[PATCHSET v3] Add direct descriptor ring passing

[PATCHSET v3] Add direct descriptor ring passing

[Jens Axboe]

Hi,

Hi,

One of the things we currently cannot do with direct descriptors is pass
it to another application or ring. This adds support for doing so, through
the IORING_OP_MSG_RING ring-to-ring messaging opcode.

Changes since v2:
- Add flag for controlling whether to post a CQE to the target ring or
  not.

-- 
Jens Axboe

[PATCH 1/2] io_uring: split out fixed file installation and removal

[Jens Axboe]

Put it with the filetable code, which is where it belongs. While doing
so, have the helpers take a ctx rather than an io_kiocb. It doesn't make
sense to use a request, as it's not an operation on the request itself.
It applies to the ring itself.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 io_uring/filetable.c | 72 +++++++++++++++++++++++++++++++++-----------
 io_uring/filetable.h |  3 ++
 io_uring/openclose.c | 35 +++------------------
 io_uring/openclose.h |  2 +-
 io_uring/rsrc.c      |  2 +-
 5 files changed, 63 insertions(+), 51 deletions(-)

diff --git a/io_uring/filetable.c b/io_uring/filetable.c
index 534e1a3c625d..abaa5ba7f655 100644
--- a/io_uring/filetable.c
+++ b/io_uring/filetable.c
@@ -58,11 +58,10 @@ void io_free_file_tables(struct io_file_table *table)
 	table->bitmap = NULL;
 }
 
-static int io_install_fixed_file(struct io_kiocb *req, struct file *file,
-				 unsigned int issue_flags, u32 slot_index)
+static int io_install_fixed_file(struct io_ring_ctx *ctx, struct file *file,
+				 u32 slot_index)
 	__must_hold(&req->ctx->uring_lock)
 {
-	struct io_ring_ctx *ctx = req->ctx;
 	bool needs_switch = false;
 	struct io_fixed_file *file_slot;
 	int ret;
@@ -108,6 +107,26 @@ static int io_install_fixed_file(struct io_kiocb *req, struct file *file,
 	return ret;
 }
 
+int __io_fixed_fd_install(struct io_ring_ctx *ctx, struct file *file,
+			  unsigned int file_slot)
+{
+	bool alloc_slot = file_slot == IORING_FILE_INDEX_ALLOC;
+	int ret;
+
+	if (alloc_slot) {
+		ret = io_file_bitmap_get(ctx);
+		if (unlikely(ret < 0))
+			return ret;
+		file_slot = ret;
+	} else {
+		file_slot--;
+	}
+
+	ret = io_install_fixed_file(ctx, file, file_slot);
+	if (!ret && alloc_slot)
+		ret = file_slot;
+	return ret;
+}
 /*
  * Note when io_fixed_fd_install() returns error value, it will ensure
  * fput() is called correspondingly.
@@ -115,27 +134,44 @@ static int io_install_fixed_file(struct io_kiocb *req, struct file *file,
 int io_fixed_fd_install(struct io_kiocb *req, unsigned int issue_flags,
 			struct file *file, unsigned int file_slot)
 {
-	bool alloc_slot = file_slot == IORING_FILE_INDEX_ALLOC;
 	struct io_ring_ctx *ctx = req->ctx;
 	int ret;
 
 	io_ring_submit_lock(ctx, issue_flags);
-
-	if (alloc_slot) {
-		ret = io_file_bitmap_get(ctx);
-		if (unlikely(ret < 0))
-			goto err;
-		file_slot = ret;
-	} else {
-		file_slot--;
-	}
-
-	ret = io_install_fixed_file(req, file, issue_flags, file_slot);
-	if (!ret && alloc_slot)
-		ret = file_slot;
-err:
+	ret = __io_fixed_fd_install(ctx, file, file_slot);
 	io_ring_submit_unlock(ctx, issue_flags);
+
 	if (unlikely(ret < 0))
 		fput(file);
 	return ret;
 }
+
+int io_fixed_fd_remove(struct io_ring_ctx *ctx, unsigned int offset)
+{
+	struct io_fixed_file *file_slot;
+	struct file *file;
+	int ret;
+
+	if (unlikely(!ctx->file_data))
+		return -ENXIO;
+	if (offset >= ctx->nr_user_files)
+		return -EINVAL;
+	ret = io_rsrc_node_switch_start(ctx);
+	if (ret)
+		return ret;
+
+	offset = array_index_nospec(offset, ctx->nr_user_files);
+	file_slot = io_fixed_file_slot(&ctx->file_table, offset);
+	if (!file_slot->file_ptr)
+		return -EBADF;
+
+	file = (struct file *)(file_slot->file_ptr & FFS_MASK);
+	ret = io_queue_rsrc_removal(ctx->file_data, offset, ctx->rsrc_node, file);
+	if (ret)
+		return ret;
+
+	file_slot->file_ptr = 0;
+	io_file_bitmap_clear(&ctx->file_table, offset);
+	io_rsrc_node_switch(ctx, ctx->file_data);
+	return 0;
+}
diff --git a/io_uring/filetable.h b/io_uring/filetable.h
index fb5a274c08ff..79eb50c1980e 100644
--- a/io_uring/filetable.h
+++ b/io_uring/filetable.h
@@ -29,6 +29,9 @@ void io_free_file_tables(struct io_file_table *table);
 
