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

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Date:   Thu, 22 Jun 2023 11:11:42 +0100
From:   Steven Price <steven.price@....com>
To:     "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>
Cc:     cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org, Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH][next] cgroup: Avoid -Wstringop-overflow warnings

On 15/06/2023 02:18, Gustavo A. R. Silva wrote:
> 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=]
> 
> These changes are based on commit d20d30ebb199 ("cgroup: Avoid compiler
> warnings with no subsystems").
> 
> Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
> ---
>  kernel/cgroup/cgroup.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
> index cd497b90e11a..1ee76e62eb98 100644
> --- a/kernel/cgroup/cgroup.c
> +++ b/kernel/cgroup/cgroup.c
> @@ -1200,6 +1200,9 @@ static struct css_set *find_css_set(struct css_set *old_cset,
>  	unsigned long key;
>  	int ssid;
>  
> +	if (!CGROUP_HAS_SUBSYS_CONFIG)
> +		return NULL;
> +
>  	lockdep_assert_held(&cgroup_mutex);
>  
>  	/* First see if we already have a cgroup group that matches
> @@ -6045,6 +6048,9 @@ int __init cgroup_init(void)
>  	struct cgroup_subsys *ss;
>  	int ssid;
>  
> +	if (!CGROUP_HAS_SUBSYS_CONFIG)
> +		return -EINVAL;
> +
>  	BUILD_BUG_ON(CGROUP_SUBSYS_COUNT > 16);
>  	BUG_ON(cgroup_init_cftypes(NULL, cgroup_base_files));
>  	BUG_ON(cgroup_init_cftypes(NULL, cgroup_psi_files));

This change (which landed in linux-next) causes a boot failure on my
(arm32) board because the cgroup filesystem isn't created which upsets
systemd:

[   11.474767] systemd[1]: Failed to mount tmpfs at /sys/fs/cgroup: No such file or directory
[   11.489933] systemd[1]: Failed to mount cgroup at /sys/fs/cgroup/systemd: No such file or directory
[!!!!!!] Failed to mount API filesystems.

Reverting this commit on the head of linux-next gets the board working
again.

Thanks,

Steve