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

Nathan Chancellor
2020-11-15T23:52:17Z

On Sun, Nov 15, 2020 at 11:38:14PM +0000, Al Viro wrote:
> 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?

Looks good to me on top of d4d50710a8b46082224376ef119a4dbb75b25c56,
thanks for quickly looking into this!

Tested-by: Nathan Chancellor <natechancellor@gmail.com>

> 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);
>