Re: [PATCH][BUGFIX] cgroups: fix pid namespace bug

Serge E. Hallyn
2009-07-02T13:59:13Z

Quoting Li Zefan (lizf@cn.fujitsu.com):
> The bug was introduced by commit cc31edceee04a7b87f2be48f9489ebb72d264844
> ("cgroups: convert tasks file to use a seq_file with shared pid array").
>
> We cache a pid array for all threads that are opening the same "tasks"
> file, but the pids in the array are always from the namespace of the
> last process that opened the file, so all other threads will read pids
> from that namespace instead of their own namespaces.
>
> To fix it, we maintain a list of pid arrays, which is keyed by pid_ns.
> The list will be of length 1 at most time.
>
> Reported-by: Paul Menage <menage@google.com>
> Idea-by: Paul Menage <menage@google.com>
> Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>

Reviewed-by: Serge Hallyn <serue@us.ibm.com>

> ---
> include/linux/cgroup.h | 11 ++----
> kernel/cgroup.c | 94 +++++++++++++++++++++++++++++++++++------------
> 2 files changed, 74 insertions(+), 31 deletions(-)
>
> diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
> index 665fa70..20411d2 100644
> --- a/include/linux/cgroup.h
> +++ b/include/linux/cgroup.h
> @@ -179,14 +179,11 @@ struct cgroup {
> */
> struct list_head release_list;
>
> - /* pids_mutex protects the fields below */
> + /* pids_mutex protects pids_list and cached pid arrays. */
> struct rw_semaphore pids_mutex;
> - /* Array of process ids in the cgroup */
> - pid_t *tasks_pids;
> - /* How many files are using the current tasks_pids array */
> - int pids_use_count;
> - /* Length of the current tasks_pids array */
> - int pids_length;
> +
> + /* Linked list of struct cgroup_pids */
> + struct list_head pids_list;
>
> /* For RCU-protected deletion */
> struct rcu_head rcu_head;
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index 3737a68..13dddb4 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -47,6 +47,7 @@
> #include <linux/hash.h>
> #include <linux/namei.h>
> #include <linux/smp_lock.h>
> +#include <linux/pid_namespace.h>
>
> #include <asm/atomic.h>
>
> @@ -960,6 +961,7 @@ static void init_cgroup_housekeeping(struct cgroup *cgrp)
> INIT_LIST_HEAD(&cgrp->children);
> INIT_LIST_HEAD(&cgrp->css_sets);
> INIT_LIST_HEAD(&cgrp->release_list);
> + INIT_LIST_HEAD(&cgrp->pids_list);
> init_rwsem(&cgrp->pids_mutex);
> }
> static void init_cgroup_root(struct cgroupfs_root *root)
> @@ -2201,12 +2203,30 @@ err:
> return ret;
> }
>
> +/*
> + * Cache pids for all threads in the same pid namespace that are
> + * opening the same "tasks" file.
> + */
> +struct cgroup_pids {
> + /* The node in cgrp->pids_list */
> + struct list_head list;
> + /* The cgroup those pids belong to */
> + struct cgroup *cgrp;
> + /* The namepsace those pids belong to */
> + struct pid_namespace *pid_ns;
> + /* Array of process ids in the cgroup */
> + pid_t *tasks_pids;
> + /* How many files are using the this tasks_pids array */
> + int pids_use_count;
> + /* Length of the current tasks_pids array */
> + int pids_length;
> +};
> +
> static int cmppid(const void *a, const void *b)
> {
> return *(pid_t *)a - *(pid_t *)b;
> }
>
> -
> /*
> * seq_file methods for the "tasks" file. The seq_file position is the
> * next pid to display; the seq_file iterator is a pointer to the pid
> @@ -2221,45 +2241,47 @@ static void *cgroup_tasks_start(struct seq_file *s, loff_t *pos)
> * after a seek to the start). Use a binary-search to find the
> * next pid to display, if any
> */
> - struct cgroup *cgrp = s->private;
> + struct cgroup_pids *cp = s->private;
> + struct cgroup *cgrp = cp->cgrp;
> int index = 0, pid = *pos;
> int *iter;
>
> down_read(&cgrp->pids_mutex);
> if (pid) {
> - int end = cgrp->pids_length;
> + int end = cp->pids_length;
