io_uring_cmd_is_kmbuf_ring() returns true if there is a kernel-managed
buffer ring at the specified buffer group.

This is a preparatory patch for upcoming fuse kernel-managed buffer
support, which needs to ensure the buffer ring registered by the server
is a kernel-managed buffer ring.

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
---
 include/linux/io_uring/cmd.h |  9 +++++++++
 io_uring/kbuf.c              | 19 +++++++++++++++++++
 io_uring/kbuf.h              |  2 ++
 io_uring/uring_cmd.c         |  9 +++++++++
 4 files changed, 39 insertions(+)

diff --git a/include/linux/io_uring/cmd.h b/include/linux/io_uring/cmd.h
index 2988592e045c..a94f1dbc89c7 100644
--- a/include/linux/io_uring/cmd.h
+++ b/include/linux/io_uring/cmd.h
@@ -99,6 +99,9 @@ int io_uring_cmd_kmbuffer_recycle(struct io_uring_cmd *cmd,
 				  unsigned int buf_group, u64 addr,
 				  unsigned int len, unsigned int bid,
 				  unsigned int issue_flags);
+
+int io_uring_cmd_is_kmbuf_ring(struct io_uring_cmd *ioucmd,
+			       unsigned int buf_group, unsigned int issue_flags);
 #else
 static inline int
 io_uring_cmd_import_fixed(u64 ubuf, unsigned long len, int rw,
@@ -176,6 +179,12 @@ static inline int io_uring_cmd_kmbuffer_recycle(struct io_uring_cmd *cmd,
 {
 	return -EOPNOTSUPP;
 }
+static inline int io_uring_cmd_is_kmbuf_ring(struct io_uring_cmd *ioucmd,
+					     unsigned int buf_group,
+					     unsigned int issue_flags)
+{
+	return -EOPNOTSUPP;
+}
 #endif
 
 static inline struct io_uring_cmd *io_uring_cmd_from_tw(struct io_tw_req tw_req)
diff --git a/io_uring/kbuf.c b/io_uring/kbuf.c
index f494d896c17e..b16f6a6aa872 100644
--- a/io_uring/kbuf.c
+++ b/io_uring/kbuf.c
@@ -960,3 +960,22 @@ int io_register_kmbuf_ring(struct io_ring_ctx *ctx, void __user *arg)
 
 	return 0;
 }
+
+bool io_is_kmbuf_ring(struct io_kiocb *req, unsigned int buf_group,
+		      unsigned int issue_flags)
+{
+	struct io_ring_ctx *ctx = req->ctx;
+	struct io_buffer_list *bl;
+	bool is_kmbuf_ring = false;
+
+	io_ring_submit_lock(ctx, issue_flags);
+
+	bl = io_buffer_get_list(ctx, buf_group);
+	if (likely(bl) && (bl->flags & IOBL_KERNEL_MANAGED)) {
+		WARN_ON_ONCE(!(bl->flags & IOBL_BUF_RING));
+		is_kmbuf_ring = true;
+	}
+
+	io_ring_submit_unlock(ctx, issue_flags);
+	return is_kmbuf_ring;
+}
diff --git a/io_uring/kbuf.h b/io_uring/kbuf.h
index 4d8b7491628e..68c4c78fbb44 100644
--- a/io_uring/kbuf.h
+++ b/io_uring/kbuf.h
@@ -149,4 +149,6 @@ int io_kbuf_ring_unpin(struct io_kiocb *req, unsigned buf_group,
 int io_kmbuf_recycle(struct io_kiocb *req, unsigned int bgid, u64 addr,
 		     unsigned int len, unsigned int bid,
 		     unsigned int issue_flags);
+bool io_is_kmbuf_ring(struct io_kiocb *req, unsigned int buf_group,
+		      unsigned int issue_flags);
 #endif
diff --git a/io_uring/uring_cmd.c b/io_uring/uring_cmd.c
index ee95d1102505..4534710252da 100644
--- a/io_uring/uring_cmd.c
+++ b/io_uring/uring_cmd.c
@@ -448,3 +448,12 @@ int io_uring_cmd_kmbuffer_recycle(struct io_uring_cmd *ioucmd,
 	return io_kmbuf_recycle(req, buf_group, addr, len, bid, issue_flags);
 }
 EXPORT_SYMBOL_GPL(io_uring_cmd_kmbuffer_recycle);
+
+int io_uring_cmd_is_kmbuf_ring(struct io_uring_cmd *ioucmd,
+			       unsigned int buf_group, unsigned int issue_flags)
+{
+	struct io_kiocb *req = cmd_to_io_kiocb(ioucmd);
+
+	return io_is_kmbuf_ring(req, buf_group, issue_flags);
+}
+EXPORT_SYMBOL_GPL(io_uring_cmd_is_kmbuf_ring);
-- 
2.47.3