libbpf: fix __arg_ctx type enforcement for perf_event programs

Adjust PERF_EVENT type enforcement around __arg_ctx to match exactly
what kernel is doing.

Fixes: 76ec90a ("libbpf: warn on unexpected __arg_ctx type when rewriting BTF")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20240125205510.3642094-3-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/lib/bpf/libbpf.c

@@ -33,6 +33,7 @@
 #include <linux/filter.h>
 #include <linux/limits.h>
 #include <linux/perf_event.h>
+#include <linux/bpf_perf_event.h>
 #include <linux/ring_buffer.h>
 #include <sys/epoll.h>
 #include <sys/ioctl.h>
@@ -6339,6 +6340,14 @@ static struct {
 	/* all other program types don't have "named" context structs */
 };
 
+/* forward declarations for arch-specific underlying types of bpf_user_pt_regs_t typedef,
+ * for below __builtin_types_compatible_p() checks;
+ * with this approach we don't need any extra arch-specific #ifdef guards
+ */
+struct pt_regs;
+struct user_pt_regs;
+struct user_regs_struct;
+
 static bool need_func_arg_type_fixup(const struct btf *btf, const struct bpf_program *prog,
 				     const char *subprog_name, int arg_idx,
 				     int arg_type_id, const char *ctx_name)
@@ -6379,11 +6388,21 @@ static bool need_func_arg_type_fixup(const struct btf *btf, const struct bpf_pro
 	/* special cases */
 	switch (prog->type) {
 	case BPF_PROG_TYPE_KPROBE:
-	case BPF_PROG_TYPE_PERF_EVENT:
 		/* `struct pt_regs *` is expected, but we need to fix up */
 		if (btf_is_struct(t) && strcmp(tname, "pt_regs") == 0)
 			return true;
 		break;
+	case BPF_PROG_TYPE_PERF_EVENT:
+		if (__builtin_types_compatible_p(bpf_user_pt_regs_t, struct pt_regs) &&
+		    btf_is_struct(t) && strcmp(tname, "pt_regs") == 0)
+			return 0;
+		if (__builtin_types_compatible_p(bpf_user_pt_regs_t, struct user_pt_regs) &&
+		    btf_is_struct(t) && strcmp(tname, "user_pt_regs") == 0)
+			return 0;
+		if (__builtin_types_compatible_p(bpf_user_pt_regs_t, struct user_regs_struct) &&
+		    btf_is_struct(t) && strcmp(tname, "user_regs_struct") == 0)
+			return 0;
+		break;
 	case BPF_PROG_TYPE_RAW_TRACEPOINT:
 	case BPF_PROG_TYPE_RAW_TRACEPOINT_WRITABLE:
 		/* allow u64* as ctx */