Feature request: return the maximum opcode supported by kernel for easier feature detection about liburing HOT 10 CLOSED

diff --git a/fs/io_uring.c b/fs/io_uring.c
index ee14a0fcd59f..44a08536a76e 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -6508,6 +6508,7 @@ static int io_uring_create(unsigned entries, struct io_uring_params *p)
 
 	p->features = IORING_FEAT_SINGLE_MMAP | IORING_FEAT_NODROP |
 			IORING_FEAT_SUBMIT_STABLE | IORING_FEAT_RW_CUR_POS;
+	p->last_opcode = IORING_OP_LAST - 1;
 	trace_io_uring_create(ret, ctx, p->sq_entries, p->cq_entries, p->flags);
 	return ret;
 err:
@@ -6528,8 +6529,10 @@ static long io_uring_setup(u32 entries, struct io_uring_params __user *params)
 
 	if (copy_from_user(&p, params, sizeof(p)))
 		return -EFAULT;
-	for (i = 0; i < ARRAY_SIZE(p.resv); i++) {
-		if (p.resv[i])
+	if (p.resv1 || p.resv2)
+		return -EINVAL;
+	for (i = 0; i < ARRAY_SIZE(p.resv3); i++) {
+		if (p.resv3[i])
 			return -EINVAL;
 	}
 
diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
index fea7da182851..4f0b5cd347b6 100644
--- a/include/uapi/linux/io_uring.h
+++ b/include/uapi/linux/io_uring.h
@@ -170,7 +170,10 @@ struct io_uring_params {
 	__u32 sq_thread_cpu;
 	__u32 sq_thread_idle;
 	__u32 features;
-	__u32 resv[4];
+	__u8 last_opcode;
+	__u8 resv1;
+	__u16 resv2;
+	__u32 resv3[3];
 	struct io_sqring_offsets sq_off;
 	struct io_cqring_offsets cq_off;
 };

#define IO_URING_OP_SUPPORTED (1U << 0)
struct io_uring_op {
    __u8 opcode;
    __u8 flags; /* IO_URING_OP_* flags */
};

struct io_uring_probe {
    __u32 bla;
    __u32 foo;
    __u32 bar;
    __u32 len; /* length of array below */
    struct io_uring_op ops[0];
};

diff --git a/fs/io_uring.c b/fs/io_uring.c
index ee14a0fcd59f..c67d7500abae 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -6554,6 +6554,33 @@ SYSCALL_DEFINE2(io_uring_setup, u32, entries,
 	return io_uring_setup(entries, params);
 }
 
+static int io_probe(struct io_ring_ctx *ctx, void __user *arg, unsigned nr_args)
+{
+	struct io_uring_probe *p;
+	size_t size;
+	int i, ret;
+
+	size = struct_size(p, ops, nr_args);
+	p = kzalloc(size, GFP_KERNEL);
+	if (!p)
+		return -ENOMEM;
+
+	p->last_op = IORING_OP_LAST - 1;
+	if (nr_args) {
+		for (i = 0; i < nr_args; i++) {
+			p->ops[i].op = i;
+			p->ops[i].flags = IO_URING_OP_SUPPORTED;
+		}
+		p->ops_len = i;
+	}
+
+	ret = 0;
+	if (copy_to_user(arg, p, size))
+		ret = -EFAULT;
+	kfree(p);
+	return ret;
+}
+
 static int __io_uring_register(struct io_ring_ctx *ctx, unsigned opcode,
 			       void __user *arg, unsigned nr_args)
 	__releases(ctx->uring_lock)
@@ -6570,7 +6597,8 @@ static int __io_uring_register(struct io_ring_ctx *ctx, unsigned opcode,
 		return -ENXIO;
 
 	if (opcode != IORING_UNREGISTER_FILES &&
-	    opcode != IORING_REGISTER_FILES_UPDATE) {
+	    opcode != IORING_REGISTER_FILES_UPDATE &&
+	    opcode != IORING_REGISTER_PROBE) {
 		percpu_ref_kill(&ctx->refs);
 
 		/*
@@ -6632,6 +6660,12 @@ static int __io_uring_register(struct io_ring_ctx *ctx, unsigned opcode,
 			break;
 		ret = io_eventfd_unregister(ctx);
 		break;
+	case IORING_REGISTER_PROBE:
+		ret = -EINVAL;
+		if (!arg)
+			break;
+		ret = io_probe(ctx, arg, nr_args);
+		break;
 	default:
 		ret = -EINVAL;
 		break;
@@ -6639,7 +6673,8 @@ static int __io_uring_register(struct io_ring_ctx *ctx, unsigned opcode,
 
 
 	if (opcode != IORING_UNREGISTER_FILES &&
-	    opcode != IORING_REGISTER_FILES_UPDATE) {
+	    opcode != IORING_REGISTER_FILES_UPDATE &&
+	    opcode != IORING_REGISTER_PROBE) {
 		/* bring the ctx back to life */
 		percpu_ref_reinit(&ctx->refs);
 out:
diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
index fea7da182851..1a48d2c4664b 100644
--- a/include/uapi/linux/io_uring.h
+++ b/include/uapi/linux/io_uring.h
@@ -194,6 +194,7 @@ struct io_uring_params {
 #define IORING_UNREGISTER_EVENTFD	5
 #define IORING_REGISTER_FILES_UPDATE	6
 #define IORING_REGISTER_EVENTFD_ASYNC	7
+#define IORING_REGISTER_PROBE		8
 
 struct io_uring_files_update {
 	__u32 offset;
@@ -201,4 +202,19 @@ struct io_uring_files_update {
 	__aligned_u64 /* __s32 * */ fds;
 };
 
+#define IO_URING_OP_SUPPORTED	(1U << 0)
+
+struct io_uring_probe_op {
+	__u8 op;
+	__u8 flags;	/* IO_URING_OP_* flags */
+};
+
+struct io_uring_probe {
+	__u8 last_op;	/* last opcode supported */
+	__u8 ops_len;	/* length of ops[] array below */
+	__u16 resv;
+	__u32 resv2[4];
+	struct io_uring_probe_op ops[0];
+};
+
 #endif