[PATCH v2 2/3] clone3: allow spawning processes into cgroups

Christian Brauner
2019-12-23T06:15:03Z

This adds support for creating a process in a different cgroup than its
parent. Callers can limit and account processes and threads right from
the moment they are spawned:
- A service manager can directly spawn new services into dedicated
cgroups.
- A process can be directly created in a frozen cgroup and will be
frozen as well.
- The initial accounting jitter experienced by process supervisors and
daemons is eliminated with this.
- Threaded applications or even thread implementations can choose to
create a specific cgroup layout where each thread is spawned
directly into a dedicated cgroup.

This feature is limited to the unified hierarchy. Callers need to pass
an directory file descriptor for the target cgroup. The caller can
choose to pass an O_PATH file descriptor. All usual migration
restrictions apply, i.e. there can be no processes in inner nodes. In
general, creating a process directly in a target cgroup adheres to all
migration restrictions.

Cc: Tejun Heo <tj@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Li Zefan <lizefan@huawei.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: cgroups@vger.kernel.org
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
---
/* v1 */
Link: https://lore.kernel.org/r/20191218173516.7875-3-christian.brauner@ubuntu.com

/* v2 */
- Oleg Nesterov <oleg@redhat.com>:
- prevent deadlock from wrong locking order
- Christian Brauner <christian.brauner@ubuntu.com>:
- Rework locking. In the previous patch version we would have already
acquired the cgroup_threadgroup_rwsem before we grabbed cgroup mutex
we need to hold when CLONE_INTO_CGROUP is specified. This meant we
could deadlock with other codepaths that all require it to be done
the other way around. Fix this by first grabbing cgroup mutex when
CLONE_INTO_CGROUP is specified and then grabbing
cgroup_threadgroup_rwsem unconditionally after. This way we don't
require the cgroup mutex be held in codepaths that don't need it.
- Switch from mutex_lock() to mutex_lock_killable().
---
include/linux/cgroup-defs.h | 7 +-
include/linux/cgroup.h | 25 +++-
include/linux/sched/task.h | 4 +
include/uapi/linux/sched.h | 5 +
kernel/cgroup/cgroup.c | 231 ++++++++++++++++++++++++++++++------
kernel/cgroup/pids.c | 25 +++-
kernel/fork.c | 19 +--
7 files changed, 260 insertions(+), 56 deletions(-)

diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index 63097cb243cb..cd848c6bac4a 100644
--- a/include/linux/cgroup-defs.h
+++ b/include/linux/cgroup-defs.h
@@ -33,6 +33,7 @@ struct kernfs_ops;
struct kernfs_open_file;
struct seq_file;
struct poll_table_struct;
+struct kernel_clone_args;

#define MAX_CGROUP_TYPE_NAMELEN 32
#define MAX_CGROUP_ROOT_NAMELEN 64
@@ -628,8 +629,10 @@ struct cgroup_subsys {
void (*cancel_attach)(struct cgroup_taskset *tset);
void (*attach)(struct cgroup_taskset *tset);
void (*post_attach)(void);
- int (*can_fork)(struct task_struct *task);
- void (*cancel_fork)(struct task_struct *task);
+ int (*can_fork)(struct task_struct *parent, struct task_struct *child,
+ struct kernel_clone_args *kargs);
+ void (*cancel_fork)(struct task_struct *child,
+ struct kernel_clone_args *kargs);
void (*fork)(struct task_struct *task);
void (*exit)(struct task_struct *task);
void (*release)(struct task_struct *task);
diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index d7ddebd0cdec..250afd24746c 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -70,6 +70,7 @@ struct css_task_iter {

extern struct cgroup_root cgrp_dfl_root;
extern struct css_set init_css_set;
+struct kernel_clone_args;

