[Dnsmasq-discuss] [PATCH] Set pointers to NULL after memory is freed

[Simon Kelley]

On 08/10/2023 09:44, renmingshuai via Dnsmasq-discuss wrote:
> Set pointers to NULL after memory is freed to reduce dangling pointers, 
> although they are later set to new values.
> 
>  From 5567d99099191f0cdb2922555e6ade2634f94f30 Mon Sep 17 00:00:00 2001
> 
> From: renmingshuai <renmingshuai at huawei.com>
> 
> Date: Sun, 8 Oct 2023 16:06:46 +0800
> 
> Subject: [PATCH] Set pointers to NULL after memory is freed
> 
> ---
> 
> src/option.c | 3 +++
> 
> 1 file changed, 3 insertions(+)
> 
> diff --git a/src/option.c b/src/option.c
> 
> index 286f06b..23601e1 100644
> 
> --- a/src/option.c
> 
> +++ b/src/option.c
> 
> @@ -5732,6 +5732,7 @@ static void clear_dynamic_conf(void)
> 
>         else
> 
>          up = &configs->next;
> 
>       }
> 
> +  daemon->dhcp_conf = NULL;
> 
> }
> 
> static void clear_dhcp_opt(struct dhcp_opt **dhcp_opts)
> 
> @@ -5755,8 +5756,10 @@ static void clear_dhcp_opt(struct dhcp_opt 
> **dhcp_opts)
> 
> static void clear_dynamic_opt(void)
> 
> {
> 
>     clear_dhcp_opt(&daemon->dhcp_opts);
> 
> +  daemon->dhcp_opts = NULL;
> 
> #ifdef HAVE_DHCP6
> 
>     clear_dhcp_opt(&daemon->dhcp_opts6);
> 
> +  daemon->dhcp_opts6 = NULL;
> 
> #endif
> 
> }
> 


This doesn't work. The daemon->dhcp_opts etc linked lists have a mixture 
of dynamic (re-readable) and static (from command line and dnsmasq.conf) 
entries. After clearing the re-readable entries, there may be static 
ones left in the linked list, so NULLing the pointers will leak those 
and stop them working.


Cheers,

Simon.

> --
> 
> 2.33.0
> 
> 
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