aarch64 5.15.68 regression in topology/thread_siblings (huge file size and no content)

[Petr Štetiar]

Hi,

we've got a recent bug report[1], that lscpu segfaults on aarch64 board running
5.15.y kernel. It is working fine on 5.10.y kernel. 

I've tracked it down[2] to the issue with `topology/thread_siblings` which
apart from very strange file size returns empty content. I assume, that it's
somehow related to the changes done in commit bb9ec13d156e ("topology: use
bin_attribute to break the size limitation of cpumap ABI"), but I didn't tried
to revert it yet to verify it.

Kernel 5.15.68:

  root@OpenWrt:/# uname -a
  Linux OpenWrt 5.15.68 #0 SMP Wed Sep 21 05:54:21 2022 aarch64 GNU/Linux

  root@OpenWrt:/# find /sys -name thread_siblings -exec ls -al {} \;
  -r--r--r--    1 root     root     18446744073709551615 Sep 22 08:37 /sys/devices/system/cpu/cpu1/topology/thread_siblings
  -r--r--r--    1 root     root     18446744073709551615 Sep 22 08:37 /sys/devices/system/cpu/cpu0/topology/thread_siblings

  root@OpenWrt:/# find /sys -name thread_siblings -exec cat {} \;
  root@OpenWrt:/# 

Kernel 5.10.138:

  root@OpenWrt:/# uname -a
  Linux OpenWrt 5.10.138 #0 SMP Sat Sep 3 02:55:34 2022 aarch64 GNU/Linux

  root@OpenWrt:/# find /sys -name thread_siblings -exec cat {} \;
  2
  1

  root@OpenWrt:/# find /sys -name thread_siblings -exec ls -al {} \;
  -r--r--r--    1 root     root          4096 Sep 22 11:12 /sys/devices/system/cpu/cpu1/topology/thread_siblings
  -r--r--r--    1 root     root          4096 Sep 22 11:12 /sys/devices/system/cpu/cpu0/topology/thread_siblings


1. https://github.com/openwrt/openwrt/issues/10737
2. https://github.com/util-linux/util-linux/pull/1821


Cheers,

Petr