#define SUBSYS(_x) extern struct cgroup_subsys _x ## _cgrp_subsys;
#include <linux/cgroup_subsys.h>
@@ -121,9 +122,15 @@ int proc_cgroup_show(struct seq_file *m, struct pid_namespace *ns,
struct pid *pid, struct task_struct *tsk);

void cgroup_fork(struct task_struct *p);
-extern int cgroup_can_fork(struct task_struct *p);
-extern void cgroup_cancel_fork(struct task_struct *p);
-extern void cgroup_post_fork(struct task_struct *p);
+extern int cgroup_can_fork(struct task_struct *parent,
+ struct task_struct *child,
+ struct kernel_clone_args *kargs);
+extern void cgroup_cancel_fork(struct task_struct *parent,
+ struct task_struct *child,
+ struct kernel_clone_args *kargs);
+extern void cgroup_post_fork(struct task_struct *parent,
+ struct task_struct *child,
+ struct kernel_clone_args *kargs);
void cgroup_exit(struct task_struct *p);
void cgroup_release(struct task_struct *p);
void cgroup_free(struct task_struct *p);
@@ -707,9 +714,15 @@ static inline int cgroupstats_build(struct cgroupstats *stats,
struct dentry *dentry) { return -EINVAL; }

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 int cgroup_can_fork(struct task_struct *parent,
+ struct task_struct *child,
+ struct kernel_clone_args *kargs) { return 0; }
+static inline void cgroup_cancel_fork(struct task_struct *parent,
+ struct task_struct *child,
+ struct kernel_clone_args *kargs) {};
+static inline void cgroup_post_fork(struct task_struct *parent,
+ struct task_struct *child,
+ struct kernel_clone_args *kargs) {};
static inline void cgroup_exit(struct task_struct *p) {}
static inline void cgroup_release(struct task_struct *p) {}
static inline void cgroup_free(struct task_struct *p) {}
diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
index f1879884238e..38359071236a 100644
--- a/include/linux/sched/task.h
+++ b/include/linux/sched/task.h
@@ -13,6 +13,7 @@
struct task_struct;
struct rusage;
union thread_union;
+struct css_set;

/* All the bits taken by the old clone syscall. */
#define CLONE_LEGACY_FLAGS 0xffffffffULL
@@ -29,6 +30,9 @@ struct kernel_clone_args {
pid_t *set_tid;
/* Number of elements in *set_tid */
size_t set_tid_size;
+ int cgroup;
+ struct cgroup *cgrp;
+ struct css_set *cset;
};

/*
diff --git a/include/uapi/linux/sched.h b/include/uapi/linux/sched.h
index 4a0217832464..08620c220f30 100644
--- a/include/uapi/linux/sched.h
+++ b/include/uapi/linux/sched.h
@@ -35,6 +35,7 @@

/* Flags for the clone3() syscall. */
#define CLONE_CLEAR_SIGHAND 0x100000000ULL /* Clear any signal handler and reset to SIG_DFL. */
+#define CLONE_INTO_CGROUP 0x200000000ULL /* Clone into a specific cgroup given the right permissions. */

#ifndef __ASSEMBLY__
/**
@@ -75,6 +76,8 @@
* @set_tid_size: This defines the size of the array referenced
* in @set_tid. This cannot be larger than the
* kernel's limit of nested PID namespaces.
+ * @cgroup: If CLONE_INTO_CGROUP is specified set this to
+ * a file descriptor for the cgroup.
*
* The structure is versioned by size and thus extensible.
* New struct members must go at the end of the struct and
@@ -91,11 +94,13 @@ struct clone_args {
__aligned_u64 tls;
__aligned_u64 set_tid;
__aligned_u64 set_tid_size;
+ __aligned_u64 cgroup;
};
#endif

#define CLONE_ARGS_SIZE_VER0 64 /* sizeof first published struct */
#define CLONE_ARGS_SIZE_VER1 80 /* sizeof second published struct */
+#define CLONE_ARGS_SIZE_VER2 88 /* sizeof third published struct */

