linux-hardening - Re: [PATCH v3][next] cgroup: Avoid -Wstringop-overflow warnings

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Date:   Thu, 17 Aug 2023 12:44:35 -0700
From:   Kees Cook <keescook@...omium.org>
To:     "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc:     Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v3][next] cgroup: Avoid -Wstringop-overflow warnings

On Thu, Aug 17, 2023 at 11:19:13AM -0600, Gustavo A. R. Silva wrote:
> Change the notation from pointer-to-array to pointer-to-pointer.
> With this, we avoid the compiler complaining about trying
> to access a region of size zero as an argument during function
> calls.
> 
> This is a workaround to prevent the compiler complaining about
> accessing an array of size zero when evaluating the arguments
> of a couple of function calls. See below:
> 
> kernel/cgroup/cgroup.c: In function 'find_css_set':
> kernel/cgroup/cgroup.c:1206:16: warning: 'find_existing_css_set' accessing 4 bytes in a region of size 0 [-Wstringop-overflow=]
>  1206 |         cset = find_existing_css_set(old_cset, cgrp, template);
>       |                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> kernel/cgroup/cgroup.c:1206:16: note: referencing argument 3 of type 'struct cgroup_subsys_state *[0]'
> kernel/cgroup/cgroup.c:1071:24: note: in a call to function 'find_existing_css_set'
>  1071 | static struct css_set *find_existing_css_set(struct css_set *old_cset,
>       |                        ^~~~~~~~~~~~~~~~~~~~~
> 
> With the change to pointer-to-pointer, the functions are not prevented
> from being executed, and they will do what they have to do when
> CGROUP_SUBSYS_COUNT == 0.
> 
> Address the following -Wstringop-overflow warnings seen when
> built with ARM architecture and aspeed_g4_defconfig configuration
> (notice that under this configuration CGROUP_SUBSYS_COUNT == 0):
> 
> kernel/cgroup/cgroup.c:1208:16: warning: 'find_existing_css_set' accessing 4 bytes in a region of size 0 [-Wstringop-overflow=]
> kernel/cgroup/cgroup.c:1258:15: warning: 'css_set_hash' accessing 4 bytes in a region of size 0 [-Wstringop-overflow=]
> kernel/cgroup/cgroup.c:6089:18: warning: 'css_set_hash' accessing 4 bytes in a region of size 0 [-Wstringop-overflow=]
> kernel/cgroup/cgroup.c:6153:18: warning: 'css_set_hash' accessing 4 bytes in a region of size 0 [-Wstringop-overflow=]
> 
> This results in no differences in binary output.
> 
> Link: https://github.com/KSPP/linux/issues/316
> Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>

Reviewed-by: Kees Cook <keescook@...omium.org>

-- 
Kees Cook