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

Al Viro
2020-11-15T23:38:42Z

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);