/*
* Scheduling policies
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index ad1f9fea5c14..a9fedcfeae4b 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -5882,21 +5882,176 @@ void cgroup_fork(struct task_struct *child)
INIT_LIST_HEAD(&child->cg_list);
}

+static struct cgroup *cgroup_get_from_file(struct file *f)
+{
+ struct cgroup_subsys_state *css;
+ struct cgroup *cgrp;
+
+ css = css_tryget_online_from_dir(f->f_path.dentry, NULL);
+ if (IS_ERR(css))
+ return ERR_CAST(css);
+
+ cgrp = css->cgroup;
+ if (!cgroup_on_dfl(cgrp)) {
+ cgroup_put(cgrp);
+ return ERR_PTR(-EBADF);
+ }
+
+ return cgrp;
+}
+
/**
- * cgroup_can_fork - called on a new task before the process is exposed
- * @child: the task in question.
+ * cgroup_css_set_fork - find or create a css_set for a child process
+ * @parent: the parent of the child process
+ * @kargs: the arguments passed to create the child process
+ *
+ * This functions finds or creates a new css_set which the child
+ * process will be attached to in cgroup_post_fork(). By default,
+ * the child process will be given the same css_set as its parent.
+ *
+ * If CLONE_INTO_CGROUP is specified this function will try to find an
+ * existing css_set which includes the request cgroup and if not create
+ * a new css_set that the child will be attached to. If this function
+ * succeeds it will hold cgroup_threadgroup_rwsem on return. If
+ * CLONE_INTO_CGROUP is requested this function will grab cgroup mutex
+ * before grabbing cgroup_threadgroup_rwsem and will hold a reference
+ * to the target cgroup.
+ */
+static int cgroup_css_set_fork(struct task_struct *parent,
+ struct kernel_clone_args *kargs)
+ __acquires(&cgroup_mutex) __acquires(&cgroup_threadgroup_rwsem)
+{
+ int ret;
+ struct cgroup *dst_cgrp = NULL, *src_cgrp;
+ struct css_set *cset;
+ struct super_block *sb;
+ struct file *f;
+
+ if (kargs->flags & CLONE_INTO_CGROUP) {
+ ret = mutex_lock_killable(&cgroup_mutex);
+ if (ret)
+ return ret;
+ }
+
+ cgroup_threadgroup_change_begin(parent);
+
+ spin_lock_irq(&css_set_lock);
+ cset = task_css_set(parent);
+ get_css_set(cset);
+ spin_unlock_irq(&css_set_lock);
+
+ if (!(kargs->flags & CLONE_INTO_CGROUP)) {
+ kargs->cset = cset;
+ return 0;
+ }
+
+ f = fget_raw(kargs->cgroup);
+ if (!f) {
+ ret = -EBADF;
+ goto err;
+ }
+ sb = f->f_path.dentry->d_sb;
+
+ dst_cgrp = cgroup_get_from_file(f);
+ if (IS_ERR(dst_cgrp)) {
+ ret = PTR_ERR(dst_cgrp);
+ dst_cgrp = NULL;
+ goto err;
+ }
+
+ spin_lock_irq(&css_set_lock);
+ src_cgrp = task_cgroup_from_root(parent, &cgrp_dfl_root);
+ spin_unlock_irq(&css_set_lock);
+
+ ret = cgroup_attach_permissions(src_cgrp, dst_cgrp, sb,
+ !!(kargs->flags & CLONE_THREAD));
+ if (ret)
+ goto err;
+
+ kargs->cset = find_css_set(cset, dst_cgrp);
+ if (!kargs->cset) {
+ ret = -ENOMEM;
+ goto err;
+ }
+
+ if (cgroup_is_dead(dst_cgrp)) {
+ ret = -ENODEV;
+ goto err;
+ }
+
+ put_css_set(cset);
+ fput(f);
+ kargs->cgrp = dst_cgrp;
+ return ret;
+
+err:
+ cgroup_threadgroup_change_end(parent);
+ mutex_unlock(&cgroup_mutex);
+ if (f)
+ fput(f);
+ if (dst_cgrp)
+ cgroup_put(dst_cgrp);
+ put_css_set(cset);
+ return ret;
+}
+
+/**
+ * cgroup_css_set_put_fork - drop references we took during fork
+ * @parent: the parent of the child process
+ * @kargs: the arguments passed to create the child process
*
- * 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.
+ * Drop references to the prepared css_set and target cgroup if
+ * CLONE_INTO_CGROUP was requested. This function can only be
+ * called before fork()'s point of no return.
+ */
+static void cgroup_css_set_put_fork(struct task_struct *parent,
+ struct kernel_clone_args *kargs)
+ __releases(&cgroup_threadgroup_rwsem) __releases(&cgroup_mutex)
+{
+ cgroup_threadgroup_change_end(parent);
+
+ if (kargs->flags & CLONE_INTO_CGROUP) {
+ struct cgroup *cgrp = kargs->cgrp;
+ struct css_set *cset = kargs->cset;
+
+ mutex_unlock(&cgroup_mutex);
+
+ if (cset) {
+ put_css_set(cset);
+ kargs->cset = NULL;
+ }
+
+ if (cgrp) {
+ cgroup_put(cgrp);
+ kargs->cgrp = NULL;
+ }
+ }
+}
+
+/**
+ * cgroup_can_fork - called on a new task before the process is exposed
+ * @parent: the parent process of @child
+ * @child: the child process of @parent
+ * @kargs: the arguments passed to create the child process
+ *
+ * This prepares a new css_set for the child process which the child will
+ * be attached to in cgroup_post_fork().
+ * This calls the subsystem can_fork() callbacks. If the cgroup_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)
+int cgroup_can_fork(struct task_struct *parent, struct task_struct *child,
+ struct kernel_clone_args *kargs)
{
struct cgroup_subsys *ss;
int i, j, ret;

