cgroup files: add write_string cgroup control file method

This patch adds a write_string() method for cgroups control files. The
semantics are that a buffer is copied from userspace to kernelspace
and the handler function invoked on that buffer.  The buffer is
guaranteed to be nul-terminated, and no longer than max_write_len
(defaulting to 64 bytes if unspecified). Later patches will convert
existing raw file write handlers in control group subsystems to use
this method.

Signed-off-by: Paul Menage <menage@google.com>
Cc: Paul Jackson <pj@sgi.com>
Cc: Pavel Emelyanov <xemul@openvz.org>
Acked-by: 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>

1 files changed, 14 insertions, 0 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 88a734edccbc..f5379455bb59 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -205,6 +205,13 @@ struct cftype {
          * subsystem, followed by a period */
         char name[MAX_CFTYPE_NAME];
         int private;
+
+        /*
+         * If non-zero, defines the maximum length of string that can
+         * be passed to write_string; defaults to 64
+         */
+        size_t max_write_len;
+
         int (*open)(struct inode *inode, struct file *file);
         ssize_t (*read)(struct cgroup *cgrp, struct cftype *cft,
                         struct file *file,
@@ -249,6 +256,13 @@ struct cftype {
         int (*write_s64)(struct cgroup *cgrp, struct cftype *cft, s64 val);
 
         /*
+         * write_string() is passed a nul-terminated kernelspace
+         * buffer of maximum length determined by max_write_len.
+         * Returns 0 or -ve error code.
+         */
+        int (*write_string)(struct cgroup *cgrp, struct cftype *cft,
+                            const char *buffer);
+        /*
          * trigger() callback can be used to get some kick from the
          * userspace, when the actual string written is not important
          * at all. The private field can be used to determine the