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>

2 files changed, 49 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
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 70d083c6fb6b..3a99cc2df860 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1363,6 +1363,39 @@ static ssize_t cgroup_write_X64(struct cgroup *cgrp, struct cftype *cft,
         return retval;
 }
 
+static ssize_t cgroup_write_string(struct cgroup *cgrp, struct cftype *cft,
+                                   struct file *file,
+                                   const char __user *userbuf,
+                                   size_t nbytes, loff_t *unused_ppos)
+{
+        char local_buffer[64];
+        int retval = 0;
+        size_t max_bytes = cft->max_write_len;
+        char *buffer = local_buffer;
+
+        if (!max_bytes)
+                max_bytes = sizeof(local_buffer) - 1;
+        if (nbytes >= max_bytes)
+                return -E2BIG;
+        /* Allocate a dynamic buffer if we need one */
+        if (nbytes >= sizeof(local_buffer)) {
+                buffer = kmalloc(nbytes + 1, GFP_KERNEL);
+                if (buffer == NULL)
+                        return -ENOMEM;
+        }
+        if (nbytes && copy_from_user(buffer, userbuf, nbytes))
+                return -EFAULT;
+
+        buffer[nbytes] = 0;     /* nul-terminate */
+        strstrip(buffer);
+        retval = cft->write_string(cgrp, cft, buffer);
+        if (!retval)
+                retval = nbytes;
+        if (buffer != local_buffer)
+                kfree(buffer);
+        return retval;
+}
+
 static ssize_t cgroup_common_file_write(struct cgroup *cgrp,
                                            struct cftype *cft,
                                            struct file *file,
@@ -1440,6 +1473,8 @@ static ssize_t cgroup_file_write(struct file *file, const char __user *buf,
                 return cft->write(cgrp, cft, file, buf, nbytes, ppos);
         if (cft->write_u64 || cft->write_s64)
                 return cgroup_write_X64(cgrp, cft, file, buf, nbytes, ppos);
+        if (cft->write_string)
+                return cgroup_write_string(cgrp, cft, file, buf, nbytes, ppos);
         if (cft->trigger) {
                 int ret = cft->trigger(cgrp, (unsigned int)cft->private);
                 return ret ? ret : nbytes;