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Subject[PATCH v5 2/3] cgroups: add can_attach callback for checking all threads in a group
Add cgroup wrapper for safely calling can_attach on all threads in a threadgroup

From: Ben Blum <bblum@andrew.cmu.edu>

This patch adds a function cgroup_can_attach_per_thread which handles iterating
over each thread in a threadgroup safely with respect to the invariants that
will be used in cgroup_attach_proc. Also, subsystems whose can_attach calls
require per-thread validation are modified to use the per_thread wrapper to
avoid duplicating cgroup-internal code.

This is a pre-patch for cgroup-procs-writable.patch.

Signed-off-by: Ben Blum <bblum@andrew.cmu.edu>
---
include/linux/cgroup.h | 12 ++++++++++++
kernel/cgroup.c | 35 +++++++++++++++++++++++++++++++++++
kernel/cgroup_freezer.c | 27 ++++++++++++---------------
kernel/cpuset.c | 20 +++++++-------------
kernel/ns_cgroup.c | 27 +++++++++++++--------------
kernel/sched.c | 21 ++++++---------------
6 files changed, 85 insertions(+), 57 deletions(-)

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index e3d00fd..f040d66 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -580,6 +580,18 @@ int cgroup_scan_tasks(struct cgroup_scanner *scan);
int cgroup_attach_task(struct cgroup *, struct task_struct *);

/*
+ * For use in subsystems whose can_attach functions need to run an operation
+ * on every task in the threadgroup. Calls the given callback once if the
+ * 'threadgroup' flag is false, or once per thread in the group if true.
+ * The callback should return 0/-ERR; this will return 0/-ERR.
+ * The callback will run within an rcu_read section, so must not sleep.
+ */
+int cgroup_can_attach_per_thread(struct cgroup *cgrp, struct task_struct *task,
+ int (*cb)(struct cgroup *cgrp,
+ struct task_struct *task),
+ bool threadgroup);
+
+/*
* CSS ID is ID for cgroup_subsys_state structs under subsys. This only works
* if cgroup_subsys.use_id == true. It can be used for looking up and scanning.
* CSS ID is assigned at cgroup allocation (create) automatically
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index f91d7dd..e8b8f71 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1688,6 +1688,41 @@ int cgroup_path(const struct cgroup *cgrp, char *buf, int buflen)
}
EXPORT_SYMBOL_GPL(cgroup_path);

+int cgroup_can_attach_per_thread(struct cgroup *cgrp, struct task_struct *task,
+ int (*cb)(struct cgroup *cgrp,
+ struct task_struct *task),
+ bool threadgroup)
+{
+ /* Start by running on the leader, in all cases. */
+ int ret = cb(cgrp, task);
+ if (ret < 0)
+ return ret;
+
+ if (threadgroup) {
+ /* Run on each task in the threadgroup. */
+ struct task_struct *c;
+ rcu_read_lock();
+ /*
+ * It is necessary for the given task to still be the leader
+ * to safely traverse thread_group. See cgroup_attach_proc.
+ */
+ if (!thread_group_leader(task)) {
+ rcu_read_unlock();
+ return -EAGAIN;
+ }
+ list_for_each_entry_rcu(c, &task->thread_group, thread_group) {
+ ret = cb(cgrp, c);
+ if (ret < 0) {
+ rcu_read_unlock();
+ return ret;
+ }
+ }
+ rcu_read_unlock();
+ }
+ return 0;
+}
+EXPORT_SYMBOL_GPL(cgroup_can_attach_per_thread);
+
/**
* cgroup_attach_task - attach task 'tsk' to cgroup 'cgrp'
* @cgrp: the cgroup the task is attaching to
diff --git a/kernel/cgroup_freezer.c b/kernel/cgroup_freezer.c
index ce71ed5..677b24e 100644
--- a/kernel/cgroup_freezer.c
+++ b/kernel/cgroup_freezer.c
@@ -161,6 +161,13 @@ static bool is_task_frozen_enough(struct task_struct *task)
(task_is_stopped_or_traced(task) && freezing(task));
}

+static int freezer_can_attach_cb(struct cgroup *cgrp, struct task_struct *task)
+{
+ if (is_task_frozen_enough(task))
+ return -EBUSY;
+ return 0;
+}
+
/*
* The call to cgroup_lock() in the freezer.state write method prevents
* a write to that file racing against an attach, and hence the
@@ -171,6 +178,7 @@ static int freezer_can_attach(struct cgroup_subsys *ss,
struct task_struct *task, bool threadgroup)
{
struct freezer *freezer;
+ int ret;

/*
* Anything frozen can't move or be moved to/from.
@@ -179,26 +187,15 @@ static int freezer_can_attach(struct cgroup_subsys *ss,
* frozen, so it's sufficient to check the latter condition.
*/

