[RFC 6/7] io_uring/rsrc: introduce helper installing one buffer

[Pavel Begunkov <asml.silence@gmail.com>]

Add a new helper called io_install_buffer(), which will be used
later for operations willing to install buffers into the registered
buffer table.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
 io_uring/rsrc.c | 15 +++++++++++++++
 io_uring/rsrc.h |  3 +++
 2 files changed, 18 insertions(+)

diff --git a/io_uring/rsrc.c b/io_uring/rsrc.c
index 3799470fd45e..db4286b42dce 100644
--- a/io_uring/rsrc.c
+++ b/io_uring/rsrc.c
@@ -517,6 +517,21 @@ static int __io_sqe_buffers_update(struct io_ring_ctx *ctx,
 	return done ? done : err;
 }
 
+int io_install_buffer(struct io_ring_ctx *ctx,
+		      struct io_mapped_ubuf *imu,
+		      unsigned i)
+{
+	if (unlikely(i >= ctx->nr_user_bufs))
+		return -EFAULT;
+
+	i = array_index_nospec(i, ctx->nr_user_bufs);
+	if (unlikely(ctx->user_bufs[i] != ctx->dummy_ubuf))
+		return -EINVAL;
+
+	ctx->user_bufs[i] = imu;
+	return 0;
+}
+
 static int __io_register_rsrc_update(struct io_ring_ctx *ctx, unsigned type,
 				     struct io_uring_rsrc_update2 *up,
 				     unsigned nr_args)
diff --git a/io_uring/rsrc.h b/io_uring/rsrc.h
index 29ce9a8a2277..aba95bdd060e 100644
--- a/io_uring/rsrc.h
+++ b/io_uring/rsrc.h
@@ -75,6 +75,9 @@ void __io_sqe_buffers_unregister(struct io_ring_ctx *ctx);
 int io_sqe_buffers_unregister(struct io_ring_ctx *ctx);
 int io_sqe_buffers_register(struct io_ring_ctx *ctx, void __user *arg,
 			    unsigned int nr_args, u64 __user *tags);
+int io_install_buffer(struct io_ring_ctx *ctx, struct io_mapped_ubuf *imu,
+		      unsigned i);
+
 void __io_sqe_files_unregister(struct io_ring_ctx *ctx);
 int io_sqe_files_unregister(struct io_ring_ctx *ctx);
 int io_sqe_files_register(struct io_ring_ctx *ctx, void __user *arg,
-- 
2.40.0