block: only read from sqe on initial invocation of blkdev_uring_cmd()

This passthrough helper currently only supports discards. Part of that
command is the start and length, which is read from the SQE. It does
so on every invocation, where it really should just make it stable
on the first invocation. This avoids needing to copy the SQE upfront,
as we only really need those two 8b values stored in our per-req
payload.

Cc: stable@vger.kernel.org # 6.17+
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: axboe

block/ioctl.c

@@ -857,6 +857,8 @@ long compat_blkdev_ioctl(struct file *file, unsigned cmd, unsigned long arg)
 #endif
 
 struct blk_iou_cmd {
+	u64 start;
+	u64 len;
 	int res;
 	bool nowait;
 };
@@ -946,23 +948,27 @@ int blkdev_uring_cmd(struct io_uring_cmd *cmd, unsigned int issue_flags)
 {
 	struct block_device *bdev = I_BDEV(cmd->file->f_mapping->host);
 	struct blk_iou_cmd *bic = io_uring_cmd_to_pdu(cmd, struct blk_iou_cmd);
-	const struct io_uring_sqe *sqe = cmd->sqe;
 	u32 cmd_op = cmd->cmd_op;
-	uint64_t start, len;
 
-	if (unlikely(sqe->ioprio || sqe->__pad1 || sqe->len ||
-		     sqe->rw_flags || sqe->file_index))
-		return -EINVAL;
+	/* Read what we need from the SQE on the first issue */
+	if (!(issue_flags & IORING_URING_CMD_REISSUE)) {
+		const struct io_uring_sqe *sqe = cmd->sqe;
+
+		if (unlikely(sqe->ioprio || sqe->__pad1 || sqe->len ||
+			     sqe->rw_flags || sqe->file_index))
+			return -EINVAL;
+
+		bic->start = READ_ONCE(sqe->addr);
+		bic->len = READ_ONCE(sqe->addr3);
+	}
 
 	bic->res = 0;
 	bic->nowait = issue_flags & IO_URING_F_NONBLOCK;
 
-	start = READ_ONCE(sqe->addr);
-	len = READ_ONCE(sqe->addr3);
-
 	switch (cmd_op) {
 	case BLOCK_URING_CMD_DISCARD:
-		return blkdev_cmd_discard(cmd, bdev, start, len, bic->nowait);
+		return blkdev_cmd_discard(cmd, bdev, bic->start, bic->len,
+					  bic->nowait);
 	}
 	return -EINVAL;
 }