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authorAl Viro <viro@zeniv.linux.org.uk>2020-11-12 22:40:37 +0300
committerAl Viro <viro@zeniv.linux.org.uk>2020-11-16 06:12:53 +0300
commit4bbf439b09c5ac3f8b3e9584fe080375d8d0ad2d (patch)
tree4c22e0ed68c83609d95949c7a7f2645b0513a8b2 /fs/seq_file.c
parentd4d50710a8b46082224376ef119a4dbb75b25c56 (diff)
downloadlinux-4bbf439b09c5ac3f8b3e9584fe080375d8d0ad2d.tar.xz
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 <natechancellor@gmail.com> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Diffstat (limited to 'fs/seq_file.c')
-rw-r--r--fs/seq_file.c57
1 files changed, 27 insertions, 30 deletions
diff --git a/fs/seq_file.c b/fs/seq_file.c
index 3b20e21604e7..03a369ccd28c 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);
/*
@@ -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);