[PATCH] bpf: Allow small structs to be type of function argument

[Jiri Olsa <jolsa@xxxxxxxxxx>]

This way we can have trampoline on function
that has arguments with types like:

  kuid_t uid
  kgid_t gid

which unwind into small structs like:

  typedef struct {
        uid_t val;
  } kuid_t;

  typedef struct {
        gid_t val;
  } kgid_t;

And we can use them in bpftrace like:
(assuming d_path changes are in)

  # bpftrace -e 'lsm:path_chown { printf("uid %d, gid %d\n", args->uid, args->gid) }'
  Attaching 1 probe...
  uid 0, gid 0
  uid 1000, gid 1000
  ...

Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx>
---
 kernel/bpf/btf.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 58c9af1d4808..f8fee5833684 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -362,6 +362,14 @@ static bool btf_type_is_struct(const struct btf_type *t)
 	return kind == BTF_KIND_STRUCT || kind == BTF_KIND_UNION;
 }
 
+/* type is struct and its size is within 8 bytes
+ * and it can be value of function argument
+ */
+static bool btf_type_is_struct_arg(const struct btf_type *t)
+{
+	return btf_type_is_struct(t) && (t->size <= sizeof(u64));
+}
+
 static bool __btf_type_is_struct(const struct btf_type *t)
 {
 	return BTF_INFO_KIND(t->info) == BTF_KIND_STRUCT;
@@ -3768,7 +3776,7 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
 	/* skip modifiers */
 	while (btf_type_is_modifier(t))
 		t = btf_type_by_id(btf, t->type);
-	if (btf_type_is_int(t) || btf_type_is_enum(t))
+	if (btf_type_is_int(t) || btf_type_is_enum(t) || btf_type_is_struct_arg(t))
 		/* accessing a scalar */
 		return true;
 	if (!btf_type_is_ptr(t)) {
@@ -4161,6 +4169,8 @@ static int __get_type_size(struct btf *btf, u32 btf_id,
 		return sizeof(void *);
 	if (btf_type_is_int(t) || btf_type_is_enum(t))
 		return t->size;
+	if (btf_type_is_struct_arg(t))
+		return t->size;
 	*bad_type = t;
 	return -EINVAL;
 }
-- 
2.25.4