io_buffer_register_bvec() currently uses blk_rq_nr_phys_segments() as
the number of bvecs in the request. However, bvecs may be split into
multiple segments depending on the queue limits. Thus, the number of
segments may overestimate the number of bvecs. For ublk devices, the
only current users of io_buffer_register_bvec(), virt_boundary_mask,
seg_boundary_mask, max_segments, and max_segment_size can all be set
arbitrarily by the ublk server process.
Set imu->nr_bvecs based on the number of bvecs the rq_for_each_bvec()
loop actually yields. However, continue using blk_rq_nr_phys_segments()
as an upper bound on the number of bvecs when allocating imu to avoid
needing to iterate the bvecs a second time.

Signed-off-by: Caleb Sander Mateos <csander@purestorage.com>
Fixes: 27cb27b6d5ea ("io_uring: add support for kernel registered bvecs")
---
 io_uring/rsrc.c | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/io_uring/rsrc.c b/io_uring/rsrc.c
index d787c16dc1c3..301c6899d240 100644
--- a/io_uring/rsrc.c
+++ b/io_uring/rsrc.c
@@ -941,12 +941,12 @@ int io_buffer_register_bvec(struct io_uring_cmd *cmd, struct request *rq,
 	struct io_ring_ctx *ctx = cmd_to_io_kiocb(cmd)->ctx;
 	struct io_rsrc_data *data = &ctx->buf_table;
 	struct req_iterator rq_iter;
 	struct io_mapped_ubuf *imu;
 	struct io_rsrc_node *node;
-	struct bio_vec bv, *bvec;
-	u16 nr_bvecs;
+	struct bio_vec bv;
+	unsigned int nr_bvecs = 0;
 	int ret = 0;
 
 	io_ring_submit_lock(ctx, issue_flags);
 	if (index >= data->nr) {
 		ret = -EINVAL;
@@ -963,32 +963,34 @@ int io_buffer_register_bvec(struct io_uring_cmd *cmd, struct request *rq,
 	if (!node) {
 		ret = -ENOMEM;
 		goto unlock;
 	}
 
-	nr_bvecs = blk_rq_nr_phys_segments(rq);
-	imu = io_alloc_imu(ctx, nr_bvecs);
+	/*
+	 * blk_rq_nr_phys_segments() may overestimate the number of bvecs
+	 * but avoids needing to iterate over the bvecs
+	 */
+	imu = io_alloc_imu(ctx, blk_rq_nr_phys_segments(rq));
 	if (!imu) {
 		kfree(node);
 		ret = -ENOMEM;
 		goto unlock;
 	}
 
 	imu->ubuf = 0;
 	imu->len = blk_rq_bytes(rq);
 	imu->acct_pages = 0;
 	imu->folio_shift = PAGE_SHIFT;
-	imu->nr_bvecs = nr_bvecs;
 	refcount_set(&imu->refs, 1);
 	imu->release = release;
 	imu->priv = rq;
 	imu->is_kbuf = true;
 	imu->dir = 1 << rq_data_dir(rq);
 
-	bvec = imu->bvec;
 	rq_for_each_bvec(bv, rq, rq_iter)
-		*bvec++ = bv;
+		imu->bvec[nr_bvecs++] = bv;
+	imu->nr_bvecs = nr_bvecs;
 
 	node->buf = imu;
 	data->nodes[index] = node;
 unlock:
 	io_ring_submit_unlock(ctx, issue_flags);
-- 
2.45.2