Re: [PATCH v2] drivers/base/node.c: fix userspace break from using bin_attributes for cpumap and cpulist

[Phil Auld]

On Wed, Jul 13, 2022 at 03:58:43PM +0200 Greg Kroah-Hartman wrote:
> On Wed, Jul 13, 2022 at 09:45:45AM -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. 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 * 6. 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 49152 Jul 13 09:26 /sys/devices/system/node/node0/cpulist
> > -r--r--r--. 1 root root  4096 Jul 13 09:24 /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@xxxxxxxxxxxxxxxxxxx>
> > Cc: "Rafael J. Wysocki" <rafael@xxxxxxxxxx>
> > Signed-off-by: Phil Auld <pauld@xxxxxxxxxx>
> > ---
> >  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..3577f4eb4ac6 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, (((NR_CPUS>>1) > PAGE_SIZE) ? NR_CPUS >> 1 : PAGE_SIZE));
> 
> Why not just PAGE_SIZE?
>

This one is likely to grow beyond page size if NR_CPUS is larger than 8192. It's about 2300 bytes
at 8192 if my math is right.

Let me ask again before I write the comments... what values of NR_CPUS are you seeing? Are people
using an order of magnitude above 8192? Two orders?  This patch (with the 6 below) handles about 1
order more.

> If not, a HUGE comment explaining what you are doing here is going to be
> required.

Sure, will do. See above :)

> 
> >  
> >  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 * 6) > PAGE_SIZE) ? NR_CPUS *6 : PAGE_SIZE));
> 
> "* 6" in both places here?
>

It's compile time so it's cheap. But yes, 6 in both places.  That's to support NR_CPUS=100000.

> And again, we need a huge comment explaining this so that we can
> understand it in a few weeks, and in 10 years...

Will do.


Thanks,
Phil

> 
> thanks,
> 
> greg k-h
> 

--