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

[Larry Finger <Larry.Finger@lwfinger.net>]

On 05/23/2016 07:18 PM, Al Viro wrote:
> 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?

As I wrote earlier, i->count was greater than zero, but size was zero, which 
caused the bulk of iterate_and_advance() to be skipped.

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.

Larry