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Message-ID: <20160524001854.GW14480@ZenIV.linux.org.uk> Date: Tue, 24 May 2016 01:18:54 +0100 From: Al Viro <viro@...IV.linux.org.uk> To: Larry Finger <Larry.Finger@...inger.net> Cc: LKML <linux-kernel@...r.kernel.org> Subject: Re: Regression in 4.6.0-git - bisected to commit dd254f5a382c 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?
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