linux-kernel - Re: [PATCH] mm: add NULL check to avoid potential NULL pointer dereference

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Date:   Wed, 31 May 2017 17:09:23 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     "Gustavo A. R. Silva" <garsilva@...eddedor.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: add NULL check to avoid potential NULL pointer
 dereference

On Tue 30-05-17 16:24:36, Gustavo A. R. Silva wrote:
> NULL check at line 1226: if (!pgdat), implies that pointer pgdat
> might be NULL.
> Function rollback_node_hotadd() dereference this pointer.
> Add NULL check to avoid a potential NULL pointer dereference.

The changelog is quite cryptic to be honest. Well the code is as well
but what do you say about the following replacement.

"
If a new pgdat has to be allocated in add_memory_resource
and the initialization fails for some reason we have to
rollback_node_hotadd. This, however, assumes that pgdat allocation
itself is successful which cannot be assumed. Add a check for pgdat
to cover that case and skip rollback_node_hotadd altogether because
there is nothing to roll back.

This has been pointed out by coverity.
"
> 
> Addresses-Coverity-ID: 1369133
> Signed-off-by: Gustavo A. R. Silva <garsilva@...eddedor.com>

Acked-by: Michal Hocko <mhocko@...e.com>

> ---
>  mm/memory_hotplug.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 599c675..ea3bc3e 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1273,7 +1273,7 @@ int __ref add_memory_resource(int nid, struct resource *res, bool online)
>  
>  error:
>  	/* rollback pgdat allocation and others */
> -	if (new_pgdat)
> +	if (new_pgdat && pgdat)
>  		rollback_node_hotadd(nid, pgdat);
>  	memblock_remove(start, size);
>  
> -- 
> 2.5.0
> 
> --
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-- 
Michal Hocko
SUSE Labs