>
> while (index < end) {
> int mid = (index + end) / 2;
> - if (cgrp->tasks_pids[mid] == pid) {
> + if (cp->tasks_pids[mid] == pid) {
> index = mid;
> break;
> - } else if (cgrp->tasks_pids[mid] <= pid)
> + } else if (cp->tasks_pids[mid] <= pid)
> index = mid + 1;
> else
> end = mid;
> }
> }
> /* If we're off the end of the array, we're done */
> - if (index >= cgrp->pids_length)
> + if (index >= cp->pids_length)
> return NULL;
> /* Update the abstract position to be the actual pid that we found */
> - iter = cgrp->tasks_pids + index;
> + iter = cp->tasks_pids + index;
> *pos = *iter;
> return iter;
> }
>
> static void cgroup_tasks_stop(struct seq_file *s, void *v)
> {
> - struct cgroup *cgrp = s->private;
> + struct cgroup_pids *cp = s->private;
> + struct cgroup *cgrp = cp->cgrp;
> up_read(&cgrp->pids_mutex);
> }
>
> static void *cgroup_tasks_next(struct seq_file *s, void *v, loff_t *pos)
> {
> - struct cgroup *cgrp = s->private;
> + struct cgroup_pids *cp = s->private;
> int *p = v;
> - int *end = cgrp->tasks_pids + cgrp->pids_length;
> + int *end = cp->tasks_pids + cp->pids_length;
>
> /*
> * Advance to the next pid in the array. If this goes off the
> @@ -2286,26 +2308,32 @@ static struct seq_operations cgroup_tasks_seq_operations = {
> .show = cgroup_tasks_show,
> };
>
> -static void release_cgroup_pid_array(struct cgroup *cgrp)
> +static void release_cgroup_pid_array(struct cgroup_pids *cp)
> {
> + struct cgroup *cgrp = cp->cgrp;
> +
> down_write(&cgrp->pids_mutex);
> - BUG_ON(!cgrp->pids_use_count);
> - if (!--cgrp->pids_use_count) {
> - kfree(cgrp->tasks_pids);
> - cgrp->tasks_pids = NULL;
> - cgrp->pids_length = 0;
> + BUG_ON(!cp->pids_use_count);
> + if (!--cp->pids_use_count) {
> + list_del(&cp->list);
> + kfree(cp->tasks_pids);
> + kfree(cp);
> }
> up_write(&cgrp->pids_mutex);
> }
>
> static int cgroup_tasks_release(struct inode *inode, struct file *file)
> {
> - struct cgroup *cgrp = __d_cgrp(file->f_dentry->d_parent);
> + struct seq_file *seq;
> + struct cgroup_pids *cp;
>
> if (!(file->f_mode & FMODE_READ))
> return 0;
>
> - release_cgroup_pid_array(cgrp);
> + seq = file->private_data;
> + cp = seq->private;
> +
> + release_cgroup_pid_array(cp);
> return seq_release(inode, file);
> }
>
> @@ -2324,6 +2352,8 @@ static struct file_operations cgroup_tasks_operations = {
> static int cgroup_tasks_open(struct inode *unused, struct file *file)
> {
> struct cgroup *cgrp = __d_cgrp(file->f_dentry->d_parent);
> + struct pid_namespace *pid_ns = task_active_pid_ns(current);
> + struct cgroup_pids *cp;
> pid_t *pidarray;
> int npids;
> int retval;
> @@ -2350,20 +2380,36 @@ static int cgroup_tasks_open(struct inode *unused, struct file *file)
> * array if necessary
> */
> down_write(&cgrp->pids_mutex);
> - kfree(cgrp->tasks_pids);
> - cgrp->tasks_pids = pidarray;
> - cgrp->pids_length = npids;
> - cgrp->pids_use_count++;
> +
> + list_for_each_entry(cp, &cgrp->pids_list, list) {
> + if (pid_ns == cp->pid_ns)
> + goto found;
> + }
> +
> + cp = kzalloc(sizeof(*cp), GFP_KERNEL);
> + if (!cp) {
> + up_write(&cgrp->pids_mutex);
> + kfree(pidarray);
> + return -ENOMEM;
> + }
> + cp->cgrp = cgrp;
> + cp->pid_ns = pid_ns;
> + list_add(&cp->list, &cgrp->pids_list);
> +found:
> + kfree(cp->tasks_pids);
> + cp->tasks_pids = pidarray;
> + cp->pids_length = npids;
> + cp->pids_use_count++;
> up_write(&cgrp->pids_mutex);
>
> file->f_op = &cgroup_tasks_operations;
>
> retval = seq_open(file, &cgroup_tasks_seq_operations);
> if (retval) {
> - release_cgroup_pid_array(cgrp);
> + release_cgroup_pid_array(cp);
> return retval;
> }
> - ((struct seq_file *)file->private_data)->private = cgrp;
> + ((struct seq_file *)file->private_data)->private = cp;
> return 0;
> }
>
> --
> 1.5.4.rc3