Add cgroup write_uint() helper method

Add write_uint() helper method for cgroup subsystems

This helper is analagous to the read_uint() helper method for
reporting u64 values to userspace. It's designed to reduce the amount
of boilerplate requierd for creating new cgroup subsystems.

Signed-off-by: Paul Menage <menage@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 46 insertions, 4 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index e2dd44f68f97..e95143c884b2 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -125,6 +125,14 @@ struct cftype {
         ssize_t (*write) (struct cgroup *cont, struct cftype *cft,
                           struct file *file,
                           const char __user *buf, size_t nbytes, loff_t *ppos);
+
+        /*
+         * write_uint() is a shortcut for the common case of accepting
+         * a single integer (as parsed by simple_strtoull) from
+         * userspace. Use in place of write(); return 0 or error.
+         */
+        int (*write_uint) (struct cgroup *cont, struct cftype *cft, u64 val);
+
         int (*release) (struct inode *inode, struct file *file);
 };
 
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 356c40d5d20a..f4c4dce9558f 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -829,6 +829,39 @@ enum cgroup_filetype {
         FILE_TASKLIST,
 };
 
+static ssize_t cgroup_write_uint(struct cgroup *cont, struct cftype *cft,
+                                 struct file *file,
+                                 const char __user *userbuf,
+                                 size_t nbytes, loff_t *unused_ppos)
+{
+        char buffer[64];
+        int retval = 0;
+        u64 val;
+        char *end;
+
+        if (!nbytes)
+                return -EINVAL;
+        if (nbytes >= sizeof(buffer))
+                return -E2BIG;
+        if (copy_from_user(buffer, userbuf, nbytes))
+                return -EFAULT;
+
+        buffer[nbytes] = 0;     /* nul-terminate */
+
+        /* strip newline if necessary */
+        if (nbytes && (buffer[nbytes-1] == '\n'))
+                buffer[nbytes-1] = 0;
+        val = simple_strtoull(buffer, &end, 0);
+        if (*end)
+                return -EINVAL;
+
+        /* Pass to subsystem */
+        retval = cft->write_uint(cont, cft, val);
+        if (!retval)
+                retval = nbytes;
+        return retval;
+}
+
 static ssize_t cgroup_common_file_write(struct cgroup *cont,
                                            struct cftype *cft,
                                            struct file *file,
@@ -886,10 +919,11 @@ static ssize_t cgroup_file_write(struct file *file, const char __user *buf,
 
         if (!cft)
                 return -ENODEV;
-        if (!cft->write)
-                return -EINVAL;
-
-        return cft->write(cont, cft, file, buf, nbytes, ppos);
+        if (cft->write)
+                return cft->write(cont, cft, file, buf, nbytes, ppos);
+        if (cft->write_uint)
+                return cgroup_write_uint(cont, cft, file, buf, nbytes, ppos);
+        return -EINVAL;
 }
 
 static ssize_t cgroup_read_uint(struct cgroup *cont, struct cftype *cft,