io_uring: add file set registration

We normally have to fget/fput for each IO we do on a file. Even with
the batching we do, the cost of the atomic inc/dec of the file usage
count adds up.

This adds IORING_REGISTER_FILES, and IORING_UNREGISTER_FILES opcodes
for the io_uring_register(2) system call. The arguments passed in must
be an array of __s32 holding file descriptors, and nr_args should hold
the number of file descriptors the application wishes to pin for the
duration of the io_uring instance (or until IORING_UNREGISTER_FILES is
called).

When used, the application must set IOSQE_FIXED_FILE in the sqe->flags
member. Then, instead of setting sqe->fd to the real fd, it sets sqe->fd
to the index in the array passed in to IORING_REGISTER_FILES.

Files are automatically unregistered when the io_uring instance is torn
down. An application need only unregister if it wishes to register a new
set of fds.

Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -49,6 +49,7 @@
 #include <linux/net.h>
 #include <net/sock.h>
 #include <net/af_unix.h>
+#include <net/scm.h>
 #include <linux/anon_inodes.h>
 #include <linux/sched/mm.h>
 #include <linux/uaccess.h>
@@ -61,6 +62,7 @@
 #include "internal.h"
 
 #define IORING_MAX_ENTRIES	4096
+#define IORING_MAX_FIXED_FILES	1024
 
 struct io_uring {
 	u32 head ____cacheline_aligned_in_smp;
@@ -123,6 +125,14 @@ struct io_ring_ctx {
 		struct fasync_struct	*cq_fasync;
 	} ____cacheline_aligned_in_smp;
 
+	/*
+	 * If used, fixed file set. Writers must ensure that ->refs is dead,
+	 * readers must ensure that ->refs is alive as long as the file* is
+	 * used. Only updated through io_uring_register(2).
+	 */
+	struct file		**user_files;
+	unsigned		nr_user_files;
+
 	/* if used, fixed mapped user buffers */
 	unsigned		nr_user_bufs;
 	struct io_mapped_ubuf	*user_bufs;
@@ -170,6 +180,7 @@ struct io_kiocb {
 	unsigned int		flags;
 #define REQ_F_FORCE_NONBLOCK	1	/* inline submission attempt */
 #define REQ_F_IOPOLL_COMPLETED	2	/* polled IO has completed */
+#define REQ_F_FIXED_FILE	4	/* ctx owns file */
 	u64			user_data;
 	u64			error;
 
@@ -404,6 +415,7 @@ static void io_iopoll_complete(struct io_ring_ctx *ctx, unsigned int *nr_events,
 		 * Batched puts of the same file, to avoid dirtying the
 		 * file usage count multiple times, if avoidable.
 		 */
+		if (!(req->flags & REQ_F_FIXED_FILE)) {
 			if (!file) {
 				file = req->rw.ki_filp;
 				file_count = 1;
@@ -414,6 +426,7 @@ static void io_iopoll_complete(struct io_ring_ctx *ctx, unsigned int *nr_events,
 				file = req->rw.ki_filp;
 				file_count = 1;
 			}
+		}
 
 		if (to_free == ARRAY_SIZE(reqs))
 			io_free_req_many(ctx, reqs, &to_free);
@@ -544,13 +557,19 @@ static void kiocb_end_write(struct kiocb *kiocb)
 	}
 }
 
+static void io_fput(struct io_kiocb *req)
+{
+	if (!(req->flags & REQ_F_FIXED_FILE))
+		fput(req->rw.ki_filp);
+}
+
 static void io_complete_rw(struct kiocb *kiocb, long res, long res2)
 {
 	struct io_kiocb *req = container_of(kiocb, struct io_kiocb, rw);
 
 	kiocb_end_write(kiocb);
 
-	fput(kiocb->ki_filp);
+	io_fput(req);
 	io_cqring_add_event(req->ctx, req->user_data, res, 0);
 	io_free_req(req);
 }
@@ -666,19 +685,29 @@ static int io_prep_rw(struct io_kiocb *req, const struct io_uring_sqe *sqe,
 {
 	struct io_ring_ctx *ctx = req->ctx;
 	struct kiocb *kiocb = &req->rw;
-	unsigned ioprio;
+	unsigned ioprio, flags;
 	int fd, ret;
 
 	/* For -EAGAIN retry, everything is already prepped */
 	if (kiocb->ki_filp)
 		return 0;
 