+ ret = cgroup_css_set_fork(parent, kargs);
+ if (ret)
+ return ret;
+
do_each_subsys_mask(ss, i, have_canfork_callback) {
- ret = ss->can_fork(child);
+ ret = ss->can_fork(parent, child, kargs);
if (ret)
goto out_revert;
} while_each_subsys_mask();
@@ -5908,50 +6063,57 @@ int cgroup_can_fork(struct task_struct *child)
if (j >= i)
break;
if (ss->cancel_fork)
- ss->cancel_fork(child);
+ ss->cancel_fork(child, kargs);
}

+ cgroup_css_set_put_fork(parent, kargs);
+
return ret;
}

/**
* cgroup_cancel_fork - called if a fork failed after cgroup_can_fork()
- * @child: the task in question
+ * @parent: the parent process of @child
+ * @child: the child process of @parent
+ * @kargs: the arguments passed to create the child process
*
* This calls the cancel_fork() callbacks if a fork failed *after*
- * cgroup_can_fork() succeded.
+ * cgroup_can_fork() succeded and cleans up references we took to
+ * prepare a new css_set for the child process in cgroup_can_fork().
*/
-void cgroup_cancel_fork(struct task_struct *child)
+void cgroup_cancel_fork(struct task_struct *parent, struct task_struct *child,
+ struct kernel_clone_args *kargs)
{
struct cgroup_subsys *ss;
int i;

for_each_subsys(ss, i)
if (ss->cancel_fork)
- ss->cancel_fork(child);
+ ss->cancel_fork(child, kargs);
+
+ cgroup_css_set_put_fork(parent, kargs);
}

