[PATCH 3/6] io_uring: allow allocated fixed files for openat/openat2

[Jens Axboe <axboe@kernel.dk>]

If the application passes in IORING_FILE_INDEX_ALLOC as the file_slot,
then that's a hint to allocate a fixed file descriptor rather than have
one be passed in directly.

This can be useful for having io_uring manage the direct descriptor space.

Normal open direct requests will complete with 0 for success, and < 0
in case of error. If io_uring is asked to allocated the direct descriptor,
then the direct descriptor is returned in case of success.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 fs/io_uring.c                 | 32 +++++++++++++++++++++++++++++---
 include/uapi/linux/io_uring.h |  9 +++++++++
 2 files changed, 38 insertions(+), 3 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 8c40411a7e78..ef999d0e09de 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -4697,7 +4697,7 @@ static int io_openat2_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
 	return __io_openat_prep(req, sqe);
 }
 
-static int __maybe_unused io_file_bitmap_get(struct io_ring_ctx *ctx)
+static int io_file_bitmap_get(struct io_ring_ctx *ctx)
 {
 	struct io_file_table *table = &ctx->file_table;
 	unsigned long nr = ctx->nr_user_files;
@@ -4722,6 +4722,32 @@ static int __maybe_unused io_file_bitmap_get(struct io_ring_ctx *ctx)
 	return -ENFILE;
 }
 
+static int io_fixed_fd_install(struct io_kiocb *req, unsigned int issue_flags,
+			       struct file *file, unsigned int file_slot)
+{
+	int alloc_slot = file_slot == IORING_FILE_INDEX_ALLOC;
+	struct io_ring_ctx *ctx = req->ctx;
+	int ret;
+
+	if (alloc_slot) {
+		io_ring_submit_lock(ctx, issue_flags);
+		file_slot = io_file_bitmap_get(ctx);
+		if (unlikely(file_slot < 0)) {
+			io_ring_submit_unlock(ctx, issue_flags);
+			return file_slot;
+		}
+	}
+
+	ret = io_install_fixed_file(req, file, issue_flags, file_slot);
+	if (alloc_slot) {
+		io_ring_submit_unlock(ctx, issue_flags);
+		if (!ret)
+			return file_slot;
+	}
+
+	return ret;
+}
+
 static int io_openat2(struct io_kiocb *req, unsigned int issue_flags)
 {
 	struct open_flags op;
@@ -4777,8 +4803,8 @@ static int io_openat2(struct io_kiocb *req, unsigned int issue_flags)
 	if (!fixed)
 		fd_install(ret, file);
 	else
-		ret = io_install_fixed_file(req, file, issue_flags,
-					    req->open.file_slot - 1);
+		ret = io_fixed_fd_install(req, issue_flags, file,
+						req->open.file_slot);
 err:
 	putname(req->open.filename);
 	req->flags &= ~REQ_F_NEED_CLEANUP;
diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
index 06621a278cb6..b7f02a55032a 100644
--- a/include/uapi/linux/io_uring.h
+++ b/include/uapi/linux/io_uring.h
@@ -63,6 +63,15 @@ struct io_uring_sqe {
 	__u64	__pad2[2];
 };
 
+/*
+ * If sqe->file_index is set to this for opcodes that instantiate a new
+ * direct descriptor (like openat/openat2/accept), then io_uring will allocate
+ * an available direct descriptor instead of having the application pass one
+ * in. The picked direct descriptor will be returned in cqe->res, or -ENFILE
+ * if the space is full.
+ */
+#define IORING_FILE_INDEX_ALLOC		(~0U)
+
 enum {
 	IOSQE_FIXED_FILE_BIT,
 	IOSQE_IO_DRAIN_BIT,
-- 
2.35.1