	size_t sz;
+	/* when a struct declares another struct within it without a
+	 * member name, force it to be an anonymous nested struct;
+	 * for example instead of
+	 *
+	 * struct bar {
+	 *	int baz;
+	 * };
+	 * struct foo {
+	 *	struct bar;
+	 *	...
+	 * };
+	 *
+	 * use
+	 *
+	 * struct foo {
+	 *	struct {
+	 *		int baz;
+	 *	};
+	 * };
+	 *
+	 * This is needed for compilation without -fms-extension .
+	 */
+	bool force_anon_struct_members;
 };
-#define btf_dump_opts__last_field sz
+#define btf_dump_opts__last_field force_anon_struct_members
 
 typedef void (*btf_dump_printf_fn_t)(void *ctx, const char *fmt, va_list args);
 
diff --git a/tools/lib/bpf/btf_dump.c b/tools/lib/bpf/btf_dump.c
index 6388392f49a0..b37062247580 100644
--- a/tools/lib/bpf/btf_dump.c
+++ b/tools/lib/bpf/btf_dump.c
@@ -54,6 +54,8 @@ struct btf_dump_type_aux_state {
 	__u8 name_resolved: 1;
 	/* whether type is referenced from any other type */
 	__u8 referenced: 1;
+	/* whether named type should be anonymized */
+	__u8 anonymize: 1;
 };
 
 /* indent string length; one indent string is added for each indent level */
@@ -84,6 +86,7 @@ struct btf_dump {
 	int ptr_sz;
 	bool strip_mods;
 	bool skip_anon_defs;
+	bool force_anon_struct_members;
 	int last_id;
 
 	/* per-type auxiliary state */
@@ -164,6 +167,7 @@ struct btf_dump *btf_dump__new(const struct btf *btf,
 	if (!d)
 		return libbpf_err_ptr(-ENOMEM);
 
+	d->force_anon_struct_members = OPTS_GET(opts, force_anon_struct_members, false);
 	d->btf = btf;
 	d->printf_fn = printf_fn;
 	d->cb_ctx = ctx;
@@ -1417,6 +1421,8 @@ static void btf_dump_emit_name(const struct btf_dump *d,
 	btf_dump_printf(d, "%s%s", separate ? " " : "", name);
 }
 
+static void btf_dump_set_anon_type(struct btf_dump *d, __u32 id, bool anon);
+
 static void btf_dump_emit_type_chain(struct btf_dump *d,
 				     struct id_stack *decls,
 				     const char *fname, int lvl)
@@ -1460,10 +1466,16 @@ static void btf_dump_emit_type_chain(struct btf_dump *d,
 		case BTF_KIND_UNION:
 			btf_dump_emit_mods(d, decls);
 			/* inline anonymous struct/union */
-			if (t->name_off == 0 && !d->skip_anon_defs)
+			if (t->name_off == 0 && !d->skip_anon_defs) {
 				btf_dump_emit_struct_def(d, id, t, lvl);
-			else
+			} else if (decls->cnt == 0 && !fname[0] && d->force_anon_struct_members) {
+				/* anonymize nested struct and emit it */
+				btf_dump_set_anon_type(d, id, true);
+				btf_dump_emit_struct_def(d, id, t, lvl);
+				btf_dump_set_anon_type(d, id, false);
+			} else {
 				btf_dump_emit_struct_fwd(d, id, t);
+			}
 			break;
 		case BTF_KIND_ENUM:
 		case BTF_KIND_ENUM64:
@@ -1661,6 +1673,13 @@ static size_t btf_dump_name_dups(struct btf_dump *d, struct hashmap *name_map,
 	return dup_cnt;
 }
 
+static void btf_dump_set_anon_type(struct btf_dump *d, __u32 id, bool anon)
+{
+	struct btf_dump_type_aux_state *s = &d->type_states[id];
+
+	s->anonymize = anon;
+}
+
 static const char *btf_dump_resolve_name(struct btf_dump *d, __u32 id,
 					 struct hashmap *name_map)
 {
@@ -1670,7 +1689,7 @@ static const char *btf_dump_resolve_name(struct btf_dump *d, __u32 id,
 	const char **cached_name = &d->cached_names[id];
 	size_t dup_cnt;
 
-	if (t->name_off == 0)
+	if (t->name_off == 0 || s->anonymize)
 		return "";
 
 	if (s->name_resolved)
-- 
2.39.3