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Message-Id: <20210828000255.3213497-1-keescook@chromium.org>
Date: Fri, 27 Aug 2021 17:02:55 -0700
From: Kees Cook <keescook@...omium.org>
To: Tejun Heo <tj@...nel.org>
Cc: Kees Cook <keescook@...omium.org>,
Stephen Rothwell <sfr@...b.auug.org.au>,
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: [PATCH v3] cgroup: Avoid compiler warnings with no subsystems
As done before in commit cb4a31675270 ("cgroup: use bitmask to filter
for_each_subsys"), avoid compiler warnings for the pathological case of
having no subsystems (i.e. CGROUP_SUBSYS_COUNT == 0). This condition is
hit for the arm multi_v7_defconfig config under -Wzero-length-bounds:
In file included from ./arch/arm/include/generated/asm/rwonce.h:1,
from include/linux/compiler.h:264,
from include/uapi/linux/swab.h:6,
from include/linux/swab.h:5,
from arch/arm/include/asm/opcodes.h:86,
from arch/arm/include/asm/bug.h:7,
from include/linux/bug.h:5,
from include/linux/thread_info.h:13,
from include/asm-generic/current.h:5,
from ./arch/arm/include/generated/asm/current.h:1,
from include/linux/sched.h:12,
from include/linux/cgroup.h:12,
from kernel/cgroup/cgroup-internal.h:5,
from kernel/cgroup/cgroup.c:31:
kernel/cgroup/cgroup.c: In function 'of_css':
kernel/cgroup/cgroup.c:651:42: warning: array subscript '<unknown>' is outside the bounds of an
interior zero-length array 'struct cgroup_subsys_state *[0]' [-Wzero-length-bounds]
651 | return rcu_dereference_raw(cgrp->subsys[cft->ss->id]);
Reported-by: Stephen Rothwell <sfr@...b.auug.org.au>
Cc: Tejun Heo <tj@...nel.org>
Cc: Zefan Li <lizefan.x@...edance.com>
Cc: Johannes Weiner <hannes@...xchg.org>
Cc: cgroups@...r.kernel.org
Signed-off-by: Kees Cook <keescook@...omium.org>
---
v3:
- encapsulate test in a macro with a big comment (tj)
v2: https://lore.kernel.org/lkml/20210827150424.2729274-1-keescook@chromium.org
v1: https://lore.kernel.org/lkml/20210827034741.2214318-1-keescook@chromium.org
---
kernel/cgroup/cgroup.c | 26 ++++++++++++++++++++------
1 file changed, 20 insertions(+), 6 deletions(-)
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 3a0161c21b6b..b320cace5342 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -67,6 +67,14 @@
/* let's not notify more than 100 times per second */
#define CGROUP_FILE_NOTIFY_MIN_INTV DIV_ROUND_UP(HZ, 100)
+/*
+ * To avoid confusing the compiler (and generating warnings) with code
+ * that attempts to access what would be a 0-element array (i.e. sized
+ * to a potentially empty array when CGROUP_SUBSYS_COUNT == 0), this
+ * constant expression can be added.
+ */
+#define CGROUP_HAS_SUBSYS_CONFIG (CGROUP_SUBSYS_COUNT > 0)
+
/*
* cgroup_mutex is the master lock. Any modification to cgroup or its
* hierarchy must be performed while holding it.
@@ -248,7 +256,7 @@ static int cgroup_addrm_files(struct cgroup_subsys_state *css,
*/
bool cgroup_ssid_enabled(int ssid)
{
- if (CGROUP_SUBSYS_COUNT == 0)
+ if (!CGROUP_HAS_SUBSYS_CONFIG)
return false;
return static_key_enabled(cgroup_subsys_enabled_key[ssid]);
@@ -472,7 +480,7 @@ static u16 cgroup_ss_mask(struct cgroup *cgrp)
static struct cgroup_subsys_state *cgroup_css(struct cgroup *cgrp,
struct cgroup_subsys *ss)
{
- if (ss)
+ if (CGROUP_HAS_SUBSYS_CONFIG && ss)
return rcu_dereference_check(cgrp->subsys[ss->id],
lockdep_is_held(&cgroup_mutex));
else
@@ -550,6 +558,9 @@ struct cgroup_subsys_state *cgroup_e_css(struct cgroup *cgrp,
{
struct cgroup_subsys_state *css;
+ if (!CGROUP_HAS_SUBSYS_CONFIG)
+ return NULL;
+
do {
css = cgroup_css(cgrp, ss);
@@ -577,6 +588,9 @@ struct cgroup_subsys_state *cgroup_get_e_css(struct cgroup *cgrp,
{
struct cgroup_subsys_state *css;
+ if (!CGROUP_HAS_SUBSYS_CONFIG)
+ return NULL;
+
rcu_read_lock();
do {
@@ -647,7 +661,7 @@ struct cgroup_subsys_state *of_css(struct kernfs_open_file *of)
* the matching css from the cgroup's subsys table is guaranteed to
* be and stay valid until the enclosing operation is complete.
*/
- if (cft->ss)
+ if (CGROUP_HAS_SUBSYS_CONFIG && cft->ss)
return rcu_dereference_raw(cgrp->subsys[cft->ss->id]);
else
return &cgrp->self;
@@ -695,7 +709,7 @@ EXPORT_SYMBOL_GPL(of_css);
*/
#define do_each_subsys_mask(ss, ssid, ss_mask) do { \
unsigned long __ss_mask = (ss_mask); \
- if (!CGROUP_SUBSYS_COUNT) { /* to avoid spurious gcc warning */ \
+ if (!CGROUP_HAS_SUBSYS_CONFIG) { \
(ssid) = 0; \
break; \
} \
@@ -2373,7 +2387,7 @@ struct task_struct *cgroup_taskset_next(struct cgroup_taskset *tset,
struct css_set *cset = tset->cur_cset;
struct task_struct *task = tset->cur_task;
- while (&cset->mg_node != tset->csets) {
+ while (CGROUP_HAS_SUBSYS_CONFIG && &cset->mg_node != tset->csets) {
if (!task)
task = list_first_entry(&cset->mg_tasks,
struct task_struct, cg_list);
@@ -4644,7 +4658,7 @@ void css_task_iter_start(struct cgroup_subsys_state *css, unsigned int flags,
it->ss = css->ss;
it->flags = flags;
- if (it->ss)
+ if (CGROUP_HAS_SUBSYS_CONFIG && it->ss)
it->cset_pos = &css->cgroup->e_csets[css->ss->id];
else
it->cset_pos = &css->cgroup->cset_links;
--
2.30.2
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