bpf: Fix global subprog context argument resolution logic

KPROBE program's user-facing context type is defined as typedef
bpf_user_pt_regs_t. This leads to a problem when trying to passing
kprobe/uprobe/usdt context argument into global subprog, as kernel
always strip away mods and typedefs of user-supplied type, but takes
expected type from bpf_ctx_convert as is, which causes mismatch.

Current way to work around this is to define a fake struct with the same
name as expected typedef:

struct bpf_user_pt_regs_t {};

__noinline my_global_subprog(struct bpf_user_pt_regs_t *ctx) { ... }

This patch fixes the issue by resolving expected type, if it's not
a struct. It still leaves the above work-around working for backwards
compatibility.

Fixes: 91cc1a99740e ("bpf: Annotate context types")
Signed-off-by: Andrii Nakryiko <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Acked-by: Stanislav Fomichev <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

kernel/bpf/btf.c

···
 	if (!ctx_struct)
 		/* should not happen */
 		return NULL;
+again:
 	ctx_tname = btf_name_by_offset(btf_vmlinux, ctx_struct->name_off);
 	if (!ctx_tname) {
 		/* should not happen */
···
 	 * int socket_filter_bpf_prog(struct __sk_buff *skb)
 	 * { // no fields of skb are ever used }
 	 */
-	if (strcmp(ctx_tname, tname))
-		return NULL;
+	if (strcmp(ctx_tname, tname)) {
+		/* bpf_user_pt_regs_t is a typedef, so resolve it to
+		 * underlying struct and check name again
+		 */
+		if (!btf_type_is_modifier(ctx_struct))
+			return NULL;
+		while (btf_type_is_modifier(ctx_struct))
+			ctx_struct = btf_type_by_id(btf_vmlinux, ctx_struct->type);
+		goto again;
+	}
 	return ctx_type;
 }