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

[Al Viro]

On Mon, May 23, 2016 at 04:30:43PM -0500, Larry Finger wrote:
> The mainline kernels past 4.6.0 fail hang when logging in. There are no
> error messages, and the machine seems to be waiting for some event that
> never happens.
> 
> The problem has been bisected to commit dd254f5a382c ("fold checks into
> iterate_and_advance()"). The bisection has been verified.
> 
> The problem is the call from iov_iter_advance(). When I reinstated the old
> macro with a new name and used it in that routine, the system works.
> Obviously, the call that seems to be incorrect has some benefits. My
> quich-and-dirty patch is attached.
> 
> I will be willing to test any patch you prepare.

Hangs where and how?  A reproducer, please...  This is really weird - the
only change there is in the cases when
	* iov_iter_advance(i, n) is called with n greater than the remaining
amount.  It's a bug, plain and simple - old variant would've been left in
seriously buggered state and at the very least we want to catch any such
places for the sake of backports
	* iov_iter_advance(i, 0) - both old and new code leave *i unchanged,
but the old one dereferences i->iov[0], which be pointing beyond the end of
array by that point.  The value read from there was not used by the old code,
at that.

	Could you slap WARN_ON(size > i->count) in the very beginning of
iov_iter_advance() (the mainline variant) and see what triggers on your
reproducer?