- if (is_task_frozen_enough(task))
- return -EBUSY;
+ ret = cgroup_can_attach_per_thread(new_cgroup, task,
+ freezer_can_attach_cb, threadgroup);
+ if (ret < 0)
+ return ret;

freezer = cgroup_freezer(new_cgroup);
if (freezer->state == CGROUP_FROZEN)
return -EBUSY;

- if (threadgroup) {
- struct task_struct *c;
-
- rcu_read_lock();
- list_for_each_entry_rcu(c, &task->thread_group, thread_group) {
- if (is_task_frozen_enough(c)) {
- rcu_read_unlock();
- return -EBUSY;
- }
- }
- rcu_read_unlock();
- }
-
return 0;
}

diff --git a/kernel/cpuset.c b/kernel/cpuset.c
index b23c097..cc4b1f7 100644
--- a/kernel/cpuset.c
+++ b/kernel/cpuset.c
@@ -1376,6 +1376,11 @@ static int fmeter_getrate(struct fmeter *fmp)
/* Protected by cgroup_lock */
static cpumask_var_t cpus_attach;

+static int cpuset_can_attach_cb(struct cgroup *cgrp, struct task_struct *task)
+{
+ return security_task_setscheduler(task, 0, NULL);
+}
+
/* Called by cgroups to determine if a cpuset is usable; cgroup_mutex held */
static int cpuset_can_attach(struct cgroup_subsys *ss, struct cgroup *cont,
struct task_struct *tsk, bool threadgroup)
@@ -1397,22 +1402,11 @@ static int cpuset_can_attach(struct cgroup_subsys *ss, struct cgroup *cont,
if (tsk->flags & PF_THREAD_BOUND)
return -EINVAL;

- ret = security_task_setscheduler(tsk, 0, NULL);
+ ret = cgroup_can_attach_per_thread(cont, tsk, cpuset_can_attach_cb,
+ threadgroup);
if (ret)
return ret;
- if (threadgroup) {
- struct task_struct *c;

- rcu_read_lock();
- list_for_each_entry_rcu(c, &tsk->thread_group, thread_group) {
- ret = security_task_setscheduler(c, 0, NULL);
- if (ret) {
- rcu_read_unlock();
- return ret;
- }
- }
- rcu_read_unlock();
- }
return 0;
}

diff --git a/kernel/ns_cgroup.c b/kernel/ns_cgroup.c
index 2a5dfec..af0accf 100644
--- a/kernel/ns_cgroup.c
+++ b/kernel/ns_cgroup.c
@@ -42,9 +42,18 @@ int ns_cgroup_clone(struct task_struct *task, struct pid *pid)
* (hence either you are in the same cgroup as task, or in an
* ancestor cgroup thereof)
*/
+static int ns_can_attach_cb(struct cgroup *new_cgroup, struct task_struct *task)
+{
+ if (!cgroup_is_descendant(new_cgroup, task))
+ return -EPERM;
+ return 0;
+}
+
static int ns_can_attach(struct cgroup_subsys *ss, struct cgroup *new_cgroup,
struct task_struct *task, bool threadgroup)
{
+ int ret;
+
if (current != task) {
if (!capable(CAP_SYS_ADMIN))
return -EPERM;
@@ -53,20 +62,10 @@ static int ns_can_attach(struct cgroup_subsys *ss, struct cgroup *new_cgroup,
return -EPERM;
}

- if (!cgroup_is_descendant(new_cgroup, task))
- return -EPERM;
-
- if (threadgroup) {
- struct task_struct *c;
- rcu_read_lock();
- list_for_each_entry_rcu(c, &task->thread_group, thread_group) {
- if (!cgroup_is_descendant(new_cgroup, c)) {
- rcu_read_unlock();
- return -EPERM;
- }
- }
- rcu_read_unlock();
- }
+ ret = cgroup_can_attach_per_thread(new_cgroup, task, ns_can_attach_cb,
+ threadgroup);
+ if (ret < 0)
+ return ret;

return 0;
}
diff --git a/kernel/sched.c b/kernel/sched.c
index 70fa78d..8330e6f 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -8715,21 +8715,12 @@ static int
cpu_cgroup_can_attach(struct cgroup_subsys *ss, struct cgroup *cgrp,
struct task_struct *tsk, bool threadgroup)
{
- int retval = cpu_cgroup_can_attach_task(cgrp, tsk);
- if (retval)
- return retval;
- if (threadgroup) {
- struct task_struct *c;
- rcu_read_lock();
- list_for_each_entry_rcu(c, &tsk->thread_group, thread_group) {
- retval = cpu_cgroup_can_attach_task(cgrp, c);
- if (retval) {
- rcu_read_unlock();
- return retval;
- }
- }
- rcu_read_unlock();
- }
+ int ret = cgroup_can_attach_per_thread(cgrp, tsk,
+ cpu_cgroup_can_attach_task,
+ threadgroup);
+ if (ret)
+ return ret;
+
return 0;
}


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