cgroup files: move the release_agent file to use typed handlers

Adds cgroup_release_agent_write() and cgroup_release_agent_show()
methods to handle writing/reading the path to a cgroup hierarchy's
release agent. As a result, cgroup_common_file_read() is now unnecessary.

As part of the change, a previously-tolerated race in
cgroup_release_agent() is avoided by copying the current
release_agent_path prior to calling call_usermode_helper().

Signed-off-by: Paul Menage <menage@google.com>
Cc: Paul Jackson <pj@sgi.com>
Cc: Pavel Emelyanov <xemul@openvz.org>
Cc: Balbir Singh <balbir@in.ibm.com>
Acked-by: Serge Hallyn <serue@us.ibm.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 59 insertions, 68 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index f5379455bb59..e78377a91a74 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -295,6 +295,8 @@ int cgroup_add_files(struct cgroup *cgrp,
 
 int cgroup_is_removed(const struct cgroup *cgrp);
 
+int cgroup_lock_live_group(struct cgroup *cgrp);
+
 int cgroup_path(const struct cgroup *cgrp, char *buf, int buflen);
 
 int cgroup_task_count(const struct cgroup *cgrp);
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 3a99cc2df860..0120b5d67a73 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -89,11 +89,7 @@ struct cgroupfs_root {
         /* Hierarchy-specific flags */
         unsigned long flags;
 
-        /* The path to use for release notifications. No locking
-         * between setting and use - so if userspace updates this
-         * while child cgroups exist, you could miss a
-         * notification. We ensure that it's always a valid
-         * NUL-terminated string */
+        /* The path to use for release notifications. */
         char release_agent_path[PATH_MAX];
 };
 
@@ -1329,6 +1325,45 @@ enum cgroup_filetype {
         FILE_RELEASE_AGENT,
 };
 
+/**
+ * cgroup_lock_live_group - take cgroup_mutex and check that cgrp is alive.
+ * @cgrp: the cgroup to be checked for liveness
+ *
+ * Returns true (with lock held) on success, or false (with no lock
+ * held) on failure.
+ */
+int cgroup_lock_live_group(struct cgroup *cgrp)
+{
+        mutex_lock(&cgroup_mutex);
+        if (cgroup_is_removed(cgrp)) {
+                mutex_unlock(&cgroup_mutex);
+                return false;
+        }
+        return true;
+}
+
+static int cgroup_release_agent_write(struct cgroup *cgrp, struct cftype *cft,
+                                      const char *buffer)
+{
+        BUILD_BUG_ON(sizeof(cgrp->root->release_agent_path) < PATH_MAX);
+        if (!cgroup_lock_live_group(cgrp))
+                return -ENODEV;
+        strcpy(cgrp->root->release_agent_path, buffer);
+        mutex_unlock(&cgroup_mutex);
+        return 0;
+}
+
+static int cgroup_release_agent_show(struct cgroup *cgrp, struct cftype *cft,
+                                     struct seq_file *seq)
+{
+        if (!cgroup_lock_live_group(cgrp))
+                return -ENODEV;
+        seq_puts(seq, cgrp->root->release_agent_path);
+        seq_putc(seq, '\n');
+        mutex_unlock(&cgroup_mutex);
+        return 0;
+}
+
 static ssize_t cgroup_write_X64(struct cgroup *cgrp, struct cftype *cft,
                                 struct file *file,
                                 const char __user *userbuf,
@@ -1443,10 +1478,6 @@ static ssize_t cgroup_common_file_write(struct cgroup *cgrp,
                 else
                         clear_bit(CGRP_NOTIFY_ON_RELEASE, &cgrp->flags);
                 break;
-        case FILE_RELEASE_AGENT:
-                BUILD_BUG_ON(sizeof(cgrp->root->release_agent_path) < PATH_MAX);
-                strcpy(cgrp->root->release_agent_path, buffer);
-                break;
         default:
                 retval = -EINVAL;
                 goto out2;
@@ -1506,49 +1537,6 @@ static ssize_t cgroup_read_s64(struct cgroup *cgrp, struct cftype *cft,
         return simple_read_from_buffer(buf, nbytes, ppos, tmp, len);
 }
 
