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Message-Id: <1426307835-5893-3-git-send-email-cyphar@cyphar.com>
Date: Sat, 14 Mar 2015 15:37:14 +1100
From: Aleksa Sarai <cyphar@...har.com>
To: tj@...nel.org, lizefan@...wei.com, mingo@...hat.com,
peterz@...radead.org
Cc: richard@....at, fweisbec@...il.com, linux-kernel@...r.kernel.org,
cgroups@...r.kernel.org, Aleksa Sarai <cyphar@...har.com>
Subject: [PATCH v6 2/3] cgroups: allow a cgroup subsystem to reject a fork
Add a new cgroup subsystem callback can_fork that conditionally
states whether or not the fork is accepted or rejected by a cgroup
policy.
In addition, add a cancel_fork callback so that if an error occurs later
in the forking process, any state modified by can_fork can be reverted.
In order to ensure that the fork charged the right hierarchy, save the
"current" css_set before doing ss->can_fork and compare it with the
"current" css_set that gets committed to the task *proper* in post_fork.
If they do not match, revert the can_fork's charging of the wrong
hierarchy and forcefully reapply it to the right hierarchy using the
reapply_fork callback. Since a changing "current" css_set in
copy_process indicates an organisation operation took place, we can
break the cgroup policy in this case.
This is in preparation for implementing the pids cgroup subsystem.
Signed-off-by: Aleksa Sarai <cyphar@...har.com>
---
include/linux/cgroup.h | 13 ++++++-
kernel/cgroup.c | 92 ++++++++++++++++++++++++++++++++++++++++++++++++--
kernel/fork.c | 18 ++++++++--
3 files changed, 117 insertions(+), 6 deletions(-)
diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index b9cb94c..9592f6e 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -32,7 +32,9 @@ struct cgroup;
extern int cgroup_init_early(void);
extern int cgroup_init(void);
extern void cgroup_fork(struct task_struct *p);
-extern void cgroup_post_fork(struct task_struct *p);
+extern int cgroup_can_fork(struct task_struct *p, void **state);
+extern void cgroup_cancel_fork(struct task_struct *p, void **state);
+extern void cgroup_post_fork(struct task_struct *p, void **old_state);
extern void cgroup_exit(struct task_struct *p);
extern int cgroupstats_build(struct cgroupstats *stats,
struct dentry *dentry);
@@ -649,6 +651,13 @@ struct cgroup_subsys {
struct cgroup_taskset *tset);
void (*attach)(struct cgroup_subsys_state *css,
struct cgroup_taskset *tset);
+ int (*can_fork)(struct cgroup_subsys_state *css,
+ struct task_struct *task);
+ void (*cancel_fork)(struct cgroup_subsys_state *css,
+ struct task_struct *task);
+ void (*reapply_fork)(struct cgroup_subsys_state *css,
+ struct cgroup_subsys_state *old_css,
+ struct task_struct *task);
void (*fork)(struct task_struct *task);
void (*exit)(struct cgroup_subsys_state *css,
struct cgroup_subsys_state *old_css,
@@ -948,6 +957,8 @@ struct cgroup_subsys_state;
static inline int cgroup_init_early(void) { return 0; }
static inline int cgroup_init(void) { return 0; }
static inline void cgroup_fork(struct task_struct *p) {}
+static inline int cgroup_can_fork(struct task_struct *p) { return 0; }
+static inline void cgroup_cancel_fork(struct task_struct *p) {}
static inline void cgroup_post_fork(struct task_struct *p) {}
static inline void cgroup_exit(struct task_struct *p) {}
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index d60107e..1a77790 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -183,6 +183,10 @@ static u64 css_serial_nr_next = 1;
*/
static int need_forkexit_callback __read_mostly;
+/* Ditto for the can_fork/cancel_fork/reapply_fork callbacks. */
+static int need_canfork_callback __read_mostly = 0,
+ need_reapplyfork_callback __read_mostly = 0;
+
static struct cftype cgroup_dfl_base_files[];
static struct cftype cgroup_legacy_base_files[];
@@ -4947,6 +4951,8 @@ static void __init cgroup_init_subsys(struct cgroup_subsys *ss, bool early)
init_css_set.subsys[ss->id] = css;
need_forkexit_callback |= !!(ss->fork || ss->exit) << ss->id;
+ need_canfork_callback |= !!(ss->can_fork || ss->cancel_fork) << ss->id;
+ need_reapplyfork_callback |= !!(ss->reapply_fork) << ss->id;
/* At system boot, before all subsystems have been
* registered, no tasks have been forked, so we don't
@@ -5200,6 +5206,66 @@ void cgroup_fork(struct task_struct *child)
}
/**
+ * cgroup_can_fork - called on a new task before the process is exposed.
+ * @child: the task in question.
+ *
+ * This calls the subsystem can_fork() callbacks. If the can_fork() callback
+ * returns an error, the fork aborts with that error code. This allows for
+ * a cgroup subsystem to conditionally allow or deny new forks.
