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Message-ID: <b3eb050d-9451-4b60-b06c-ace7dab57497@embeddedor.com>
Date: Sat, 30 Aug 2025 15:30:11 +0200
From: "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To: 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: [RFC] cgroup: Avoid thousands of -Wflex-array-member-not-at-end
warnings
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 */
/* size: 2112, cachelines: 33, members: 43 */
/* sum members: 2000, holes: 3, sum holes: 56 */
/* padding: 56 */
/* paddings: 2, sum paddings: 8 */
/* forced alignments: 1 */
} __attribute__((__aligned__(64)));
The offset for `ancestors` is at 2056, but sizeof(struct group) == 2112 due
to the 56 bytes of tailing padding. This looks buggy. (thinkingface)
So, one solution for this is to use the TRAILING_OVERLAP() helper and
move these members at the end of `struct cgroup_root`. With this the
misalignment disappears (together with the 14722 warnings :) ), and now
both cgrp->ancestors[0] and cgrp_ancestor_storage share the same address
in memory. See below:
diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index 539c64eeef38..901a46f70a02 100644
--- a/include/linux/cgroup-defs.h
+++ b/include/linux/cgroup-defs.h
@@ -630,16 +630,6 @@ struct cgroup_root {
struct list_head root_list;
struct rcu_head rcu; /* Must be near the top */
- /*
- * The root cgroup. The containing cgroup_root will be destroyed on its
- * release. cgrp->ancestors[0] will be used overflowing into the
- * following field. cgrp_ancestor_storage must immediately follow.
- */
- struct cgroup cgrp;
-
- /* must follow cgrp for cgrp->ancestors[0], see above */
- struct cgroup *cgrp_ancestor_storage;
-
/* Number of cgroups in the hierarchy, used only for /proc/cgroups */
atomic_t nr_cgrps;
@@ -651,6 +641,18 @@ struct cgroup_root {
/* The name for this hierarchy - may be empty */
char name[MAX_CGROUP_ROOT_NAMELEN];
+
+ /*
+ * The root cgroup. The containing cgroup_root will be destroyed on its
+ * release. cgrp->ancestors[0] will be used overflowing into the
+ * following field. cgrp_ancestor_storage must immediately follow.
+ *
+ * Must be last --ends in a flexible-array members.
+ */
+ TRAILING_OVERLAP(struct cgroup, cgrp, ancestors,
+ /* must follow cgrp for cgrp->ancestors[0], see above */
+ struct cgroup *cgrp_ancestor_storage;
+ );
};
However, this causes the size of struct cgroup_root to increase from 6400
bytes to 16384 bytes due to struct cgroup to be aligned to page size 4096
bytes. See below:
struct cgroup_root {
struct kernfs_root * kf_root; /* 0 8 */
unsigned int subsys_mask; /* 8 4 */
int hierarchy_id; /* 12 4 */
struct list_head root_list; /* 16 16 */
struct callback_head rcu __attribute__((__aligned__(8))); /* 32 16 */
atomic_t nr_cgrps; /* 48 4 */
unsigned int flags; /* 52 4 */
char name[64]; /* 56 64 */
/* --- cacheline 1 boundary (64 bytes) was 56 bytes ago --- */
char release_agent_path[4096]; /* 120 4096 */
/* XXX 3976 bytes hole, try to pack */
/* --- cacheline 128 boundary (8192 bytes) --- */
union {
struct cgroup cgrp __attribute__((__aligned__(4096))); /* 8192 8192 */
struct {
unsigned char __offset_to_ancestors[5784]; /* 8192 5784 */
/* --- cacheline 218 boundary (13952 bytes) was 24 bytes ago --- */
struct cgroup * cgrp_ancestor_storage; /* 13976 8 */
}; /* 8192 5792 */
} __attribute__((__aligned__(4096))); /* 8192 8192 */
/* size: 16384, cachelines: 256, members: 10 */
/* sum members: 12408, holes: 1, sum holes: 3976 */
/* forced alignments: 2, forced holes: 1, sum forced holes: 3976 */
} __attribute__((__aligned__(4096)));
I've tried with the struct_group_tagged()/container_of() technique:
https://git.kernel.org/pub/scm/linux/kernel/git/gustavoars/linux.git/commit/?h=testing/wfamnae-next20250723&id=03da6b0772af1a62778400f26fe57796fe1ebf27
but cgroup_root grows up to 20K in this case.
So, I guess my question here is... what do you think?... (thinkingface)
Thanks!
-Gustavo
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