linux-kernel - Re: [bisected] 3c8ba0d61d04ced9f8d9ff93977995a9e4e96e91 oopses on s390

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Message-Id: <alpine.LFD.2.21.1804091809310.1698@schleppi>
Date:   Mon, 9 Apr 2018 18:18:16 +0200 (CEST)
From:   Sebastian Ott <sebott@...ux.ibm.com>
To:     Kees Cook <keescook@...omium.org>
cc:     Sebastian Ott <sebott@...ux.vnet.ibm.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Martin Uecker <Martin.Uecker@....uni-goettingen.de>,
        Ingo Molnar <mingo@...nel.org>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
Subject: Re: [bisected] 3c8ba0d61d04ced9f8d9ff93977995a9e4e96e91 oopses on
 s390

On Fri, 6 Apr 2018, Kees Cook wrote:
> On Fri, Apr 6, 2018 at 2:47 AM, Sebastian Ott <sebott@...ux.vnet.ibm.com> wrote:
> > Today's kernel oopsed on s390. Bisect points to:
> > 3c8ba0d61d04 ("kernel.h: Retain constant expression output for max()/min()")
> >
> > [    1.898277] dasd-eckd 0.0.3304: DASD with 4 KB/block, 21636720 KB total size, 48 KB/track, compatible disk layout
> > [    1.898308] ------------[ cut here ]------------
> > [    1.898310] kernel BUG at block/bio.c:1798!
> 
> Well that's extremely bad. :(

What happened is that the bio build by the partition detection code was
attempted to be split by the block layer because the block queue had a
max_sector setting of 0. blk_queue_max_hw_sectors uses min_not_zero.

Both of the following return 0 on my machine:
+       pr_warn("%u\n", min_not_zero(100, 1000));
+       pr_warn("%u\n", min_not_zero(1000, 100));

So, we now know what failed...the question is why?

Sebastian