[PATCH v2] io_uring/zcrx: reorder fd allocation in zcrx_export()

[Bertie Tryner <bertietryner@gmail.com>]

Currently, zcrx_export() allocates a file descriptor and copies the
control structure to userspace before the backing file is created.

While the operation returns an error on failure, it is cleaner to
follow the standard kernel pattern of performing the copy_to_user()
and fd_install() only after all resource allocations (like the
anon_inode) have succeeded. This aligns the code with other
fd-publishing paths in the VFS.

Signed-off-by: Bertie Tryner <Bertie.Tryner@warwick.ac.uk>
---
 io_uring/zcrx.c | 25 ++++++++++++++-----------
 1 file changed, 14 insertions(+), 11 deletions(-)

diff --git a/io_uring/zcrx.c b/io_uring/zcrx.c
index 262ac73..700eff9 100644
--- a/io_uring/zcrx.c
+++ b/io_uring/zcrx.c
@@ -637,19 +637,10 @@ static int zcrx_export(struct io_ring_ctx *ctx, struct io_zcrx_ifq *ifq,
 {
 	struct zcrx_ctrl_export *ce = &ctrl->zc_export;
 	struct file *file;
-	int fd = -1;
+	int fd;
 
 	if (!mem_is_zero(ce, sizeof(*ce)))
 		return -EINVAL;
-	fd = get_unused_fd_flags(O_CLOEXEC);
-	if (fd < 0)
-		return fd;
-
-	ce->zcrx_fd = fd;
-	if (copy_to_user(arg, ctrl, sizeof(*ctrl))) {
-		put_unused_fd(fd);
-		return -EFAULT;
-	}
 
 	refcount_inc(&ifq->refs);
 	refcount_inc(&ifq->user_refs);
@@ -657,11 +648,23 @@ static int zcrx_export(struct io_ring_ctx *ctx, struct io_zcrx_ifq *ifq,
 	file = anon_inode_create_getfile("[zcrx]", &zcrx_box_fops,
 					 ifq, O_CLOEXEC, NULL);
 	if (IS_ERR(file)) {
-		put_unused_fd(fd);
 		zcrx_unregister(ifq);
 		return PTR_ERR(file);
 	}
 
+	fd = get_unused_fd_flags(O_CLOEXEC);
+	if (fd < 0) {
+		fput(file);
+		return fd;
+	}
+
+	ce->zcrx_fd = fd;
+	if (copy_to_user(arg, ctrl, sizeof(*ctrl))) {
+		fput(file);
+		put_unused_fd(fd);
+		return -EFAULT;
+	}
+
 	fd_install(fd, file);
 	return 0;
 }
-- 
2.39.5 (Apple Git-154)