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

[Phil Auld <pauld@redhat.com>]

On Fri, Jul 15, 2022 at 05:37:13PM +0200 Greg Kroah-Hartman wrote:
> 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...
>

Erm, counting is hard...  I thought I did have 12 :)

> thanks for sticking with this.

Thanks for the patience.


Cheers,
Phil

> 
> greg k-h
> 

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