On Mon, Oct 25, 2021 at 08:33:06AM +0000, Yafang Shao wrote:

connector comm was introduced in commit
f786ecba4158 ("connector: add comm change event report to proc connector").
struct comm_proc_event was defined in include/linux/cn_proc.h first and
then been moved into file include/uapi/linux/cn_proc.h in commit
607ca46e97a1 ("UAPI: (Scripted) Disintegrate include/linux").

As this is the UAPI code, we can't change it without potentially breaking
things (i.e. userspace binaries have this size built in, so we can't just
change the size). To prepare for the followup change - extending task
comm, we have to use __get_task_comm() to avoid the BUILD_BUG_ON() in
proc_comm_connector().

I wonder, looking at this again, if it might make more sense to avoid
this cn_proc.c change, and instead, adjust get_task_comm() like so:

#define get_task_comm(buf, tsk)
        __get_task_comm(buf, __must_be_array(buf) + sizeof(buf), tsk)

This would still enforce the original goal of making sure
get_task_comm() is being used on a char array, and now that
__get_task_comm() will truncate & pad, it's safe to use on both
too-small and too-big arrays.

-- 
Kees Cook