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

[Greg Kroah-Hartman]

On Wed, Jul 13, 2022 at 07:37:27AM -0400, Phil Auld wrote:
> On Wed, Jul 13, 2022 at 11:18:59AM +1200 Barry Song wrote:
> > On Wed, Jul 13, 2022 at 9:58 AM Phil Auld <pauld@xxxxxxxxxx> 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. 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 system (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
> > 
> > it is a fundamental problem of bin_attr, isn't it? when we don't know the
> > exact size of an attribute, and this size might become more than one
> > PAGE_SIZE, we use bin_attr to break the limitation. but the fact is that
> > we really don't know or it is really hard to know the actual size of the
> > attribute.
> >
> 
> But it broke userspace applications. I figured rather than revert it maybe
> we can find a max size to put in there and make it continue to work.

Yes, we need to do this, we can't break userspace.

thanks,

greg k-h