Re: Regression in 4.6.0-git - bisected to commit dd254f5a382c

[Larry Finger]

On 05/24/2016 11:28 AM, Al Viro wrote:
> On Tue, May 24, 2016 at 11:10:09AM -0500, Larry Finger wrote:
>
> > For now, the following one-line hack allows my system to boot:
> >
> > diff --git a/fs/read_write.c b/fs/read_write.c
> > index 933b53a..d5d64d9 100644
> > --- a/fs/read_write.c
> > +++ b/fs/read_write.c
> > @@ -721,6 +721,7 @@ static ssize_t do_loop_readv_writev(struct file *filp,
> > struct iov_iter *iter,
> >                  ret += nr;
> >                  if (nr != iovec.iov_len)
> >                          break;
> > +               nr = max_t(ssize_t, nr, 1);
> >                  iov_iter_advance(iter, nr);
> >          }
> >
> > I have no idea what subtle bug in do_loop_readv_writev() is causing nr to be
> > zero, but it seems to have been exposed by commit dd254f5a382c.
>
> This is definitely broken.  What happens is that something calls readv()
> or writev() with one of the iovecs in the middle of the vector having
> zero ->iov_len.  The call of iov_iter_advance(iter, 0) used to step to
> the next iovec in such situation; the change in this commit leaves that to the
> next primitive called, and most of the time that works just fine.
> do_loop_readv_writev(), though, is picking the size to be passed to
> ->write()/->read() from the current iovec, and if it had been zero from
> the very beginning... guess what's going to happen.
>
> I'm somewhat curious about the source of that syscall - it's a good testcase
> to add to ltp/xfstests, but it smells very odd for normal userland code.
> It certainly needs to be fixed kernel-side, but the code issuing that is
> probably worth looking into.  Oddities are like mushrooms - found one, look
> around for more and don't assume that the rest will be of the same species...
>
> I'm looking into iterate_and_advance right now; hopefully will have a sane
> replacement shortly...

I did a pull from mainline this morning (commit 84787c572d402) and the problem 
has somehow been fixed. I did not see any likely candidates in the commit 
messages, and I'm bisecting again to see which change did it.

Larry