Re: [PATCH v5] drivers/base: fix userspace break from using bin_attributes for cpumap and cpulist

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Fri, Jul 15, 2022 at 09:49:24AM -0400, Phil Auld wrote:
> 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.
> 
> For cpulist the maximum size is on the order of
> 	NR_CPUS * (ceil(log10(NR_CPUS)) + 1)/2
> 
> which for 8192 is 20480 (8192 * 5)/2. In order to get near that you'd need
> a system with every other CPU on one node. For example: (0,2,4,8, ... ).
> To simplify the math and support larger NR_CPUS in the future we are using
> (NR_CPUS * 7)/2. We also set it to a min of PAGE_SIZE to retain the older
> behavior for smaller NR_CPUS.
> 
> The cpumap file the size works out to be NR_CPUS/4 + NR_CPUS/32 - 1
> (or NR_CPUS * 9/32 - 1) including the ","s.
> 
> Add a set of macros for these values to cpumask.h so they can be used in
> multiple places. Apply these to the handful of such files in
> drivers/base/topology.c as well as node.c.
> 
> As an example, on an 80 cpu 4-node system (NR_CPUS == 8192):
> 
> before:
> 
> -r--r--r--. 1 root root 0 Jul 12 14:08 system/node/node0/cpulist
> -r--r--r--. 1 root root 0 Jul 11 17:25 system/node/node0/cpumap
> 
> after:
> 
> -r--r--r--. 1 root root 28672 Jul 13 11:32 system/node/node0/cpulist
> -r--r--r--. 1 root root  4096 Jul 13 11:31 system/node/node0/cpumap
> 
> CONFIG_NR_CPUS = 16384
> -r--r--r--. 1 root root 57344 Jul 13 14:03 system/node/node0/cpulist
> -r--r--r--. 1 root root  4607 Jul 13 14:02 system/node/node0/cpumap
> 
> The actual number of cpus doesn't matter for the reported size since they
> are based on NR_CPUS.
> 
> Fixes: 75bd50fa841 ("drivers/base/node.c: use bin_attribute to break the size limitation of cpumap ABI")
> Fixes: bb9ec13d156 ("topology: use bin_attribute to break the size limitation of cpumap ABI")

Nit, use the full 12 characters otherwise our tools will complain.  I'll
go fix it up by hand...

thanks for sticking with this.

greg k-h