Commit 4bbf439b authored by Al Viro's avatar Al Viro
Browse files

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: d4d50710 (seq_file: add seq_read_iter)
Tested-by: default avatarNathan Chancellor <natechancellor@gmail.com>
Signed-off-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
parent d4d50710

fs/seq_file.c

+27 −30
Original line number Diff line number Diff line
@@ -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);



	/*
@@ -206,36 +208,34 @@ ssize_t seq_read_iter(struct kiocb *iocb, struct iov_iter *iter) 
		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;
@@ -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 once for every record shown.

	while (1) {

		size_t offs = m->count;

		loff_t pos = m->index;
@@ -259,30 +262,27 @@ Fill: 
					    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;

		} else if (err || seq_has_overflowed(m)) {

			m->count = offs;

			if (likely(err <= 0))

			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;

	}
@@ -291,9 +291,6 @@ Done: 
Enomem:

	err = -ENOMEM;

	goto Done;

Efault:

	err = -EFAULT;

	goto Done;

}

EXPORT_SYMBOL(seq_read_iter);

CRA Git | Maintained and supported by SUSTech CRA and CCSE