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Message-ID: <65a443d1-be01-4e5b-9086-b4ce2746ab63@embeddedor.com>
Date: Mon, 1 Sep 2025 09:44:27 +0200
From: "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To: Chen Ridong <chenridong@...weicloud.com>, Tejun Heo <tj@...nel.org>,
Johannes Weiner <hannes@...xchg.org>, Michal Koutný
<mkoutny@...e.com>
Cc: cgroups@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
linux-hardening@...r.kernel.org, "Gustavo A. R. Silva"
<gustavoars@...nel.org>
Subject: Re: [RFC] cgroup: Avoid thousands of -Wflex-array-member-not-at-end
warnings
On 9/1/25 03:29, Chen Ridong wrote:
>
>
> On 2025/8/30 21:30, Gustavo A. R. Silva wrote:
>> Hi all,
>>
>> I'm working on enabling -Wflex-array-member-not-at-end in mainline, and
>> I ran into thousands (yes, 14722 to be precise) of these warnings caused
>> by an instance of `struct cgroup` in the middle of `struct cgroup_root`.
>> See below:
>>
>> 620 struct cgroup_root {
>> ...
>> 633 /*
>> 634 * The root cgroup. The containing cgroup_root will be destroyed on its
>> 635 * release. cgrp->ancestors[0] will be used overflowing into the
>> 636 * following field. cgrp_ancestor_storage must immediately follow.
>> 637 */
>> 638 struct cgroup cgrp;
>> 639
>> 640 /* must follow cgrp for cgrp->ancestors[0], see above */
>> 641 struct cgroup *cgrp_ancestor_storage;
>> ...
>> };
>>
>> Based on the comments above, it seems that the original code was expecting
>> cgrp->ancestors[0] and cgrp_ancestor_storage to share the same addres in
>> memory.
>>
>> However when I take a look at the pahole output, I see that these two members
>> are actually misaligned by 56 bytes. See below:
>>
>> struct cgroup_root {
>> ...
>>
>> /* --- cacheline 1 boundary (64 bytes) --- */
>> struct cgroup cgrp __attribute__((__aligned__(64))); /* 64 2112 */
>>
>> /* XXX last struct has 56 bytes of padding */
>>
>> /* --- cacheline 34 boundary (2176 bytes) --- */
>> struct cgroup * cgrp_ancestor_storage; /* 2176 8 */
>>
>> ...
>>
>> /* size: 6400, cachelines: 100, members: 11 */
>> /* sum members: 6336, holes: 1, sum holes: 16 */
>> /* padding: 48 */
>> /* paddings: 1, sum paddings: 56 */
>> /* forced alignments: 1, forced holes: 1, sum forced holes: 16 */
>> } __attribute__((__aligned__(64)));
>>
>> This is due to the fact that struct cgroup have some tailing padding after
>> flexible-array member `ancestors` due to alignment to 64 bytes, see below:
>>
>> struct cgroup {
>> ...
>>
>> struct cgroup * ancestors[]; /* 2056 0 */
>>
>
> Instead of using a flexible array member, could we convert this to a pointer and handle the memory
> allocation explicitly?
>
Yep, that's always an option. However, I also wanted to see what people
think about the current misalignment between cgrp->ancestors[0] and
cgrp_ancestor_storage I describe above.
And if the heap allocation is an acceptable solution in this case, I'm
happy to go that route.
Thanks
-Gustavo
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