[Bug target/93654] New: Inappropriate "- -fcf-protection and -mindirect-branch=thunk are incompatible on x86_64" restriction

[andrew.cooper3 at citrix dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93654

            Bug ID: 93654
           Summary: Inappropriate "- -fcf-protection and
                    -mindirect-branch=thunk are incompatible on x86_64"
                    restriction
           Product: gcc
           Version: 9.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: andrew.cooper3 at citrix dot com
  Target Milestone: ---

Bug https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87412 prohibited the use of
-fcf-protection and -mindirect-branch=thunk in combination.

However, it also breaks kernels which use -mindirect-branch=thunk-extern

When retpoline protections were developed, I specifically requested
thunk-extern to exist for kernels which provide their own, so that it can be
made compatible with CET.

A kernel which provides its own thunks will boot-time modify them to be
appropriate for the system, and may not be a retpoline gadget.  (They may be
lfence; jmp *%reg which is recommended on AMD, or just jmp *%reg with IBRS)

-mindirect-branch=thunk-extern specifically should be permitted with
-fcf-protection, because this *was* the plan to make a single binary capable of
using CET on applicable hardware, yet being safe to Spectre v2 on older
hardware.