 int io_fixed_fd_install(struct io_kiocb *req, unsigned int issue_flags,
 			struct file *file, unsigned int file_slot);
+int __io_fixed_fd_install(struct io_ring_ctx *ctx, struct file *file,
+				unsigned int file_slot);
+int io_fixed_fd_remove(struct io_ring_ctx *ctx, unsigned int offset);
 
 unsigned int io_file_get_flags(struct file *file);
 
diff --git a/io_uring/openclose.c b/io_uring/openclose.c
index 099a5ec84dfd..d1818ec9169b 100644
--- a/io_uring/openclose.c
+++ b/io_uring/openclose.c
@@ -173,42 +173,15 @@ void io_open_cleanup(struct io_kiocb *req)
 		putname(open->filename);
 }
 
-int __io_close_fixed(struct io_kiocb *req, unsigned int issue_flags,
+int __io_close_fixed(struct io_ring_ctx *ctx, unsigned int issue_flags,
 		     unsigned int offset)
 {
-	struct io_ring_ctx *ctx = req->ctx;
-	struct io_fixed_file *file_slot;
-	struct file *file;
 	int ret;
 
 	io_ring_submit_lock(ctx, issue_flags);
-	ret = -ENXIO;
-	if (unlikely(!ctx->file_data))
-		goto out;
-	ret = -EINVAL;
-	if (offset >= ctx->nr_user_files)
-		goto out;
-	ret = io_rsrc_node_switch_start(ctx);
-	if (ret)
-		goto out;
-
-	offset = array_index_nospec(offset, ctx->nr_user_files);
-	file_slot = io_fixed_file_slot(&ctx->file_table, offset);
-	ret = -EBADF;
-	if (!file_slot->file_ptr)
-		goto out;
-
-	file = (struct file *)(file_slot->file_ptr & FFS_MASK);
-	ret = io_queue_rsrc_removal(ctx->file_data, offset, ctx->rsrc_node, file);
-	if (ret)
-		goto out;
-
-	file_slot->file_ptr = 0;
-	io_file_bitmap_clear(&ctx->file_table, offset);
-	io_rsrc_node_switch(ctx, ctx->file_data);
-	ret = 0;
-out:
+	ret = io_fixed_fd_remove(ctx, offset);
 	io_ring_submit_unlock(ctx, issue_flags);
+
 	return ret;
 }
 
@@ -216,7 +189,7 @@ static inline int io_close_fixed(struct io_kiocb *req, unsigned int issue_flags)
 {
 	struct io_close *close = io_kiocb_to_cmd(req);
 
-	return __io_close_fixed(req, issue_flags, close->file_slot - 1);
+	return __io_close_fixed(req->ctx, issue_flags, close->file_slot - 1);
 }
 
 int io_close_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
diff --git a/io_uring/openclose.h b/io_uring/openclose.h
index 9f578f3fad87..4b1c28d3a66c 100644
--- a/io_uring/openclose.h
+++ b/io_uring/openclose.h
@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: GPL-2.0
 
-int __io_close_fixed(struct io_kiocb *req, unsigned int issue_flags,
+int __io_close_fixed(struct io_ring_ctx *ctx, unsigned int issue_flags,
 		     unsigned int offset);
 
 int io_openat_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe);
diff --git a/io_uring/rsrc.c b/io_uring/rsrc.c
index 3a2a5ef263f0..c49217f9cfc6 100644
--- a/io_uring/rsrc.c
+++ b/io_uring/rsrc.c
@@ -700,7 +700,7 @@ static int io_files_update_with_index_alloc(struct io_kiocb *req,
 		if (ret < 0)
 			break;
 		if (copy_to_user(&fds[done], &ret, sizeof(ret))) {
-			__io_close_fixed(req, issue_flags, ret);
+			__io_close_fixed(req->ctx, issue_flags, ret);
 			ret = -EFAULT;
 			break;
 		}
-- 
2.35.1

[PATCH 2/2] io_uring: add support for passing fixed file descriptors

[Jens Axboe]

With IORING_OP_MSG_RING, one ring can send a message to another ring.
Extend that support to also allow sending a fixed file descriptor to
that ring, enabling one ring to pass a registered descriptor to another
one.

