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

Al Viro
2020-11-10T21:33:01Z

On Wed, Nov 04, 2020 at 09:27:33AM +0100, Christoph Hellwig wrote:

> ssize_t seq_read(struct file *file, char __user *buf, size_t size, loff_t *ppos)
> {
> - struct seq_file *m = file->private_data;
> + struct iovec iov = { .iov_base = buf, .iov_len = size};
> + struct kiocb kiocb;
> + struct iov_iter iter;
> + ssize_t ret;
> +
> + init_sync_kiocb(&kiocb, file);
> + iov_iter_init(&iter, READ, &iov, 1, size);
> +
> + kiocb.ki_pos = *ppos;
> + ret = seq_read_iter(&kiocb, &iter);
> + *ppos = kiocb.ki_pos;
> + return ret;
> +}
> +EXPORT_SYMBOL(seq_read);

This is basically an open-coded copy of new_sync_read()...

> if (m->count) {
> n = min(m->count, size);
> - err = copy_to_user(buf, m->buf + m->from, n);
> - if (err)
> + if (copy_to_iter(m->buf + m->from, n, iter) != n)
> goto Efault;
> m->count -= n;
> m->from += n;
> size -= n;
> - buf += n;
> copied += n;
> if (!size)
> goto Done;

> n = min(m->count, size);
> - err = copy_to_user(buf, m->buf, n);
> - if (err)
> + if (copy_to_iter(m->buf, n, iter) != n)
> goto Efault;

This is actually broken from generic_file_splice_read() POV; if you've
already emitted something, you will end up with more data spewed into
pipe than you report to caller. You want something similar to
copy_to_iter_full() here, with iterator _not_ advanced in case of
failure. The first call is not an issue (you have no data copied
yet, so you'll end up with -EFAULT, aka "discard everything you've
put there and return -EAGAIN"), but the second really is a problem.

BTW, other ->read_iter() instances might need to be careful with that
pattern as well; drivers/gpu/drm/drm_dp_aux_dev.c:auxdev_read_iter()
would appear to have the same problem.

<greps some more>
if (unlikely(iov_iter_is_pipe(iter))) {
void *addr = kmap_atomic(page);

written = copy_to_iter(addr, copy, iter);
kunmap_atomic(addr);
} else
in fs/cifs/file.c looks... interesting, considering the fact that
copy_to_iter() for pipe destination might very well have to do
allocations. With GFP_USER. Under kmap_atomic()...

Note that we have this:
static inline int copy_linear_skb(struct sk_buff *skb, int len, int off,
struct iov_iter *to)
{
int n;

n = copy_to_iter(skb->data + off, len, to);
if (n == len)
return 0;

iov_iter_revert(to, n);
return -EFAULT;
}
i.e. the same "do not advance on short copy" kind of thing.

AFAICS, not all callers want that semantics, but I think it's worth
a new primitive. I'm not saying it should be a prereq for your
series, but either that or an explicit iov_iter_revert() is needed.