/**
- * cgroup_post_fork - called on a new task after adding it to the task list
- * @child: the task in question
- *
- * Adds the task to the list running through its css_set if necessary and
- * call the subsystem fork() callbacks. Has to be after the task is
- * visible on the task list in case we race with the first call to
- * cgroup_task_iter_start() - to guarantee that the new task ends up on its
- * list.
+ * cgroup_post_fork - finalize cgroup setup for the child process
+ * @parent: the parent process of @child
+ * @child: the child process of @parent
+ * @kargs: the arguments passed to create the child process
+ *
+ * Attach the child process to its css_set calling the subsystem fork()
+ * callbacks.
*/
-void cgroup_post_fork(struct task_struct *child)
+void cgroup_post_fork(struct task_struct *parent, struct task_struct *child,
+ struct kernel_clone_args *kargs)
+ __releases(&cgroup_threadgroup_rwsem) __releases(&cgroup_mutex)
{
struct cgroup_subsys *ss;
- struct css_set *cset;
+ struct css_set *cset = kargs->cset;
int i;

spin_lock_irq(&css_set_lock);

WARN_ON_ONCE(!list_empty(&child->cg_list));
- cset = task_css_set(current); /* current is @child's parent */
- get_css_set(cset);
cset->nr_tasks++;
css_set_move_task(child, NULL, cset, false);

@@ -5984,6 +6146,15 @@ void cgroup_post_fork(struct task_struct *child)
do_each_subsys_mask(ss, i, have_fork_callback) {
ss->fork(child);
} while_each_subsys_mask();
+
+ cgroup_threadgroup_change_end(parent);
+
+ if (kargs->flags & CLONE_INTO_CGROUP) {
+ mutex_unlock(&cgroup_mutex);
+
+ cgroup_put(kargs->cgrp);
+ kargs->cgrp = NULL;
+ }
}

/**
@@ -6170,7 +6341,6 @@ EXPORT_SYMBOL_GPL(cgroup_get_from_path);
*/
struct cgroup *cgroup_get_from_fd(int fd)
{
- struct cgroup_subsys_state *css;
struct cgroup *cgrp;
struct file *f;

@@ -6178,17 +6348,8 @@ struct cgroup *cgroup_get_from_fd(int fd)
if (!f)
return ERR_PTR(-EBADF);

- css = css_tryget_online_from_dir(f->f_path.dentry, NULL);
+ cgrp = cgroup_get_from_file(f);
fput(f);
- if (IS_ERR(css))
- return ERR_CAST(css);
-
- cgrp = css->cgroup;
- if (!cgroup_on_dfl(cgrp)) {
- cgroup_put(cgrp);
- return ERR_PTR(-EBADF);
- }
-
return cgrp;
}
EXPORT_SYMBOL_GPL(cgroup_get_from_fd);
diff --git a/kernel/cgroup/pids.c b/kernel/cgroup/pids.c
index 138059eb730d..e5955bc1fb00 100644
--- a/kernel/cgroup/pids.c
+++ b/kernel/cgroup/pids.c
@@ -33,6 +33,7 @@
#include <linux/atomic.h>
#include <linux/cgroup.h>
#include <linux/slab.h>
+#include <linux/sched/task.h>

#define PIDS_MAX (PID_MAX_LIMIT + 1ULL)
#define PIDS_MAX_STR "max"
@@ -214,13 +215,21 @@ static void pids_cancel_attach(struct cgroup_taskset *tset)
* task_css_check(true) in pids_can_fork() and pids_cancel_fork() relies
* on cgroup_threadgroup_change_begin() held by the copy_process().
*/
-static int pids_can_fork(struct task_struct *task)
+static int pids_can_fork(struct task_struct *parent, struct task_struct *child,
+ struct kernel_clone_args *args)
{
+ struct css_set *new_cset = NULL;
struct cgroup_subsys_state *css;
struct pids_cgroup *pids;
int err;