+	flags = READ_ONCE(sqe->flags);
 	fd = READ_ONCE(sqe->fd);
+
+	if (flags & IOSQE_FIXED_FILE) {
+		if (unlikely(!ctx->user_files ||
+		    (unsigned) fd >= ctx->nr_user_files))
+			return -EBADF;
+		kiocb->ki_filp = ctx->user_files[fd];
+		req->flags |= REQ_F_FIXED_FILE;
+	} else {
 		kiocb->ki_filp = io_file_get(state, fd);
 		if (unlikely(!kiocb->ki_filp))
 			return -EBADF;
 		if (force_nonblock && !io_file_supports_async(kiocb->ki_filp))
 			force_nonblock = false;
+	}
 	kiocb->ki_pos = READ_ONCE(sqe->off);
 	kiocb->ki_flags = iocb_flags(kiocb->ki_filp);
 	kiocb->ki_hint = ki_hint_validate(file_write_hint(kiocb->ki_filp));
@@ -718,10 +747,14 @@ static int io_prep_rw(struct io_kiocb *req, const struct io_uring_sqe *sqe,
 	}
 	return 0;
 out_fput:
-	/* in case of error, we didn't use this file reference. drop it. */
+	if (!(flags & IOSQE_FIXED_FILE)) {
+		/*
+		 * in case of error, we didn't use this file reference. drop it.
+		 */
 		if (state)
 			state->used_refs--;
 		io_file_put(state, kiocb->ki_filp);
+	}
 	return ret;
 }
 
@@ -863,7 +896,7 @@ static ssize_t io_read(struct io_kiocb *req, const struct sqe_submit *s,
 out_fput:
 	/* Hold on to the file for -EAGAIN */
 	if (unlikely(ret && ret != -EAGAIN))
-		fput(file);
+		io_fput(req);
 	return ret;
 }
 
@@ -917,7 +950,7 @@ static ssize_t io_write(struct io_kiocb *req, const struct sqe_submit *s,
 	kfree(iovec);
 out_fput:
 	if (unlikely(ret))
-		fput(file);
+		io_fput(req);
 	return ret;
 }
 
@@ -940,7 +973,7 @@ static int io_nop(struct io_kiocb *req, u64 user_data)
 	 */
 	if (req->rw.ki_filp) {
 		err = -EBADF;
-		fput(req->rw.ki_filp);
+		io_fput(req);
 	}
 	io_cqring_add_event(ctx, user_data, err, 0);
 	io_free_req(req);
@@ -949,21 +982,32 @@ static int io_nop(struct io_kiocb *req, u64 user_data)
 
 static int io_prep_fsync(struct io_kiocb *req, const struct io_uring_sqe *sqe)
 {
+	struct io_ring_ctx *ctx = req->ctx;
+	unsigned flags;
 	int fd;
 
 	/* Prep already done */
 	if (req->rw.ki_filp)
 		return 0;
 
-	if (unlikely(req->ctx->flags & IORING_SETUP_IOPOLL))
+	if (unlikely(ctx->flags & IORING_SETUP_IOPOLL))
 		return -EINVAL;
 	if (unlikely(sqe->addr || sqe->ioprio || sqe->buf_index))
 		return -EINVAL;
 
 	fd = READ_ONCE(sqe->fd);
+	flags = READ_ONCE(sqe->flags);
+
+	if (flags & IOSQE_FIXED_FILE) {
+		if (unlikely(!ctx->user_files || fd >= ctx->nr_user_files))
+			return -EBADF;
+		req->rw.ki_filp = ctx->user_files[fd];
+		req->flags |= REQ_F_FIXED_FILE;
+	} else {
 		req->rw.ki_filp = fget(fd);
 		if (unlikely(!req->rw.ki_filp))
 			return -EBADF;
+	}
 
 	return 0;
 }
@@ -993,7 +1037,7 @@ static int io_fsync(struct io_kiocb *req, const struct io_uring_sqe *sqe,
 				end > 0 ? end : LLONG_MAX,
 				fsync_flags & IORING_FSYNC_DATASYNC);
 
-	fput(req->rw.ki_filp);
+	io_fput(req);
 	io_cqring_add_event(req->ctx, sqe->user_data, ret, 0);
 	io_free_req(req);
 	return 0;
@@ -1132,7 +1176,7 @@ static int io_submit_sqe(struct io_ring_ctx *ctx, struct sqe_submit *s,
 	ssize_t ret;
 
 	/* enforce forwards compatibility on users */
-	if (unlikely(s->sqe->flags))
+	if (unlikely(s->sqe->flags & ~IOSQE_FIXED_FILE))
 		return -EINVAL;
 
 	req = io_get_req(ctx, state);
@@ -1344,6 +1388,201 @@ static int io_cqring_wait(struct io_ring_ctx *ctx, int min_events,
 	return READ_ONCE(ring->r.head) == READ_ONCE(ring->r.tail) ? ret : 0;
 }
 
