Re: [PATCH 1/6] seq_file: add seq_read_iter

[Al Viro <viro@zeniv.linux.org.uk>]

On Sat, Nov 14, 2020 at 08:50:00PM +0000, Al Viro wrote:

OK, I think I understand what's going on.  Could you check if
reverting the variant in -next and applying the following instead
fixes what you are seeing?

diff --git a/fs/seq_file.c b/fs/seq_file.c
index 3b20e21604e7..35667112bbd1 100644
--- a/fs/seq_file.c
+++ b/fs/seq_file.c
@@ -168,7 +168,6 @@ EXPORT_SYMBOL(seq_read);
 ssize_t seq_read_iter(struct kiocb *iocb, struct iov_iter *iter)
 {
 	struct seq_file *m = iocb->ki_filp->private_data;
-	size_t size = iov_iter_count(iter);
 	size_t copied = 0;
 	size_t n;
 	void *p;
@@ -208,16 +207,15 @@ ssize_t seq_read_iter(struct kiocb *iocb, struct iov_iter *iter)
 	}
 	/* if not empty - flush it first */
 	if (m->count) {
-		n = min(m->count, size);
-		if (copy_to_iter(m->buf + m->from, n, iter) != n)
-			goto Efault;
+		n = copy_to_iter(m->buf + m->from, m->count, iter);
 		m->count -= n;
 		m->from += n;
-		size -= n;
 		copied += n;
-		if (!size)
-			goto Done;
+		if (m->count)
+			goto Efault;
 	}
+	if (!iov_iter_count(iter))
+		goto Done;
 	/* we need at least one record in buffer */
 	m->from = 0;
 	p = m->op->start(m, &m->index);
@@ -249,6 +247,7 @@ ssize_t seq_read_iter(struct kiocb *iocb, struct iov_iter *iter)
 	goto Done;
 Fill:
 	/* they want more? let's try to get some more */
+	/* m->count is positive and there's space left in iter */
 	while (1) {
 		size_t offs = m->count;
 		loff_t pos = m->index;
@@ -263,7 +262,7 @@ ssize_t seq_read_iter(struct kiocb *iocb, struct iov_iter *iter)
 			err = PTR_ERR(p);
 			break;
 		}
-		if (m->count >= size)
+		if (m->count >= iov_iter_count(iter))
 			break;
 		err = m->op->show(m, p);
 		if (seq_has_overflowed(m) || err) {
@@ -273,12 +272,12 @@ ssize_t seq_read_iter(struct kiocb *iocb, struct iov_iter *iter)
 		}
 	}
 	m->op->stop(m, p);
-	n = min(m->count, size);
-	if (copy_to_iter(m->buf, n, iter) != n)
-		goto Efault;
+	n = copy_to_iter(m->buf, m->count, iter);
 	copied += n;
-	m->count -= n;
 	m->from = n;
+	m->count -= n;
+	if (m->count)
+		goto Efault;
 Done:
 	if (!copied)
 		copied = err;