Arguments are extended to pass in:

sqe->addr3	fixed file slot in source ring
sqe->file_index	fixed file slot in destination ring

IORING_OP_MSG_RING is extended to take a command argument in sqe->addr.
If set to zero (or IORING_MSG_DATA), it sends just a message like before.
If set to IORING_MSG_SEND_FD, a fixed file descriptor is sent according
to the above arguments.

Two common use cases for this are:

1) Server needs to be shutdown or restarted, pass file descriptors to
   another onei

2) Backend is split, and one accepts connections, while others then get
  the fd passed and handle the actual connection.

Both of those are classic SCM_RIGHTS use cases, and it's not possible to
support them with direct descriptors today.

By default, this will post a CQE to the target ring, similarly to how
IORING_MSG_DATA does it. If IORING_MSG_RING_CQE_SKIP is set, no message
is posted to the target ring. The issuer is expected to notify the
receiver side separately.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 include/uapi/linux/io_uring.h |  17 +++++
 io_uring/msg_ring.c           | 130 ++++++++++++++++++++++++++++++++--
 2 files changed, 140 insertions(+), 7 deletions(-)

diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
index 8715f0942ec2..15e54e633ee2 100644
--- a/include/uapi/linux/io_uring.h
+++ b/include/uapi/linux/io_uring.h
@@ -47,6 +47,7 @@ struct io_uring_sqe {
 		__u32		unlink_flags;
 		__u32		hardlink_flags;
 		__u32		xattr_flags;
+		__u32		msg_ring_flags;
 	};
 	__u64	user_data;	/* data to be passed back at completion time */
 	/* pack this to avoid bogus arm OABI complaints */
@@ -264,6 +265,22 @@ enum io_uring_op {
  */
 #define IORING_ACCEPT_MULTISHOT	(1U << 0)
 
+/*
+ * IORING_OP_MSG_RING command types, stored in sqe->addr
+ */
+enum {
+	IORING_MSG_DATA,	/* pass sqe->len as 'res' and off as user_data */
+	IORING_MSG_SEND_FD,	/* send a registered fd to another ring */
+};
+
+/*
+ * IORING_OP_MSG_RING flags (sqe->msg_ring_flags)
+ *
+ * IORING_MSG_RING_CQE_SKIP	Don't post a CQE to the target ring. Not
+ *				applicable for IORING_MSG_DATA, obviously.
+ */
+#define IORING_MSG_RING_CQE_SKIP	(1U << 0)
+
 /*
  * IO completion data structure (Completion Queue Entry)
  */
diff --git a/io_uring/msg_ring.c b/io_uring/msg_ring.c
index b02be2349652..939205b30c8b 100644
--- a/io_uring/msg_ring.c
+++ b/io_uring/msg_ring.c
@@ -3,46 +3,162 @@
 #include <linux/errno.h>
 #include <linux/file.h>
 #include <linux/slab.h>
+#include <linux/nospec.h>
 #include <linux/io_uring.h>
 
 #include <uapi/linux/io_uring.h>
 
 #include "io_uring.h"
+#include "rsrc.h"
+#include "filetable.h"
 #include "msg_ring.h"
 
 struct io_msg {
 	struct file			*file;
 	u64 user_data;
 	u32 len;
+	u32 cmd;
+	u32 src_fd;
+	u32 dst_fd;
+	u32 flags;
 };
 