-static ssize_t cgroup_common_file_read(struct cgroup *cgrp,
-                                          struct cftype *cft,
-                                          struct file *file,
-                                          char __user *buf,
-                                          size_t nbytes, loff_t *ppos)
-{
-        enum cgroup_filetype type = cft->private;
-        char *page;
-        ssize_t retval = 0;
-        char *s;
-
-        if (!(page = (char *)__get_free_page(GFP_KERNEL)))
-                return -ENOMEM;
-
-        s = page;
-
-        switch (type) {
-        case FILE_RELEASE_AGENT:
-        {
-                struct cgroupfs_root *root;
-                size_t n;
-                mutex_lock(&cgroup_mutex);
-                root = cgrp->root;
-                n = strnlen(root->release_agent_path,
-                            sizeof(root->release_agent_path));
-                n = min(n, (size_t) PAGE_SIZE);
-                strncpy(s, root->release_agent_path, n);
-                mutex_unlock(&cgroup_mutex);
-                s += n;
-                break;
-        }
-        default:
-                retval = -EINVAL;
-                goto out;
-        }
-        *s++ = '\n';
-
-        retval = simple_read_from_buffer(buf, nbytes, ppos, page, s - page);
-out:
-        free_page((unsigned long)page);
-        return retval;
-}
-
 static ssize_t cgroup_file_read(struct file *file, char __user *buf,
                                    size_t nbytes, loff_t *ppos)
 {
@@ -1606,6 +1594,7 @@ int cgroup_seqfile_release(struct inode *inode, struct file *file)
 
 static struct file_operations cgroup_seqfile_operations = {
         .read = seq_read,
+        .write = cgroup_file_write,
         .llseek = seq_lseek,
         .release = cgroup_seqfile_release,
 };
@@ -2283,8 +2272,9 @@ static struct cftype files[] = {
 
 static struct cftype cft_release_agent = {
         .name = "release_agent",
-        .read = cgroup_common_file_read,
-        .write = cgroup_common_file_write,
+        .read_seq_string = cgroup_release_agent_show,
+        .write_string = cgroup_release_agent_write,
+        .max_write_len = PATH_MAX,
         .private = FILE_RELEASE_AGENT,
 };
 
@@ -3111,27 +3101,24 @@ static void cgroup_release_agent(struct work_struct *work)
         while (!list_empty(&release_list)) {
                 char *argv[3], *envp[3];
                 int i;
-                char *pathbuf;
+                char *pathbuf = NULL, *agentbuf = NULL;
                 struct cgroup *cgrp = list_entry(release_list.next,
                                                     struct cgroup,
                                                     release_list);
                 list_del_init(&cgrp->release_list);
                 spin_unlock(&release_list_lock);
                 pathbuf = kmalloc(PAGE_SIZE, GFP_KERNEL);
-                if (!pathbuf) {
-                        spin_lock(&release_list_lock);
-                        continue;
-                }
-
-                if (cgroup_path(cgrp, pathbuf, PAGE_SIZE) < 0) {
-                        kfree(pathbuf);
-                        spin_lock(&release_list_lock);
-                        continue;
-                }
+                if (!pathbuf)
+                        goto continue_free;
+                if (cgroup_path(cgrp, pathbuf, PAGE_SIZE) < 0)
+                        goto continue_free;
+                agentbuf = kstrdup(cgrp->root->release_agent_path, GFP_KERNEL);
+                if (!agentbuf)
+                        goto continue_free;
 
                 i = 0;
-                argv[i++] = cgrp->root->release_agent_path;
-                argv[i++] = (char *)pathbuf;
+                argv[i++] = agentbuf;
+                argv[i++] = pathbuf;
                 argv[i] = NULL;
 
                 i = 0;
@@ -3145,8 +3132,10 @@ static void cgroup_release_agent(struct work_struct *work)
                  * be a slow process */
                 mutex_unlock(&cgroup_mutex);
                 call_usermodehelper(argv[0], argv, envp, UMH_WAIT_EXEC);
-                kfree(pathbuf);
                 mutex_lock(&cgroup_mutex);
+ continue_free:
+                kfree(pathbuf);
+                kfree(agentbuf);
                 spin_lock(&release_list_lock);
         }
         spin_unlock(&release_list_lock);