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

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

On Sun, Nov 15, 2020 at 02:41:25PM -0700, Nathan Chancellor wrote:
> Hi Al,
> 
> Apologies for the delay.
> 
> On Sun, Nov 15, 2020 at 03:53:55PM +0000, Al Viro wrote:
> > 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?
> 
> The below diff on top of d4d50710a8b46082224376ef119a4dbb75b25c56 does
> not fix my issue unfortunately.

OK...  Now that I have a reproducer[1], I think I've sorted it out.
And yes, it had been too subtle for its own good ;-/

[1] I still wonder what the hell in the userland has come up with the
idea of reading through a file with readv(), each time with 2-element
iovec array, the first element covering 0 bytes and the second one - 1024.
AFAICS, nothing is systemd git appears to be _that_ weird...  Makes for
a useful testcase, though...

Anyway, could you test this replacement?

diff --git a/fs/seq_file.c b/fs/seq_file.c
index 3b20e21604e7..c0dfe2861b35 100644
--- a/fs/seq_file.c
+++ b/fs/seq_file.c
@@ -168,12 +168,14 @@ 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;
 	int err = 0;
 
+	if (!iov_iter_count(iter))
+		return 0;
+
 	mutex_lock(&m->lock);
 
 	/*
@@ -208,34 +210,32 @@ 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)
+		if (m->count)	// hadn't managed to copy everything
 			goto Done;
 	}
-	/* we need at least one record in buffer */
+	/* we need at least one non-empty record in the buffer */
 	m->from = 0;
 	p = m->op->start(m, &m->index);
 	while (1) {
 		err = PTR_ERR(p);
-		if (!p || IS_ERR(p))
+		if (!p || IS_ERR(p))	// EOF or an error
 			break;
 		err = m->op->show(m, p);
-		if (err < 0)
+		if (err < 0)		// hard error
 			break;
-		if (unlikely(err))
+		if (unlikely(err))	// ->show() says "skip it"
 			m->count = 0;
-		if (unlikely(!m->count)) {
+		if (unlikely(!m->count)) { // empty record
 			p = m->op->next(m, p, &m->index);
 			continue;
 		}
-		if (m->count < m->size)
+		if (!seq_has_overflowed(m)) // got it
 			goto Fill;
+		// need a bigger buffer
 		m->op->stop(m, p);
 		kvfree(m->buf);
 		m->count = 0;
@@ -244,11 +244,14 @@ ssize_t seq_read_iter(struct kiocb *iocb, struct iov_iter *iter)
 			goto Enomem;
 		p = m->op->start(m, &m->index);
 	}
+	// EOF or an error
 	m->op->stop(m, p);
 	m->count = 0;
 	goto Done;
 Fill:
-	/* they want more? let's try to get some more */
+	// one non-empty record is in the buffer; if they want more,
+	// try to fit more in, but in any case we need to advance
+	// the iterator at least once.
 	while (1) {
 		size_t offs = m->count;
 		loff_t pos = m->index;
@@ -259,11 +262,9 @@ ssize_t seq_read_iter(struct kiocb *iocb, struct iov_iter *iter)
 					    m->op->next);
 			m->index++;
 		}
-		if (!p || IS_ERR(p)) {
-			err = PTR_ERR(p);
+		if (!p || IS_ERR(p))	// no next record for us
 			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) {
@@ -273,16 +274,14 @@ 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;
 Done:
-	if (!copied)
-		copied = err;
-	else {
+	if (unlikely(!copied)) {
+		copied = m->count ? -EFAULT : err;
+	} else {
 		iocb->ki_pos += copied;
 		m->read_pos += copied;
 	}
@@ -291,9 +290,6 @@ ssize_t seq_read_iter(struct kiocb *iocb, struct iov_iter *iter)
 Enomem:
 	err = -ENOMEM;
 	goto Done;
-Efault:
-	err = -EFAULT;
-	goto Done;
 }
 EXPORT_SYMBOL(seq_read_iter);