Re: [RFC PATCH net-next] bpf: fix potential percpu map overcopy to user.

Documentation/cputopology.txt +106 says /sys/devices/system/cpu/possible
outputs cpu_possible_mask. That is the same as in num_possible_cpus(), so
first step would be to fix the buggy example code, imho.

What perhaps could be done in a second step to reduce overhead is an option
for bpf(2) to pass in a cpu mask similarly as for sched_{get,set}affinity()
syscalls, where user space can construct a mask via CPU_SET(3). For the
syscall time, kernel would lock hot plugging via get_online_cpus() and
put_online_cpus(), it would check whether passed CPUs are online to query
and if so then it would copy the values into the user provided buffer. I'd
think this might be useful in a number of ways anyway.

I like this idea. So in this case, the only the data at the cpu
specified by user in the CPU_SET is copied to userspace, potentially
have better performance than always copying the data *
num_possible_cpus() bytes.

Regards,
William