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Date:   Fri, 16 Jun 2023 22:44:07 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Liam Ni <zhiguangni01@gmail.com>
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
Subject: Re: [PATCH] mm/sparse:avoid null pointer access in memory_present()
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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 = __nr_to_section(section);
> -		if (!ms->section_mem_map) {
> +		if (ms && !ms->section_mem_map) {
>  			ms->section_mem_map = sparse_encode_early_nid(nid) |
>  							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?