Re: [PATCH v2 11/14] fs/resctrl: Use stricter checks on input to cpus/cpus_list file

[Reinette Chatre <reinette.chatre@intel.com>]

On 3/20/26 3:03 PM, Reinette Chatre wrote:
> Callbacks handling writes to resctrl files are called via
> kernfs_fop_write_iter() that ensures that the resctrl callback is never
> called with a NULL buffer. Even if user space issues a write() with a NULL
> buffer and zero count the resctrl callback receives a valid buffer
> terminated with '\0' and the number of bytes parameter set to zero.
> 
> The NULL buffer check at the beginning of rdtgroup_cpus_write(), while
> making no assumptions about caller behavior, is not useful. An empty
> buffer is transformed into an empty cpumask that is passed through entire
> flow that results in no changes.
> 
> Ensure that the user provided buffer contains some data before attempting
> to parse it using the same check as other resctrl files (the familiar
> "nbytes == 0"). The custom is for resctrl file callbacks to fail on an
> empty buffer and this brings interactions with the cpus/cpus_list file in
> line with this custom. The risk is that if there exists a user space that
> uses empty writes to this specific file then those successful interactions
> will start failing.
> 
> Exit right away if there was no failure yet no cpumask could be created
> from the input. It is of no use to pass an empty cpumask through the
> entire flow, just return with success to short-circuit the existing
> behavior.
> 
> Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
> ---

Chris Samuel reported that this breaks a legitimate use case where user space
could use an empty write, for example write(fd, NULL, 0), to remove all CPUs
from a resource group.

This change thus ended up being too strict and should be dropped.

Reinette