+ */
+int cgroup_can_fork(struct task_struct *child, void **state)
+{
+ struct cgroup_subsys *ss;
+ struct css_set *cset;
+ int i, j, retval;
+
+ cset = task_css_set(current);
+ get_css_set(cset);
+ *state = cset;
+
+ for_each_subsys_which(need_canfork_callback, ss, i)
+ if (ss->can_fork) {
+ retval = ss->can_fork(cset->subsys[i], child);
+ if (retval)
+ goto out_revert;
+ }
+
+ return 0;
+
+out_revert:
+ for_each_subsys_which(need_canfork_callback, ss, j) {
+ if (j == i)
+ break;
+
+ if (ss->cancel_fork)
+ ss->cancel_fork(cset->subsys[i], child);
+ }
+
+ put_css_set(cset);
+ return retval;
+}
+
+/**
+ * cgroup_cancel_fork - called if a fork failed after cgroup_can_fork()
+ * @child: the task in question
+ *
+ * This calls the cancel_fork() callbacks if a fork failed *after*
+ * cgroup_can_fork() succeded.
+ */
+void cgroup_cancel_fork(struct task_struct *child, void **state)
+{
+ struct cgroup_subsys *ss;
+ struct css_set *cset = *state;
+ int i;
+
+ for_each_subsys_which(need_canfork_callback, ss, i)
+ if (ss->cancel_fork)
+ ss->cancel_fork(cset->subsys[i], child);
+
+ put_css_set(cset);
+}
+
+/**
* cgroup_post_fork - called on a new task after adding it to the task list
* @child: the task in question
*
@@ -5209,9 +5275,10 @@ void cgroup_fork(struct task_struct *child)
* cgroup_task_iter_start() - to guarantee that the new task ends up on its
* list.
*/
-void cgroup_post_fork(struct task_struct *child)
+void cgroup_post_fork(struct task_struct *child, void **old_state)
{
struct cgroup_subsys *ss;
+ struct css_set *cset, *old_cset = *old_state;
int i;
/*
@@ -5235,9 +5302,8 @@ void cgroup_post_fork(struct task_struct *child)
* in the init_css_set before cg_links is enabled and there's no
* operation which transfers all tasks out of init_css_set.
*/
+ cset = old_cset;
if (use_task_css_set_links) {
- struct css_set *cset;
-
down_write(&css_set_rwsem);
cset = task_css_set(current);
if (list_empty(&child->cg_list)) {
@@ -5249,6 +5315,24 @@ void cgroup_post_fork(struct task_struct *child)
}
/*
+ * Deal with tasks that were migrated mid-fork. If the css_set
+ * changed between can_fork() and post_fork() an organisation
+ * operation has occurred, and we need to revert/reapply the
+ * the can_fork().
+ */
+ for_each_subsys_which(need_canfork_callback, ss, i) {
+ struct cgroup_subsys_state *css = cset->subsys[i],
+ *old_css = old_cset->subsys[i];
+
+ /*
+ * We only reapply for subsystems whose
+ * association changed in the interim.
+ */
+ if (old_css != css && ss->reapply_fork)
+ ss->reapply_fork(css, old_css, child);
+ }
+
+ /*
* Call ss->fork(). This must happen after @child is linked on
* css_set; otherwise, @child might change state between ->fork()
* and addition to css_set.
@@ -5256,6 +5340,8 @@ void cgroup_post_fork(struct task_struct *child)
for_each_subsys_which(need_forkexit_callback, ss, i)
if (ss->fork)
ss->fork(child);
+
+ put_css_set(old_cset);
}
/**
diff --git a/kernel/fork.c b/kernel/fork.c
index cf65139..c15ca74 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1196,6 +1196,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
{
int retval;
struct task_struct *p;
+ void *cfs;
if ((clone_flags & (CLONE_NEWNS|CLONE_FS)) == (CLONE_NEWNS|CLONE_FS))
return ERR_PTR(-EINVAL);
@@ -1469,6 +1470,17 @@ static struct task_struct *copy_process(unsigned long clone_flags,
p->task_works = NULL;
/*
+ * Ensure that the cgroup subsystem policies allow the new process to be
+ * forked. If this fork is happening in an organization operation, then
+ * this will not charge the correct css_set. This is fixed after
+ * cgroup_post_fork() (when the css_set has been updated) by undoing
+ * this operation and forcefully charging the correct css_set.
+ */
+ retval = cgroup_can_fork(p, &cfs);
+ if (retval)
+ goto bad_fork_free_pid;
+
+ /*
* Make it visible to the rest of the system, but dont wake it up yet.
* Need tasklist lock for parent etc handling!
*/
@@ -1504,7 +1516,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
spin_unlock(¤t->sighand->siglock);
write_unlock_irq(&tasklist_lock);
retval = -ERESTARTNOINTR;
- goto bad_fork_free_pid;
+ goto bad_fork_cgroup_cancel;
}
if (likely(p->pid)) {
@@ -1546,7 +1558,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
write_unlock_irq(&tasklist_lock);
proc_fork_connector(p);
- cgroup_post_fork(p);
+ cgroup_post_fork(p, &cfs);
if (clone_flags & CLONE_THREAD)
threadgroup_change_end(current);
perf_event_fork(p);
@@ -1556,6 +1568,8 @@ static struct task_struct *copy_process(unsigned long clone_flags,
return p;
+bad_fork_cgroup_cancel:
+ cgroup_cancel_fork(p, &cfs);
bad_fork_free_pid:
if (pid != &init_struct_pid)
free_pid(pid);
--
2.3.2
--
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