[PATCH v2] arch: Reserve map_shadow_stack() syscall number for all architectures

[Edgecombe, Rick P]

On Thu, 2023-09-14 at 18:58 +0000, Sohil Mehta wrote:
> commit c35559f94ebc ("x86/shstk: Introduce map_shadow_stack syscall")
> recently added support for map_shadow_stack() but it is limited to
> x86
> only for now. There is a possibility that other architectures
> (namely,
> arm64 and RISC-V), that are implementing equivalent support for
> shadow
> stacks, might need to add support for it.
> 
> Independent of that, reserving arch-specific syscall numbers in the
> syscall tables of all architectures is good practice and would help
> avoid future conflicts. map_shadow_stack() is marked as a conditional
> syscall in sys_ni.c. Adding it to the syscall tables of other
> architectures is harmless and would return ENOSYS when exercised.
> 
> Note, map_shadow_stack() was assigned #453 during the merge process
> since #452 was taken by fchmodat2().
> 
> For Powerpc, map it to sys_ni_syscall() as is the norm for Powerpc
> syscall tables.
> 
> For Alpha, map_shadow_stack() takes up #563 as Alpha still diverges
> from
> the common syscall numbering system in the other architectures.

Reviewed-by: Rick Edgecombe <rick.p.edgecombe at intel.com>