+static void __io_sqe_files_unregister(struct io_ring_ctx *ctx)
+{
+#if defined(CONFIG_UNIX)
+	if (ctx->ring_sock) {
+		struct sock *sock = ctx->ring_sock->sk;
+		struct sk_buff *skb;
+
+		while ((skb = skb_dequeue(&sock->sk_receive_queue)) != NULL)
+			kfree_skb(skb);
+	}
+#else
+	int i;
+
+	for (i = 0; i < ctx->nr_user_files; i++)
+		fput(ctx->user_files[i]);
+#endif
+}
+
+static int io_sqe_files_unregister(struct io_ring_ctx *ctx)
+{
+	if (!ctx->user_files)
+		return -ENXIO;
+
+	__io_sqe_files_unregister(ctx);
+	kfree(ctx->user_files);
+	ctx->user_files = NULL;
+	ctx->nr_user_files = 0;
+	return 0;
+}
+
+static void io_finish_async(struct io_ring_ctx *ctx)
+{
+	if (ctx->sqo_wq) {
+		destroy_workqueue(ctx->sqo_wq);
+		ctx->sqo_wq = NULL;
+	}
+}
+
+#if defined(CONFIG_UNIX)
+static void io_destruct_skb(struct sk_buff *skb)
+{
+	struct io_ring_ctx *ctx = skb->sk->sk_user_data;
+
+	io_finish_async(ctx);
+	unix_destruct_scm(skb);
+}
+
+/*
+ * Ensure the UNIX gc is aware of our file set, so we are certain that
+ * the io_uring can be safely unregistered on process exit, even if we have
+ * loops in the file referencing.
+ */
+static int __io_sqe_files_scm(struct io_ring_ctx *ctx, int nr, int offset)
+{
+	struct sock *sk = ctx->ring_sock->sk;
+	struct scm_fp_list *fpl;
+	struct sk_buff *skb;
+	int i;
+
+	if (!capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN)) {
+		unsigned long inflight = ctx->user->unix_inflight + nr;
+
+		if (inflight > task_rlimit(current, RLIMIT_NOFILE))
+			return -EMFILE;
+	}
+
+	fpl = kzalloc(sizeof(*fpl), GFP_KERNEL);
+	if (!fpl)
+		return -ENOMEM;
+
+	skb = alloc_skb(0, GFP_KERNEL);
+	if (!skb) {
+		kfree(fpl);
+		return -ENOMEM;
+	}
+
+	skb->sk = sk;
+	skb->destructor = io_destruct_skb;
+
+	fpl->user = get_uid(ctx->user);
+	for (i = 0; i < nr; i++) {
+		fpl->fp[i] = get_file(ctx->user_files[i + offset]);
+		unix_inflight(fpl->user, fpl->fp[i]);
+	}
+
+	fpl->max = fpl->count = nr;
+	UNIXCB(skb).fp = fpl;
+	refcount_add(skb->truesize, &sk->sk_wmem_alloc);
+	skb_queue_head(&sk->sk_receive_queue, skb);
+
+	for (i = 0; i < nr; i++)
+		fput(fpl->fp[i]);
+
+	return 0;
+}
+
+/*
+ * If UNIX sockets are enabled, fd passing can cause a reference cycle which
+ * causes regular reference counting to break down. We rely on the UNIX
+ * garbage collection to take care of this problem for us.
+ */
+static int io_sqe_files_scm(struct io_ring_ctx *ctx)
+{
+	unsigned left, total;
+	int ret = 0;
+
+	total = 0;
+	left = ctx->nr_user_files;
+	while (left) {
+		unsigned this_files = min_t(unsigned, left, SCM_MAX_FD);
+		int ret;
+
+		ret = __io_sqe_files_scm(ctx, this_files, total);
+		if (ret)
+			break;
+		left -= this_files;
+		total += this_files;
+	}
+
+	if (!ret)
+		return 0;
+
+	while (total < ctx->nr_user_files) {
+		fput(ctx->user_files[total]);
+		total++;
+	}
+
+	return ret;
+}
+#else
+static int io_sqe_files_scm(struct io_ring_ctx *ctx)
+{
+	return 0;
+}
+#endif
+
+static int io_sqe_files_register(struct io_ring_ctx *ctx, void __user *arg,
+				 unsigned nr_args)
+{
+	__s32 __user *fds = (__s32 __user *) arg;
+	int fd, ret = 0;
+	unsigned i;
+
+	if (ctx->user_files)
+		return -EBUSY;
+	if (!nr_args)
+		return -EINVAL;
+	if (nr_args > IORING_MAX_FIXED_FILES)
+		return -EMFILE;
+
+	ctx->user_files = kcalloc(nr_args, sizeof(struct file *), GFP_KERNEL);
+	if (!ctx->user_files)
+		return -ENOMEM;
+
+	for (i = 0; i < nr_args; i++) {
+		ret = -EFAULT;
+		if (copy_from_user(&fd, &fds[i], sizeof(fd)))
+			break;
+
+		ctx->user_files[i] = fget(fd);
+
+		ret = -EBADF;
+		if (!ctx->user_files[i])
+			break;
+		/*
+		 * Don't allow io_uring instances to be registered. If UNIX
+		 * isn't enabled, then this causes a reference cycle and this
+		 * instance can never get freed. If UNIX is enabled we'll
+		 * handle it just fine, but there's still no point in allowing
+		 * a ring fd as it doesn't support regular read/write anyway.
+		 */
+		if (ctx->user_files[i]->f_op == &io_uring_fops) {
+			fput(ctx->user_files[i]);
+			break;
+		}
+		ctx->nr_user_files++;
+		ret = 0;
+	}
+
+	if (ret) {
+		for (i = 0; i < ctx->nr_user_files; i++)
+			fput(ctx->user_files[i]);
+
+		kfree(ctx->user_files);
+		ctx->nr_user_files = 0;
+		return ret;
+	}
+
+	ret = io_sqe_files_scm(ctx);
+	if (ret)
+		io_sqe_files_unregister(ctx);
+
+	return ret;
+}
+
 static int io_sq_offload_start(struct io_ring_ctx *ctx)
 {
 	int ret;
@@ -1612,13 +1851,13 @@ static int io_sqe_buffer_register(struct io_ring_ctx *ctx, void __user *arg,
 
