linux-kernel - Re: kmemleak memory scanning

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Message-ID: <YMpCjmCrhPwpCiid@nuc10>
Date:   Wed, 16 Jun 2021 11:27:26 -0700
From:   Rustam Kovhaev <rkovhaev@...il.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, dvyukov@...gle.com,
        gregkh@...uxfoundation.org
Subject: Re: kmemleak memory scanning

On Tue, Jun 15, 2021 at 10:12:06AM +0200, David Hildenbrand wrote:
> On 14.06.21 22:31, Rustam Kovhaev wrote:
> > hello Catalin, Andrew!
> > 
> > while troubleshooting a false positive syzbot kmemleak report i have
> > noticed an interesting behavior in kmemleak and i wonder whether it is
> > behavior by design and should be documented, or maybe something to
> > improve.
> 
> Hi,
> 
> See below regarding documentation.
> 
> > apologies if some of the questions do not make sense, i am still going
> > through kmemleak code..
> > 
> > a) kmemleak scans struct page (kmemleak.c:1462), but it does not scan
> > the actual contents (page_address(page)) of the page.
> > if we allocate an object with kmalloc(), then allocate page with
> > alloc_page(), and if we put kmalloc pointer somewhere inside that page,
> > kmemleak will report kmalloc pointer as a false positive.
> > should we improve kmemleak and make it scan page contents?
> > or will this bring too many false negatives?
> 
> I looked into this a while ago to see which parts of the kernel end up
> reading random physical page content and was happy to see that kmemleak does
> *not* scan random physical memory :)
> 
> We have to be very careful when reading random physical page content,
> especially in virt environments this is really undesired, or when dealing
> with memory holes, memory with problematic semantics like gart memory ...

i see, makes sense, thank you for the info!

> 
> The doc (Documentation/dev-tools/kmemleak.rst) states "Page allocations and
> ioremap are not tracked.", which includes the alloc_page() example you gave
> I think.

i see it now, ty!