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Message-ID: <6ailuetf6jt66q2suzlnqo6ersf4tdqfllgltw6ef3gulvvd64@v2rg6wkulzrc>
Date: Mon, 1 Sep 2025 20:04:50 +0200
From: Michal Koutný <mkoutny@...e.com>
To: "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Cc: Tejun Heo <tj@...nel.org>, Johannes Weiner <hannes@...xchg.org>,
cgroups@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
linux-hardening@...r.kernel.org, "Gustavo A. R. Silva" <gustavoars@...nel.org>,
Chen Ridong <chenridong@...weicloud.com>
Subject: Re: [RFC] cgroup: Avoid thousands of -Wflex-array-member-not-at-end
warnings
On Mon, Sep 01, 2025 at 05:44:38PM +0200, "Gustavo A. R. Silva" <gustavo@...eddedor.com> wrote:
> Oh, a correction about this. Actually, if we need to use __packed, we would
> have to pass it as an argument to TRAILING_OVERLAP(), like this:
>
> -#define TRAILING_OVERLAP(TYPE, NAME, FAM, MEMBERS) \
> +#define TRAILING_OVERLAP(TYPE, NAME, FAM, MEMBERS, ATTRS) \
> union { \
> TYPE NAME; \
> struct { \
> unsigned char __offset_to_##FAM[offsetof(TYPE, FAM)]; \
> MEMBERS \
> - }; \
> + } ATTRS; \
> }
>
> However, in this case MEMBERS is only cgrp_ancestor_storage, and it's correctly
> aligned to __offset_to_##FAM[offsetof(TYPE, FAM)]; inside the helper. So, we
> don't really need to pack that internal struct.
My intention with the attribute was to prevent a gap (padding) occurring
between
unsigned char __offset_to_##FAM
and
MEMBERS
which would make the address of the first member to mismatch the address
of FAM (the example in struct cgroup_root notwithstanding).
(But perhaps it's guaranteed that first member's offset in the struct is
always equal to offsetof(TYPE, FAM).)
Michal
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