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References: <20230617044036.3985524-1-zhiguangni01@gmail.com> <20230616224407.863c74a3dc9d4f1427802f91@linux-foundation.org>
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From:   Liam Ni <zhiguangni01@gmail.com>
Date:   Sat, 17 Jun 2023 14:17:58 +0800
Message-ID: <CACZJ9cXM9VkJ5=euHphwM5TtX3aZqZ_QynOq10FtrqCxZfZsQA@mail.gmail.com>
Subject: Re: [PATCH] mm/sparse:avoid null pointer access in memory_present()
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     dave.hansen@linux.intel.com, luto@kernel.org, peterz@infradead.org,
        tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, x86@kernel.org,
        hpa@zytor.com, rppt@kernel.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
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On Sat, 17 Jun 2023 at 13:44, Andrew Morton <akpm@linux-foundation.org> wro=
te:
>
> On Sat, 17 Jun 2023 14:40:36 +1000 Liam Ni <zhiguangni01@gmail.com> wrote=
:
>
> > __nr_to_section() may return a null pointer,
> > before accessing the member variable section_mem_map,
> > we should first determine whether it is a null pointer.
> >
> > ...
> >
> > --- a/mm/sparse.c
> > +++ b/mm/sparse.c
> > @@ -258,7 +258,7 @@ static void __init memory_present(int nid, unsigned=
 long start, unsigned long en
> >               set_section_nid(section, nid);
> >
> >               ms =3D __nr_to_section(section);
> > -             if (!ms->section_mem_map) {
> > +             if (ms && !ms->section_mem_map) {
> >                       ms->section_mem_map =3D sparse_encode_early_nid(n=
id) |
> >                                                       SECTION_IS_ONLINE=
;
> >                       __section_mark_present(ms, section);
>
> I'm suspecting that if __nr_to_section() returns NULL here, we should
> just panic.  But a null-deref gives the same information, so why change
> things?

Do you mean if ms is a null pointer=EF=BC=8Cms->section_mem_map will cause
system panic,so we needn't change?

>