linux-kernel - Re: [PATCH v8 13/19] locking/rwsem: Make rwsem->owner an atomic_long

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Date:   Fri, 19 Jul 2019 20:45:32 +0100
From:   Luis Henriques <lhenriques@...e.com>
To:     Waiman Long <longman@...hat.com>
Cc:     Borislav Petkov <bp@...en8.de>, Will Deacon <will.deacon@....com>,
        "huang ying" <huang.ying.caritas@...il.com>,
        Peter Zijlstra <peterz@...radead.org>, <x86@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        "Ingo Molnar" <mingo@...hat.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        <linux-kernel@...r.kernel.org>, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v8 13/19] locking/rwsem: Make rwsem->owner an atomic_long_t

Waiman Long <longman@...hat.com> writes:

> On 7/19/19 2:45 PM, Luis Henriques wrote:
>> On Mon, May 20, 2019 at 04:59:12PM -0400, Waiman Long wrote:
>>> The rwsem->owner contains not just the task structure pointer, it also
>>> holds some flags for storing the current state of the rwsem. Some of
>>> the flags may have to be atomically updated. To reflect the new reality,
>>> the owner is now changed to an atomic_long_t type.
>>>
>>> New helper functions are added to properly separate out the task
>>> structure pointer and the embedded flags.
>> I started seeing KASAN use-after-free with current master, and a bisect
>> showed me that this commit 94a9717b3c40 ("locking/rwsem: Make
>> rwsem->owner an atomic_long_t") was the problem.  Does it ring any
>> bells?  I can easily reproduce it with xfstests (generic/464).
>>
>> Cheers,
>> --
>> Luís
>
> This patch shouldn't change the behavior of the rwsem code. The code
> only access data within the rw_semaphore structures. I don't know why it
> will cause a KASAN error. I will have to reproduce it and figure out
> exactly which statement is doing the invalid access.

Yeah, screwing the bisection is something I've done in the past so I may
have got the wrong commit.  Another detail is that I was running
xfstests against CephFS, I didn't tried with any other filesystem.  I
can try to reproduce with btrfs or xfs next week.

Cheers,
-- 
Luis