tracing/probes: Add a function to search a member of a struct/union

Add btf_find_struct_member() API to search a member of a given data structure
or union from the member's name.

Link: https://lore.kernel.org/all/169272156248.160970.8868479822371129043.stgit@devnote2/Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>

kernel/trace/trace_btf.c

 // SPDX-License-Identifier: GPL-2.0
 #include <linux/btf.h>
 #include <linux/kernel.h>
+#include <linux/slab.h>
 
 #include "trace_btf.h"
 
@@ -51,3 +52,71 @@ const struct btf_param *btf_get_func_param(const struct btf_type *func_proto, s3
 		return NULL;
 }
 
+#define BTF_ANON_STACK_MAX	16
+
+struct btf_anon_stack {
+	u32 tid;
+	u32 offset;
+};
+
+/*
+ * Find a member of data structure/union by name and return it.
+ * Return NULL if not found, or -EINVAL if parameter is invalid.
+ * If the member is an member of anonymous union/structure, the offset
+ * of that anonymous union/structure is stored into @anon_offset. Caller
+ * can calculate the correct offset from the root data structure by
+ * adding anon_offset to the member's offset.
+ */
+const struct btf_member *btf_find_struct_member(struct btf *btf,
+						const struct btf_type *type,
+						const char *member_name,
+						u32 *anon_offset)
+{
+	struct btf_anon_stack *anon_stack;
+	const struct btf_member *member;
+	u32 tid, cur_offset = 0;
+	const char *name;
+	int i, top = 0;
+
+	anon_stack = kcalloc(BTF_ANON_STACK_MAX, sizeof(*anon_stack), GFP_KERNEL);
+	if (!anon_stack)
+		return ERR_PTR(-ENOMEM);
+
+retry:
+	if (!btf_type_is_struct(type)) {
+		member = ERR_PTR(-EINVAL);
+		goto out;
+	}
+
+	for_each_member(i, type, member) {
+		if (!member->name_off) {
+			/* Anonymous union/struct: push it for later use */
+			type = btf_type_skip_modifiers(btf, member->type, &tid);
+			if (type && top < BTF_ANON_STACK_MAX) {
+				anon_stack[top].tid = tid;
+				anon_stack[top++].offset =
+					cur_offset + member->offset;
+			}
+		} else {
+			name = btf_name_by_offset(btf, member->name_off);
+			if (name && !strcmp(member_name, name)) {
+				if (anon_offset)
+					*anon_offset = cur_offset;
+				goto out;
+			}
+		}
+	}
+	if (top > 0) {
+		/* Pop from the anonymous stack and retry */
+		tid = anon_stack[--top].tid;
+		cur_offset = anon_stack[top].offset;
+		type = btf_type_by_id(btf, tid);
+		goto retry;
+	}
+	member = NULL;
+
+out:
+	kfree(anon_stack);
+	return member;
+}
+

kernel/trace/trace_btf.h

@@ -5,3 +5,7 @@ const struct btf_type *btf_find_func_proto(const char *func_name,
 					   struct btf **btf_p);
 const struct btf_param *btf_get_func_param(const struct btf_type *func_proto,
 					   s32 *nr);
+const struct btf_member *btf_find_struct_member(struct btf *btf,
+						const struct btf_type *type,
+						const char *member_name,
+						u32 *anon_offset);