On Thu, 20 Jul 2023 23:34:42 +0100
Alan Maguire [off-list ref] wrote:

On 17/07/2023 16:23, Masami Hiramatsu (Google) wrote:

quoted

From: Masami Hiramatsu (Google) <mhiramat@kernel.org>

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

Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

A few small things below, but

Reviewed-by: Alan Maguire <redacted>

Thanks

quoted

---
 include/linux/btf.h |    3 +++
 kernel/bpf/btf.c    |   38 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 41 insertions(+)

diff --git a/include/linux/btf.h b/include/linux/btf.h
index 98fbbcdd72ec..097fe9b51562 100644
--- a/include/linux/btf.h
+++ b/include/linux/btf.h

@@ -225,6 +225,9 @@ const struct btf_type *btf_find_func_proto(struct btf *btf,
 					   const char *func_name);
 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);
 
 #define for_each_member(i, struct_type, member)			\
 	for (i = 0, member = btf_type_member(struct_type);	\

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index e015b52956cb..452ffb0393d6 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c

@@ -1992,6 +1992,44 @@ const struct btf_param *btf_get_func_param(const struct btf_type *func_proto, s3
 		return NULL;
 }
 
+/*
+ * Find a member of data structure/union by name and return it.
+ * Return NULL if not found, or -EINVAL if parameter is invalid.
+ */
+const struct btf_member *btf_find_struct_member(struct btf *btf,
+						const struct btf_type *type,
+						const char *member_name)
+{
+	const struct btf_member *members, *ret;
+	const char *name;
+	int i, vlen;
+
+	if (!btf || !member_name || !btf_type_is_struct(type))
+		return ERR_PTR(-EINVAL);
+
+	vlen = btf_type_vlen(type);
+	members = (const struct btf_member *)(type + 1);
+
+	for (i = 0; i < vlen; i++) {

could use for_each_member() here I think, or perhaps use
btf_type_member(type) when getting member pointer above.

Thanks! I missed that macro.

quoted

+		if (!members[i].name_off) {
+			/* unnamed union: dig deeper */
+			type = btf_type_by_id(btf, members[i].type);
+			if (!IS_ERR_OR_NULL(type)) {
+				ret = btf_find_struct_member(btf, type,
+							     member_name);

You'll need to skip modifiers before calling btf_find_struct_member()
here I think; it's possible to have a const anonymous union for example,

Yeah, it is possible. Let me add it.

so to get to the union you'd need to skip the modifiers first. Otherwise
you could fail the btf_type_is_struct() test on re-entry.

Indeed. 

Thank you!

quoted

+				if (!IS_ERR_OR_NULL(ret))
+					return ret;
+			}
+		} else {
+			name = btf_name_by_offset(btf, members[i].name_off);
+			if (name && !strcmp(member_name, name))
+				return &members[i];
+		}
+	}
+
+	return NULL;
+}
+
 static u32 btf_resolved_type_id(const struct btf *btf, u32 type_id)
 {
 	while (type_id < btf->start_id)

-- 
Masami Hiramatsu (Google) [off-list ref]