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fix return values of seq_read_iter()

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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: d4d50710a8 (seq_file: add seq_read_iter)
Tested-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
This commit is contained in:
Al Viro 2020-11-12 14:40:37 -05:00
parent d4d50710a8
commit 4bbf439b09
1 changed files with 27 additions and 30 deletions
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57
fs/seq_file.c
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@ -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;
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;
}
@ -291,9 +291,6 @@ Done:
Enomem:
err = -ENOMEM;
goto Done;
Efault:
err = -EFAULT;
goto Done;
}
EXPORT_SYMBOL(seq_read_iter);