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

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

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@linuxfoundation.org>
> Cc: "Rafael J. Wysocki" <rafael@kernel.org>
> Signed-off-by: Phil Auld <pauld@redhat.com>
> ---
>  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?

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

>  
>  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?

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

thanks,

greg k-h