[PATCH v3] 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
(8192 * 5). 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...). To simplify the math and
support larger NR_CPUS we are using NR_CPUS * 7 to support a future with much larger NR_CPUS.
We also set it to a min of PAGE_SIZE to retain the older behavior for smaller NR_CPUS.
The cpumap file wants to be something like NR_CPUS/4 + NR_CPUS/32, for the ","s so for
simplicity we are using NR_CPUS/2.

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 57344 Jul 13 11:32 /sys/devices/system/node/node0/cpulist
-r--r--r--. 1 root root  4096 Jul 13 11:31 /sys/devices/system/node/node0/cpumap

NR_CPUS = 16384
-r--r--r--. 1 root root 114688 Jul 13 14:03 /sys/devices/system/node/node0/cpulist
-r--r--r--. 1 root root   8192 Jul 13 14:02 /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 | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/drivers/base/node.c b/drivers/base/node.c
index 0ac6376ef7a1..89c932a1d8ca 100644
--- a/drivers/base/node.c
+++ b/drivers/base/node.c
@@ -45,7 +45,11 @@ static inline ssize_t cpumap_read(struct file *file, struct kobject *kobj,
 	return n;
 }
 
-static BIN_ATTR_RO(cpumap, 0);
+/* Report a valid max size for this file to avoid breaking userspace. We use NR_CPUS/2 as 
+ * a simplification of NR_CPUS/8 + NR_CPUS/32.  Use PAGE_SIZE as a minimum for smaller 
+ * configurations. 
+ */ 
+static BIN_ATTR_RO(cpumap, (((NR_CPUS >> 1) > PAGE_SIZE) ? NR_CPUS >> 1 : PAGE_SIZE));
 
 static inline ssize_t cpulist_read(struct file *file, struct kobject *kobj,
 				   struct bin_attribute *attr, char *buf,
@@ -66,7 +70,15 @@ static inline ssize_t cpulist_read(struct file *file, struct kobject *kobj,
 	return n;
 }
 
-static BIN_ATTR_RO(cpulist, 0);
+/* Report a valid maximum size for this file since 0 breaks userspace, which
+ * may use the size from fstat to allocate a read buffer. 
+ * The value 7 is a hardcoded version of ceil(log10(NR_CPUS)) + 1 for future values
+ * of NR_CPUS that may be upto 2 orders of magnitude larger than 8192.
+ * In a worst case system every other cpu is on one of two nodes. This leads to 
+ * a file like "0,2,4,6,8...1024,...8190,...". Use PAGE_SIZE as a minimum for smaller
+ * NR_CPUS.  
+*/
+static BIN_ATTR_RO(cpulist, (((NR_CPUS * 7) > PAGE_SIZE) ? NR_CPUS * 7 : PAGE_SIZE));
 
 /**
  * struct node_access_nodes - Access class device to hold user visible
-- 
2.31.1