 static void io_ring_ctx_free(struct io_ring_ctx *ctx)
 {
-	if (ctx->sqo_wq)
+	io_finish_async(ctx);
-		destroy_workqueue(ctx->sqo_wq);
 	if (ctx->sqo_mm)
 		mmdrop(ctx->sqo_mm);
 
 	io_iopoll_reap_events(ctx);
 	io_sqe_buffer_unregister(ctx);
+	io_sqe_files_unregister(ctx);
 
 #if defined(CONFIG_UNIX)
 	if (ctx->ring_sock)
@@ -1858,6 +2097,7 @@ static int io_uring_get_fd(struct io_ring_ctx *ctx)
 
 #if defined(CONFIG_UNIX)
 	ctx->ring_sock->file = file;
+	ctx->ring_sock->sk->sk_user_data = ctx;
 #endif
 	fd_install(ret, file);
 	return ret;
@@ -2001,6 +2241,15 @@ static int __io_uring_register(struct io_ring_ctx *ctx, unsigned opcode,
 			break;
 		ret = io_sqe_buffer_unregister(ctx);
 		break;
+	case IORING_REGISTER_FILES:
+		ret = io_sqe_files_register(ctx, arg, nr_args);
+		break;
+	case IORING_UNREGISTER_FILES:
+		ret = -EINVAL;
+		if (arg || nr_args)
+			break;
+		ret = io_sqe_files_unregister(ctx);
+		break;
 	default:
 		ret = -EINVAL;
 		break;

include/uapi/linux/io_uring.h

@@ -16,7 +16,7 @@
  */
 struct io_uring_sqe {
 	__u8	opcode;		/* type of operation for this sqe */
-	__u8	flags;		/* as of now unused */
+	__u8	flags;		/* IOSQE_ flags */
 	__u16	ioprio;		/* ioprio for the request */
 	__s32	fd;		/* file descriptor to do IO on */
 	__u64	off;		/* offset into file */
@@ -33,6 +33,11 @@ struct io_uring_sqe {
 	};
 };
 
+/*
+ * sqe->flags
+ */
+#define IOSQE_FIXED_FILE	(1U << 0)	/* use fixed fileset */
+
 /*
  * io_uring_setup() flags
  */
@@ -113,5 +118,7 @@ struct io_uring_params {
  */
 #define IORING_REGISTER_BUFFERS		0
 #define IORING_UNREGISTER_BUFFERS	1
+#define IORING_REGISTER_FILES		2
+#define IORING_UNREGISTER_FILES		3
 
 #endif