- css = task_css_check(current, pids_cgrp_id, true);
+ if (args)
+ new_cset = args->cset;
+
+ if (!new_cset)
+ css = task_css_check(current, pids_cgrp_id, true);
+ else
+ css = new_cset->subsys[pids_cgrp_id];
pids = css_pids(css);
err = pids_try_charge(pids, 1);
if (err) {
@@ -235,12 +244,20 @@ static int pids_can_fork(struct task_struct *task)
return err;
}

-static void pids_cancel_fork(struct task_struct *task)
+static void pids_cancel_fork(struct task_struct *task,
+ struct kernel_clone_args *args)
{
+ struct css_set *new_cset = NULL;
struct cgroup_subsys_state *css;
struct pids_cgroup *pids;

- css = task_css_check(current, pids_cgrp_id, true);
+ if (args)
+ new_cset = args->cset;
+
+ if (!new_cset)
+ css = task_css_check(current, pids_cgrp_id, true);
+ else
+ css = new_cset->subsys[pids_cgrp_id];
pids = css_pids(css);
pids_uncharge(pids, 1);
}
diff --git a/kernel/fork.c b/kernel/fork.c
index 2508a4f238a3..1604552f7cd3 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2165,16 +2165,15 @@ static __latent_entropy struct task_struct *copy_process(
INIT_LIST_HEAD(&p->thread_group);
p->task_works = NULL;

- cgroup_threadgroup_change_begin(current);
/*
* Ensure that the cgroup subsystem policies allow the new process to be
* forked. It should be noted the the new process's css_set can be changed
* between here and cgroup_post_fork() if an organisation operation is in
* progress.
*/
- retval = cgroup_can_fork(p);
+ retval = cgroup_can_fork(current, p, args);
if (retval)
- goto bad_fork_cgroup_threadgroup_change_end;
+ goto bad_fork_put_pidfd;

/*
* From this point on we must avoid any synchronous user-space
@@ -2279,8 +2278,7 @@ static __latent_entropy struct task_struct *copy_process(
write_unlock_irq(&tasklist_lock);

proc_fork_connector(p);
- cgroup_post_fork(p);
- cgroup_threadgroup_change_end(current);
+ cgroup_post_fork(current, p, args);
perf_event_fork(p);

trace_task_newtask(p, clone_flags);
@@ -2291,9 +2289,7 @@ static __latent_entropy struct task_struct *copy_process(
bad_fork_cancel_cgroup:
spin_unlock(&current->sighand->siglock);
write_unlock_irq(&tasklist_lock);
- cgroup_cancel_fork(p);
-bad_fork_cgroup_threadgroup_change_end:
- cgroup_threadgroup_change_end(current);
+ cgroup_cancel_fork(current, p, args);
bad_fork_put_pidfd:
if (clone_flags & CLONE_PIDFD) {
fput(pidfile);
@@ -2612,6 +2608,9 @@ noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
!valid_signal(args.exit_signal)))
return -EINVAL;

+ if ((args.flags & CLONE_INTO_CGROUP) && args.cgroup < 0)
+ return -EINVAL;
+
*kargs = (struct kernel_clone_args){
.flags = args.flags,
.pidfd = u64_to_user_ptr(args.pidfd),
@@ -2622,6 +2621,7 @@ noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
.stack_size = args.stack_size,
.tls = args.tls,
.set_tid_size = args.set_tid_size,
+ .cgroup = args.cgroup,
};

if (args.set_tid &&
@@ -2665,7 +2665,8 @@ static inline bool clone3_stack_valid(struct kernel_clone_args *kargs)
static bool clone3_args_valid(struct kernel_clone_args *kargs)
{
/* Verify that no unknown flags are passed along. */
- if (kargs->flags & ~(CLONE_LEGACY_FLAGS | CLONE_CLEAR_SIGHAND))
+ if (kargs->flags &
+ ~(CLONE_LEGACY_FLAGS | CLONE_CLEAR_SIGHAND | CLONE_INTO_CGROUP))
return false;

/*
--
2.24.0