Work around Linux 5.8's unexporting of native_write_cr4

[solardiz]

Linux 5.8 no longer exports the native_write_cr4 symbol unless CONFIG_LKDTM is enabled. We use that symbol, and need to work around that, perhaps by using kallsyms_lookup_name to locate it.

Reference: https://www.openwall.com/lists/lkrg-users/2020/08/07/1

[solardiz]

We believe this is fixed with 671b079, although more testing is needed. Closing the issue for now (can reopen if necessary).

[morfikov]

I can confirm that it builds, at least on:

# cat /proc/version
Linux version 5.8.1-amd64 (morfik@morfikownia) (gcc (Debian 10.2.0-5) 10.2.0, GNU ld (GNU Binutils for Debian) 2.35) #1 SMP PREEMPT Thu Aug 13 13:14:40 CEST 2020