linux-kernel - Re: [PATCH v2 2/3] mm: don't hide potentially null memmap pointer in sparse_remove_one

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Message-ID: <20190626065935.GL17798@dhcp22.suse.cz>
Date:   Wed, 26 Jun 2019 08:59:35 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Alastair D'Silva <alastair@...ilva.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Pavel Tatashin <pasha.tatashin@...cle.com>,
        Oscar Salvador <osalvador@...e.de>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Baoquan He <bhe@...hat.com>, Qian Cai <cai@....pw>,
        Logan Gunthorpe <logang@...tatee.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v2 2/3] mm: don't hide potentially null memmap pointer in
 sparse_remove_one_section

On Wed 26-06-19 16:30:55, Alastair D'Silva wrote:
> On Wed, 2019-06-26 at 08:23 +0200, Michal Hocko wrote:
> > On Wed 26-06-19 16:11:22, Alastair D'Silva wrote:
> > > From: Alastair D'Silva <alastair@...ilva.org>
> > > 
> > > By adding offset to memmap before passing it in to
> > > clear_hwpoisoned_pages,
> > > we hide a potentially null memmap from the null check inside
> > > clear_hwpoisoned_pages.
> > > 
> > > This patch passes the offset to clear_hwpoisoned_pages instead,
> > > allowing
> > > memmap to successfully peform it's null check.
> > 
> > Same issue with the changelog as the previous patch (missing WHY).
> > 
> 
> The first paragraph explains what the problem is with the existing code
> (same applies to 1/3 too).

Under what conditions that happens? Is this a theoretical problem or can
you hit this by a (buggy) code? Please be much more specific.

-- 
Michal Hocko
SUSE Labs