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From: Dmitry Vyukov <dvyukov@google.com>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: syzkaller <syzkaller@googlegroups.com>,
	syzbot <syzbot+61e04e51b7ac86930589@syzkaller.appspotmail.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Jens Axboe <axboe@kernel.dk>, Borislav Petkov <bp@alien8.de>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
	KVM list <kvm@vger.kernel.org>,
	linux-block <linux-block@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	masahiroy@kernel.org, Ingo Molnar <mingo@redhat.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Sean Christopherson <seanjc@google.com>,
	syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>, Will Deacon <will@kernel.org>,
	"the arch/x86 maintainers" <x86@kernel.org>
Subject: Re: Bisections with different bug manifestations
Date: Fri, 16 Apr 2021 15:26:15 +0200
Message-ID: <CACT4Y+YutXjDarTu_J=EjsDDgt5LzXyNjN-hd1ZpWg6kDYgw6g@mail.gmail.com> (raw)
In-Reply-To: <20210416091300.0758c62a@gandalf.local.home>

On Fri, Apr 16, 2021 at 3:13 PM Steven Rostedt <rostedt@goodmis.org> wrote:
> On Fri, 16 Apr 2021 09:51:45 +0200
> Dmitry Vyukov <dvyukov@google.com> wrote:
> >
> > If you look at substantial base of bisection logs, you will find lots
> > of cases where bug types, functions don't match. Kernel crashes
> > differently even on the same revision. And obviously things change if
> > you change revisions. Also if you see presumably a different bug, what
> > does it say regarding the original bug.
> Yes, but there are also several types of cases where the issue will be the
> same. Namely lockdep. I agree that use after free warnings can have a side
> effect, and may be more difficult.

But how do we know it's lockdep, rather than a use-after-free
manifested as lockdep?
A Significant portion of kernel bugs are caused by concurrency and can
manifest in different ways, e.g. these are not lockdep, or WARN, or
use-after-free, but rather a race in nature.

> But there's many other bugs that remain
> consistent across kernels. And if you stumble on one of them, look for it
> only.

For example? Does not look to be true for WARN, BUG, KASAN,
"inconsistent lock state".

> And if you hit another bug, and if it doesn't crash, then ignore it (of
> course this could be an issue if you have panic on warning set). But
> otherwise, just skip it.

It's not possible to skip, say, BUG.
And if we skip, say, a use-after-free, how do we know we are not
making things worse? Because now we are running on corrupted memory,
so anything can happen. Definitely a stray lockdep report can happen,
or other way around not happen when it should...

> > I would very much like to improve automatic bisection quality, but it
> > does not look trivial at all.
> >
> > Some random examples where, say, your hypothesis of WARN-to-WARN,
> > BUG-to-BUG does not hold even on the same kernel revision (add to this
> At least lockdep to lockdep, as when I do manual bisects, that's exactly
> what I look for, and ignore all other warnings. And that has found the
> problem commit pretty much every time.

What lockdep bug types do you mean? All?
In the examples above you can see at least "inconsistent lock state"
mixed with 2 other completely different bug types.

> > different revisions and the fact that a different bug does not give
> > info regarding the original bug):
> >
> Can you tell me that all these examples bisected to the commit that caused
> the bug? Because if it did not, then you may have just proved my point ;-)

I don't know now what was the result, but for a single run these were
manifestations of the same root bug.
E.g. see below, that's UAF in fuse_dev_do_read vs WARNING in
request_end. request_end is also fuse. And you can see that a memory
corruption causing a random bug type, in this case WARNING, but can as
well be LOCKDEP.

> > run #0: crashed: KASAN: use-after-free Read in fuse_dev_do_read
> > run #1: crashed: WARNING in request_end
> > run #2: crashed: KASAN: use-after-free Read in fuse_dev_do_read
> > run #3: OK
> > run #4: OK

  reply index

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-13 17:33 [syzbot] possible deadlock in del_gendisk syzbot
2021-04-13 17:41 ` Steven Rostedt
2021-04-13 17:43   ` Steven Rostedt
2021-04-13 18:24     ` Dmitry Vyukov
2021-04-13 18:40       ` Steven Rostedt
2021-04-13 18:43         ` Steven Rostedt
2021-04-16  7:51           ` Bisections with different bug manifestations Dmitry Vyukov
2021-04-16 13:13             ` Steven Rostedt
2021-04-16 13:26               ` Dmitry Vyukov [this message]
2021-04-16 13:48                 ` Dmitry Vyukov

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