+static int io_msg_ring_data(struct io_kiocb *req)
+{
+	struct io_ring_ctx *target_ctx = req->file->private_data;
+	struct io_msg *msg = io_kiocb_to_cmd(req);
+
+	if (msg->src_fd || msg->dst_fd || msg->flags)
+		return -EINVAL;
+
+	if (io_post_aux_cqe(target_ctx, msg->user_data, msg->len, 0))
+		return 0;
+
+	return -EOVERFLOW;
+}
+
+static void io_double_unlock_ctx(struct io_ring_ctx *ctx,
+				 struct io_ring_ctx *octx,
+				 unsigned int issue_flags)
+{
+	if (issue_flags & IO_URING_F_UNLOCKED)
+		mutex_unlock(&ctx->uring_lock);
+	mutex_unlock(&octx->uring_lock);
+}
+
+static int io_double_lock_ctx(struct io_ring_ctx *ctx,
+			      struct io_ring_ctx *octx,
+			      unsigned int issue_flags)
+{
+	/*
+	 * To ensure proper ordering between the two ctxs, we can only
+	 * attempt a trylock on the target. If that fails and we already have
+	 * the source ctx lock, punt to io-wq.
+	 */
+	if (!(issue_flags & IO_URING_F_UNLOCKED)) {
+		if (!mutex_trylock(&octx->uring_lock))
+			return -EAGAIN;
+		return 0;
+	}
+
+	/* Always grab smallest value ctx first. We know ctx != octx. */
+	if (ctx < octx) {
+		mutex_lock(&ctx->uring_lock);
+		mutex_lock(&octx->uring_lock);
+	} else {
+		mutex_lock(&octx->uring_lock);
+		mutex_lock(&ctx->uring_lock);
+	}
+
+	return 0;
+}
+
+static int io_msg_send_fd(struct io_kiocb *req, unsigned int issue_flags)
+{
+	struct io_ring_ctx *target_ctx = req->file->private_data;
+	struct io_msg *msg = io_kiocb_to_cmd(req);
+	struct io_ring_ctx *ctx = req->ctx;
+	unsigned long file_ptr;
+	struct file *src_file;
+	int ret;
+
+	if (target_ctx == ctx)
+		return -EINVAL;
+
+	ret = io_double_lock_ctx(ctx, target_ctx, issue_flags);
+	if (unlikely(ret))
+		return ret;
+
+	ret = -EBADF;
+	if (unlikely(msg->src_fd >= ctx->nr_user_files))
+		goto out_unlock;
+
+	msg->src_fd = array_index_nospec(msg->src_fd, ctx->nr_user_files);
+	file_ptr = io_fixed_file_slot(&ctx->file_table, msg->src_fd)->file_ptr;
+	src_file = (struct file *) (file_ptr & FFS_MASK);
+	get_file(src_file);
+
+	ret = __io_fixed_fd_install(target_ctx, src_file, msg->dst_fd);
+	if (ret < 0) {
+		fput(src_file);
+		goto out_unlock;
+	}
+
+	if (msg->flags & IORING_MSG_RING_CQE_SKIP)
+		goto out_unlock;
+
+	/*
+	 * If this fails, the target still received the file descriptor but
+	 * wasn't notified of the fact. This means that if this request
+	 * completes with -EOVERFLOW, then the sender must ensure that a
+	 * later IORING_OP_MSG_RING delivers the message.
+	 */
+	if (!io_post_aux_cqe(target_ctx, msg->user_data, msg->len, 0))
+		ret = -EOVERFLOW;
+out_unlock:
+	io_double_unlock_ctx(ctx, target_ctx, issue_flags);
+	return ret;
+}
+
 int io_msg_ring_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
 {
 	struct io_msg *msg = io_kiocb_to_cmd(req);
 
-	if (unlikely(sqe->addr || sqe->rw_flags || sqe->splice_fd_in ||
-		     sqe->buf_index || sqe->personality))
+	if (unlikely(sqe->buf_index || sqe->personality))
 		return -EINVAL;
 
 	msg->user_data = READ_ONCE(sqe->off);
 	msg->len = READ_ONCE(sqe->len);
+	msg->cmd = READ_ONCE(sqe->addr);
+	msg->src_fd = READ_ONCE(sqe->addr3);
+	msg->dst_fd = READ_ONCE(sqe->file_index);
+	msg->flags = READ_ONCE(sqe->msg_ring_flags);
+	if (msg->flags & ~IORING_MSG_RING_CQE_SKIP)
+		return -EINVAL;
+
 	return 0;
 }
 
 int io_msg_ring(struct io_kiocb *req, unsigned int issue_flags)
 {
 	struct io_msg *msg = io_kiocb_to_cmd(req);
-	struct io_ring_ctx *target_ctx;
 	int ret;
 
 	ret = -EBADFD;
 	if (!io_is_uring_fops(req->file))
 		goto done;
 
-	ret = -EOVERFLOW;
-	target_ctx = req->file->private_data;
-	if (io_post_aux_cqe(target_ctx, msg->user_data, msg->len, 0))
-		ret = 0;
+	switch (msg->cmd) {
+	case IORING_MSG_DATA:
+		ret = io_msg_ring_data(req);
+		break;
+	case IORING_MSG_SEND_FD:
+		ret = io_msg_send_fd(req, issue_flags);
+		break;
+	default:
+		ret = -EINVAL;
+		break;
+	}
 
 done:
 	if (ret < 0)
-- 
2.35.1