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

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Message-ID: <20160525062434.GE14480@ZenIV.linux.org.uk>
Date:	Wed, 25 May 2016 07:24:34 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Matthew McClintock <mmcclint@...eaurora.org>
Cc:	Larry Finger <Larry.Finger@...inger.net>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Regression in 4.6.0-git - bisected to commit dd254f5a382c

On Wed, May 25, 2016 at 12:41:33AM +0100, Al Viro wrote:
> On Tue, May 24, 2016 at 05:31:51PM -0500, Matthew McClintock wrote:
> > 
> > > On May 24, 2016, at 2:36 PM, Larry Finger <Larry.Finger@...inger.net> wrote:
> > > 
> > > On 05/24/2016 02:25 PM, Matthew McClintock wrote:
> > >> On May 24, 2016, at 2:16 PM, Larry Finger <Larry.Finger@...inger.net> wrote:
> > >>> 
> > >>> On 05/24/2016 02:13 PM, Matthew McClintock wrote:
> > >>>> I’m seeing this too, same commit if you want another person to test/reproduce.
> > >>> 
> > >>> If you do a pull today, does that fix your problem?
> > >> 
> > >> Hmm, no. Which commit am I looking for? I’m on a56f489502e28caac56c8a0735549740f0ae0711
> > > 
> > > Commit 84787c572d402644dca4874aba73324d9f8e3948 is working for me. I have a fixup in lib/iov_iter.c with a dump_stack() call if the fixup was needed. That dump is not triggered. I do not seem to have a56f489502e yet.
> > 
> > Still seeing the issue on top of tree and the above commit. Re-ran bisection just to be sure.
> 
> Guys, the bug is real and definitely still there.
> 	char c;
> 	struct iovec v[2] = {{&c, 0}, {&c, 1}};
> 	readv(0, v, 2);
> will trigger it just fine with stdin on e.g. tty.  It needs fixing and I'll
> post a fix as soon as it gets through the local testing.  In the meanwhile,
> I would like to know what in userland is doing that kind of call - kernel
> certainly shouldn't end up in an infinite loop on that, but it's bloody odd
> and I wonder what's going on in userland code to result in that call.
> 
> Again, I understand what's going on kernel-side; the only tricky part is how
> to fix it without bringing the nasal daemons back.  I think I have a solution
> and I'm going to post it tonight if it survives the local beating.  In any
> case, the testcase above deserves being added to LTP - it's a real regression.

FWIW, the reproducer is
#include <sys/uio.>
main()
{
	char c;
	struct iovec v[2] = {{&c,0},{&c,1}};
	readv(0, v, 2);
}
ran with stdin from tty.  Fix for that is simply

diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index 28cb431..0cd5227 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -101,7 +101,7 @@
 #define iterate_and_advance(i, n, v, I, B, K) {			\
 	if (unlikely(i->count < n))				\
 		n = i->count;					\
-	if (n) {						\
+	if (i->count) {						\
 		size_t skip = i->iov_offset;			\
 		if (unlikely(i->type & ITER_BVEC)) {		\
 			const struct bio_vec *bvec;		\

Could you see if your reproducer is fixed by that?