cgroup: update the meaning of cftype->max_write_len

cftype->max_write_len is used to extend the maximum size of writes.
It's interpreted in such a way that the actual maximum size is one
less than the specified value.  The default size is defined by
CGROUP_LOCAL_BUFFER_SIZE.  Its interpretation is quite confusing - its
value is decremented by 1 and then compared for equality with max
size, which means that the actual default size is
CGROUP_LOCAL_BUFFER_SIZE - 2, which is 62 chars.

There's no point in having a limit that low.  Update its definition so
that it means the actual string length sans termination and anything
below PAGE_SIZE-1 is treated as PAGE_SIZE-1.

.max_write_len for "release_agent" is updated to PATH_MAX-1 and
cgroup_release_agent_write() is updated so that the redundant strlen()
check is removed and it uses strlcpy() instead of strcpy().
.max_write_len initializations in blk-throttle.c and cfq-iosched.c are
no longer necessary and removed.  The one in cpuset is kept unchanged
as it's an approximated value to begin with.

This will also make transition to kernfs smoother.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

block/blk-throttle.c

@@ -1425,28 +1425,24 @@ static struct cftype throtl_files[] = {
 		.private = offsetof(struct throtl_grp, bps[READ]),
 		.seq_show = tg_print_conf_u64,
 		.write_string = tg_set_conf_u64,
-		.max_write_len = 256,
 	},
 	{
 		.name = "throttle.write_bps_device",
 		.private = offsetof(struct throtl_grp, bps[WRITE]),
 		.seq_show = tg_print_conf_u64,
 		.write_string = tg_set_conf_u64,
-		.max_write_len = 256,
 	},
 	{
 		.name = "throttle.read_iops_device",
 		.private = offsetof(struct throtl_grp, iops[READ]),
 		.seq_show = tg_print_conf_uint,
 		.write_string = tg_set_conf_uint,
-		.max_write_len = 256,
 	},
 	{
 		.name = "throttle.write_iops_device",
 		.private = offsetof(struct throtl_grp, iops[WRITE]),
 		.seq_show = tg_print_conf_uint,
 		.write_string = tg_set_conf_uint,
-		.max_write_len = 256,
 	},
 	{
 		.name = "throttle.io_service_bytes",

block/cfq-iosched.c

@@ -1838,7 +1838,6 @@ static struct cftype cfq_blkcg_files[] = {
 		.flags = CFTYPE_ONLY_ON_ROOT,
 		.seq_show = cfqg_print_leaf_weight_device,
 		.write_string = cfqg_set_leaf_weight_device,
-		.max_write_len = 256,
 	},
 	{
 		.name = "weight",
@@ -1853,7 +1852,6 @@ static struct cftype cfq_blkcg_files[] = {
 		.flags = CFTYPE_NOT_ON_ROOT,
 		.seq_show = cfqg_print_weight_device,
 		.write_string = cfqg_set_weight_device,
-		.max_write_len = 256,
 	},
 	{
 		.name = "weight",
@@ -1866,7 +1864,6 @@ static struct cftype cfq_blkcg_files[] = {
 		.name = "leaf_weight_device",
 		.seq_show = cfqg_print_leaf_weight_device,
 		.write_string = cfqg_set_leaf_weight_device,
-		.max_write_len = 256,
 	},
 	{
 		.name = "leaf_weight",

include/linux/cgroup.h

@@ -400,8 +400,9 @@ struct cftype {
 	umode_t mode;
 
 	/*
-	 * If non-zero, defines the maximum length of string that can
-	 * be passed to write_string; defaults to 64
+	 * The maximum length of string, excluding trailing nul, that can
+	 * be passed to write_string.  If < PAGE_SIZE-1, PAGE_SIZE-1 is
+	 * assumed.
 	 */
 	size_t max_write_len;
 

kernel/cgroup.c

@@ -2213,13 +2213,14 @@ static int cgroup_procs_write(struct cgroup_subsys_state *css,
 static int cgroup_release_agent_write(struct cgroup_subsys_state *css,
 				      struct cftype *cft, const char *buffer)
 {
-	BUILD_BUG_ON(sizeof(css->cgroup->root->release_agent_path) < PATH_MAX);
-	if (strlen(buffer) >= PATH_MAX)
-		return -EINVAL;
+	struct cgroupfs_root *root = css->cgroup->root;
+
+	BUILD_BUG_ON(sizeof(root->release_agent_path) < PATH_MAX);
 	if (!cgroup_lock_live_group(css->cgroup))
 		return -ENODEV;
 	spin_lock(&release_agent_path_lock);
-	strcpy(css->cgroup->root->release_agent_path, buffer);
+	strlcpy(root->release_agent_path, buffer,
+		sizeof(root->release_agent_path));
 	spin_unlock(&release_agent_path_lock);
 	mutex_unlock(&cgroup_mutex);
 	return 0;
@@ -2245,20 +2246,17 @@ static int cgroup_sane_behavior_show(struct seq_file *seq, void *v)
 	return 0;
 }
 
-/* A buffer size big enough for numbers or short strings */
-#define CGROUP_LOCAL_BUFFER_SIZE 64
-
 static ssize_t cgroup_file_write(struct file *file, const char __user *userbuf,
 				 size_t nbytes, loff_t *ppos)
 {
 	struct cfent *cfe = __d_cfe(file->f_dentry);
 	struct cftype *cft = __d_cft(file->f_dentry);
 	struct cgroup_subsys_state *css = cfe->css;
-	size_t max_bytes = cft->max_write_len ?: CGROUP_LOCAL_BUFFER_SIZE - 1;
+	size_t max_bytes = max(cft->max_write_len, PAGE_SIZE);
 	char *buf;
 	int ret;
 
-	if (nbytes >= max_bytes)
+	if (nbytes > max_bytes)
 		return -E2BIG;
 
 	buf = kmalloc(nbytes + 1, GFP_KERNEL);
@@ -3919,7 +3917,7 @@ static struct cftype cgroup_base_files[] = {
 		.flags = CFTYPE_INSANE | CFTYPE_ONLY_ON_ROOT,
 		.seq_show = cgroup_release_agent_show,
 		.write_string = cgroup_release_agent_write,
-		.max_write_len = PATH_MAX,
+		.max_write_len = PATH_MAX - 1,
 	},
 	{ }	/* terminate */
 };