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Message-Id: <6E57F7A4-59B0-46EA-9FFF-D0A4BA2D8E51@codeaurora.org> Date: Tue, 24 May 2016 14:13:11 -0500 From: Matthew McClintock <mmcclint@...eaurora.org> To: Larry Finger <Larry.Finger@...inger.net> Cc: Al Viro <viro@...IV.linux.org.uk>, LKML <linux-kernel@...r.kernel.org> Subject: Re: Regression in 4.6.0-git - bisected to commit dd254f5a382c I’m seeing this too, same commit if you want another person to test/reproduce. -M > On May 24, 2016, at 11:10 AM, Larry Finger <Larry.Finger@...inger.net> wrote: > > 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 > >
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