io_uring_try_setup_flags: New function to probe kernel flag support

man/io_uring_try_setup_flags.3

@@ -0,0 +1,47 @@
+.\" Copyright (C) 2024 Tavian Barnes <[email protected]>
+.\"
+.\" SPDX-License-Identifier: LGPL-2.0-or-later
+.\"
+.TH io_uring_try_setup_flags 3 "December 4, 2024" "liburing-2.9" "liburing Manual"
+.SH NAME
+io_uring_try_setup_flags \- check if the kernel supports some setup flags
+.SH SYNOPSIS
+.nf
+.B #include <liburing.h>
+.PP
+.BI "int io_uring_try_setup_flags(struct io_uring_params *" params ","
+.BI "                             unsigned " flags ");"
+.fi
+.SH DESCRIPTION
+.PP
+The
+.BR io_uring_try_setup_flags (3)
+function checks whether the running kernel supports a combination of
+.B IORING_SETUP_*
+flags. On success,
+.BR io_uring_try_setup_flags (3)
+performs
+.PP
+.nf
+.RS
+.I params\->flags |= flags;
+.RE
+.fi
+.PP
+and returns 0. On failure,
+.I params
+remains unchanged and
+.B \-errno
+is returned.
+.SH RETURN VALUE
+.BR io_uring_try_setup_flags (3)
+returns 0 on success and
+.BR -errno
+on failure.
+.SH ERRORS
+.TP
+.B EINVAL
+The flag combination is invalid.
+.SH SEE ALSO
+.BR io_uring_setup (2),
+.BR io_uring_queue_init (3)

src/include/liburing.h

@@ -158,6 +158,8 @@ IOURINGINLINE int io_uring_opcode_supported(const struct io_uring_probe *p,
 	return (p->ops[op].flags & IO_URING_OP_SUPPORTED) != 0;
 }
 
+int io_uring_try_setup_flags(struct io_uring_params *p, unsigned flags);
+
 int io_uring_queue_init_mem(unsigned entries, struct io_uring *ring,
 				struct io_uring_params *p,
 				void *buf, size_t buf_size);

src/liburing.map

@@ -110,4 +110,5 @@ LIBURING_2.9 {
 		io_uring_submit_and_wait_reg;
 		io_uring_clone_buffers_offset;
 		io_uring_register_region;
+		io_uring_try_setup_flags;
 } LIBURING_2.8;

src/setup.c

@@ -496,6 +496,23 @@ __cold void io_uring_free_probe(struct io_uring_probe *probe)
 	free(probe);
 }
 
+int io_uring_try_setup_flags(struct io_uring_params *p, unsigned flags)
+{
+	struct io_uring ring;
+	unsigned saved = p->flags;
+	int r;
+
+	p->flags |= flags;
+	r = io_uring_queue_init_params(2, &ring, p);
+	if (r == 0) {
+		io_uring_queue_exit(&ring);
+	} else {
+		p->flags = saved;
+	}
+
+	return r;
+}
+
 static size_t npages(size_t size, long page_size)
 {
 	size--;

test/Makefile

@@ -228,6 +228,7 @@ test_srcs := \
 	timeout.c \
 	timeout-new.c \
 	truncate.c \
+	try-setup-flags.c \
 	tty-write-dpoll.c \
 	unlink.c \
 	uring_cmd_ublk.c \

test/try-setup-flags.c

@@ -0,0 +1,33 @@
+/* SPDX-License-Identifier: MIT */
+/*
+ * Description: test io_uring_try_setup_flags()
+ *
+ */
+#include <stdio.h>
+
+#include "liburing.h"
+#include "helpers.h"
+
+int main(int argc, char *argv[])
+{
+	struct io_uring_params params = {0};
+	int ret;
+
+	if (argc > 1)
+		return T_EXIT_SKIP;
+
+	ret = io_uring_try_setup_flags(&params, IORING_SETUP_CLAMP);
+	if (ret != 0) {
+		fprintf(stderr, "IORING_SETUP_CLAMP failed\n");
+		return T_EXIT_FAIL;
+	}
+
+	/* should fail without IORING_SETUP_SINGLE_ISSUER */
+	ret = io_uring_try_setup_flags(&params, IORING_SETUP_DEFER_TASKRUN);
+	if (ret != -EINVAL) {
+		fprintf(stderr, "IORING_SETUP_DEFER_TASKRUN failed\n");
+		return T_EXIT_FAIL;
+	}
+
+	return T_EXIT_PASS;
+}