mem-hotplug: avoid illegal state prefixed with legal state when changing state of...

mem-hotplug: avoid illegal state prefixed with legal state when changing state of memory_block

We use the following command to online a memory_block:

echo online|online_kernel|online_movable > /sys/devices/system/memory/memoryXXX/state

But, if we do the following:

echo online_fhsjkghfkd > /sys/devices/system/memory/memoryXXX/state

the block will also be onlined.

This is because the following code in store_mem_state() does not compare
the whole string, but only the prefix of the string.

store_mem_state()
{
	......
 328         if (!strncmp(buf, "online_kernel", min_t(int, count, 13)))

Here, only compare the first 13 letters of the string. If we give "online_kernelXXXXXX",
it will be recognized as online_kernel, which is incorrect.

 329                 online_type = ONLINE_KERNEL;
 330         else if (!strncmp(buf, "online_movable", min_t(int, count, 14)))

We have the same problem here,

 331                 online_type = ONLINE_MOVABLE;
 332         else if (!strncmp(buf, "online", min_t(int, count, 6)))

here,

(Here is more problematic. If we give online_movalbe, which is a typo of online_movable,
 it will be recognized as online without noticing the author.)

 333                 online_type = ONLINE_KEEP;
 334         else if (!strncmp(buf, "offline", min_t(int, count, 7)))

and here.

 335                 online_type = -1;
 336         else {
 337                 ret = -EINVAL;
 338                 goto err;
 339         }
	......
}

This patch fix this problem by using sysfs_streq() to compare the whole string.

Signed-off-by: Tang Chen <tangchen@cn.fujitsu.com>
Reported-by: Hu Tao <hutao@cn.fujitsu.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
Cc: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
Cc: Gu Zheng <guz.fnst@cn.fujitsu.com>
Acked-by: Toshi Kani <toshi.kani@hp.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>