io_uring: update kiocb->ki_pos at execution time

commit d34e1e5b39 upstream.

Update kiocb->ki_pos at execution time rather than in io_prep_rw().
io_prep_rw() happens before the job is enqueued to a worker and so the
offset might be read multiple times before being executed once.

Ensures that the file position in a set of _linked_ SQEs will be only
obtained after earlier SQEs have completed, and so will include their
incremented file position.

Signed-off-by: Dylan Yudaken <dylany@fb.com>
Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>

io_uring/io_uring.c

@@ -2922,14 +2922,6 @@ static int io_prep_rw(struct io_kiocb *req, const struct io_uring_sqe *sqe,
 		req->flags |= REQ_F_ISREG;
 
 	kiocb->ki_pos = READ_ONCE(sqe->off);
-	if (kiocb->ki_pos == -1) {
-		if (!(file->f_mode & FMODE_STREAM)) {
-			req->flags |= REQ_F_CUR_POS;
-			kiocb->ki_pos = file->f_pos;
-		} else {
-			kiocb->ki_pos = 0;
-		}
-	}
 	kiocb->ki_hint = ki_hint_validate(file_write_hint(kiocb->ki_filp));
 	kiocb->ki_flags = iocb_flags(kiocb->ki_filp);
 	ret = kiocb_set_rw_flags(kiocb, READ_ONCE(sqe->rw_flags));
@@ -3011,6 +3003,20 @@ static inline void io_rw_done(struct kiocb *kiocb, ssize_t ret)
 	}
 }
 
+static inline void io_kiocb_update_pos(struct io_kiocb *req)
+{
+	struct kiocb *kiocb = &req->rw.kiocb;
+
+	if (kiocb->ki_pos == -1) {
+		if (!(req->file->f_mode & FMODE_STREAM)) {
+			req->flags |= REQ_F_CUR_POS;
+			kiocb->ki_pos = req->file->f_pos;
+		} else {
+			kiocb->ki_pos = 0;
+		}
+	}
+}
+
 static void kiocb_done(struct kiocb *kiocb, ssize_t ret,
 		       unsigned int issue_flags)
 {
@@ -3566,6 +3572,8 @@ static int io_read(struct io_kiocb *req, unsigned int issue_flags)
 		return ret ?: -EAGAIN;
 	}
 
+	io_kiocb_update_pos(req);
+
 	ret = rw_verify_area(READ, req->file, io_kiocb_ppos(kiocb), req->result);
 	if (unlikely(ret)) {
 		kfree(iovec);
@@ -3700,6 +3708,8 @@ static int io_write(struct io_kiocb *req, unsigned int issue_flags)
 	    (req->flags & REQ_F_ISREG))
 		goto copy_iov;
 
+	io_kiocb_update_pos(req);
+
 	ret = rw_verify_area(WRITE, req->file, io_kiocb_ppos(kiocb), req->result);
 	if (unlikely(ret))
 		goto out_free;