fix return values of seq_read_iter()

Unlike ->read(), ->read_iter() instances *must* return the amount
of data they'd left in iterator. For ->read() returning less than
it has actually copied is a QoI issue; read(fd, unmapped_page - 5, 8)
is allowed to fill all 5 bytes of destination and return 4; it's
not nice to caller, but POSIX allows pretty much anything in such
situation, up to and including a SIGSEGV.

generic_file_splice_read() uses pipe-backed iterator as destination;
there a short copy comes from pipe being full, not from running into
an un{mapped,writable} page in the middle of destination as we
have for iovec-backed iterators read(2) uses. And there we rely
upon the ->read_iter() reporting the actual amount it has left
in destination.

Conversion of a ->read() instance into ->read_iter() has to watch
out for that. If you really need an "all or nothing" kind of
behaviour somewhere, you need to do iov_iter_revert() to prune
the partial copy.

In case of seq_read_iter() we can handle short copy just fine;
the data is in m->buf and next call will fetch it from there.

Fixes: d4d50710a8b4 (seq_file: add seq_read_iter)
Tested-by: Nathan Chancellor <[e-mail]>
Signed-off-by: Al Viro <[e-mail]>

fs/seq_file.c

···
 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);
 
···
 		if (!m->buf)
 			goto Enomem;
 	}
-	/* if not empty - flush it first */
+	// something left in the buffer - copy it out 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 */
+	// get a 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;
···
 			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 once for every record shown.
 	while (1) {
 		size_t offs = m->count;
 		loff_t pos = m->index;
···
 					    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) {
+		if (err > 0) {		// ->show() says "skip it"
 			m->count = offs;
-			if (likely(err <= 0))
-				break;
+		} else if (err || seq_has_overflowed(m)) {
+			m->count = offs;
+			break;
 		}
 	}
 	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;
 	}
···
 	return copied;
 Enomem:
 	err = -ENOMEM;
-	goto Done;
-Efault:
-	err = -EFAULT;
 	goto Done;
 }
 EXPORT_SYMBOL(seq_read_iter);