[PATCH] drivers/base/node.c: fix userspace break from using bin_attributes for cpumap and cpulist

[Phil Auld <pauld@redhat.com>]

Using bin_attributes with a 0 size causes fstat and friends to return that 0 size.
This breaks userspace code that retrieves the size before reading the file. Rather
than reverting 75bd50fa841 ("drivers/base/node.c: use bin_attribute to break the size
limitation of cpumap ABI") let's put in a size value at compile time. Use direct
comparison and a worst-case maximum to ensure compile time constants. For cpulist the 
max is on the order of NR_CPUS * (ceil(log10(NR_CPUS)) + 1) which for 8192 is 40960. 
In order to get near that you'd need a system with every other CPU on one node or 
something similar. e.g. (0,2,4,... 1024,1026...). We set it to a min of PAGE_SIZE 
to retain the older behavior. For cpumap, PAGE_SIZE is plenty big.

On an 80 cpu 4-node sytem (NR_CPUS == 8192)

before:

-r--r--r--. 1 root root 0 Jul 12 14:08 /sys/devices/system/node/node0/cpulist
-r--r--r--. 1 root root 0 Jul 11 17:25 /sys/devices/system/node/node0/cpumap

after:

-r--r--r--. 1 root root 40960 Jul 12 16:48 /sys/devices/system/node/node0/cpulist
-r--r--r--. 1 root root  4096 Jul 12 15:50 /sys/devices/system/node/node0/cpumap

Fixes: 75bd50fa841 ("drivers/base/node.c: use bin_attribute to break the size limitation of cpumap ABI")
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Signed-off-by: Phil Auld <pauld@redhat.com>
---
 drivers/base/node.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/base/node.c b/drivers/base/node.c
index 0ac6376ef7a1..291c69671f23 100644
--- a/drivers/base/node.c
+++ b/drivers/base/node.c
@@ -45,7 +45,7 @@ static inline ssize_t cpumap_read(struct file *file, struct kobject *kobj,
 	return n;
 }
 
-static BIN_ATTR_RO(cpumap, 0);
+static BIN_ATTR_RO(cpumap, PAGE_SIZE);
 
 static inline ssize_t cpulist_read(struct file *file, struct kobject *kobj,
 				   struct bin_attribute *attr, char *buf,
@@ -66,7 +66,7 @@ static inline ssize_t cpulist_read(struct file *file, struct kobject *kobj,
 	return n;
 }
 
-static BIN_ATTR_RO(cpulist, 0);
+static BIN_ATTR_RO(cpulist, (((NR_CPUS * 5) > PAGE_SIZE) ? NR_CPUS *5 : PAGE_SIZE));
 
 /**
  * struct node_access_nodes - Access class device to hold user visible
-- 
2.31.1