Re: [fuse-devel] [PATCH] fuse: break infinite loop in fuse_fill_write_pages()

[Maxim Patlasov]

On 10/02/2015 04:21 AM, Konstantin Khlebnikov wrote:
> Bump. Add more peopple in CC.
>
> On Mon, Sep 21, 2015 at 1:02 PM, Roman Gushchin <klamm@xxxxxxxxxxxxxx> wrote:
> > I got a report about unkillable task eating CPU. Thge further
> > investigation shows, that the problem is in the fuse_fill_write_pages()
> > function. If iov's first segment has zero length, we get an infinite
> > loop, because we never reach iov_iter_advance() call.

iov_iter_copy_from_user_atomic() eventually calls iterate_iovec(). The 
latter silently consumes zero-length iov. So I don't think "iov's first 
segment has zero length" can cause infinite loop.

Thanks,
Maxim

> > Fix this by calling iov_iter_advance() before repeating an attempt to
> > copy data from userspace.
> >
> > A similar problem is described in 124d3b7041f ("fix writev regression:
> > pan hanging unkillable and un-straceable").
> >
> > Signed-off-by: Roman Gushchin <klamm@xxxxxxxxxxxxxx>
> > Cc: Miklos Szeredi <miklos@xxxxxxxxxx>
> > ---
> >   fs/fuse/file.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/fs/fuse/file.c b/fs/fuse/file.c
> > index f523f2f..195476a 100644
> > --- a/fs/fuse/file.c
> > +++ b/fs/fuse/file.c
> > @@ -1049,6 +1049,7 @@ static ssize_t fuse_fill_write_pages(struct fuse_req *req,
> >                  tmp = iov_iter_copy_from_user_atomic(page, ii, offset, bytes);
> >                  flush_dcache_page(page);
> >
> > +               iov_iter_advance(ii, tmp);
> >                  if (!tmp) {
> >                          unlock_page(page);
> >                          page_cache_release(page);
> > @@ -1061,7 +1062,6 @@ static ssize_t fuse_fill_write_pages(struct fuse_req *req,
> >                  req->page_descs[req->num_pages].length = tmp;
> >                  req->num_pages++;
> >
> > -               iov_iter_advance(ii, tmp);
> >                  count += tmp;
> >                  pos += tmp;
> >                  offset += tmp;
> > --
> > 2.4.3
> >
> > --
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