linux-kernel - Re: [syzbot] memory leak in xas

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Date:   Thu, 14 Jul 2022 17:27:28 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        syzbot <syzbot+a785d07959bc94837d51@...kaller.appspotmail.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        syzkaller-bugs@...glegroups.com, Zach O'Keefe <zokeefe@...gle.com>,
        Yang Shi <shy828301@...il.com>,
        Liam Howlett <liam.howlett@...cle.com>
Subject: Re: [syzbot] memory leak in xas_create

On Tue, Jul 12, 2022 at 03:29:29PM +0200, Dmitry Vyukov wrote:
> Then I think it's still possible that this is a KMEMLEAK false
> positive. IIRC it may have some false positives since it does not do
> full stop-the-world before scanning memory/registers. syzkaller tries
> to circumvent this by doing multiple scans with some delays, but it
> does not give 100% guarantee.
> And I am assuming this code does not try to hide pointers by storing
> something in low/high bits, etc.

Oh, I meant to answer this.  The XArray does set bit 1 of the pointer
when it's stored in the tree.  However, this shouldn't affect kmemleak
(I would think) because it looks like a pointer to the third byte of the
allocation, so the allocation is still referenced, even if the first
byte of the allocation isn't referenced.

Also, I would expect kmemleak to report bugs all over if this were the
problem, because every node no matter how it's allocated gets its bit 1
set.