RFA: PR 84154: Fix checking -mibt and -mshstk options for control flow protection

[Nick Clifton <nickc at redhat dot com>]

Hi H.J.

  Attached is a potential patch for PR 84145:
  
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84145

  The problem was that the code to check that the --mibt and/or -mshstk
  options have been correctly enabled for control flow protection did
  not take into account that the wrong option might have been enabled.

  So the patch inverts the test, looking at the value of
  flag_cf_protection first and then checking to see if the needed x86
  specific options have been enabled.  This gives results like this:


   % gcc -c main.c
   % gcc -c main.c -fcf-protection=full
cc1: error: '-fcf-protection=full' requires CET support on this target. Use -mcet or both of -mibt and -mshstk options to enable CET
   % gcc -c main.c -fcf-protection=full -mcet
   % gcc -c main.c -fcf-protection=full -mibt
cc1: error: '-fcf-protection=full' requires CET support on this target. Use -mcet or both of -mibt and -mshstk options to enable CET
   % gcc -c main.c -fcf-protection=full -mibt -mshstk
   % gcc -c main.c -fcf-protection=branch
cc1: error: '-fcf-protection=branch' requires CET support on this target. Use -mcet or -mibt to enable CET
   % gcc -c main.c -fcf-protection=branch -mcet
   % gcc -c main.c -fcf-protection=branch -mibt
   % gcc -c main.c -fcf-protection=branch -mshstk
cc1: error: '-fcf-protection=branch' requires CET support on this target. Use -mcet or -mibt to enable CET
   % gcc -c main.c -fcf-protection=return
cc1: error: '-fcf-protection=return' requires CET support on this target. Use -mcet or -mshstk to enable CET
   % gcc -c main.c -fcf-protection=return -mcet
   % gcc -c main.c -fcf-protection=return -mibt
cc1: error: '-fcf-protection=return' requires CET support on this target. Use -mcet or -mshstk to enable CET
   % gcc -c main.c -fcf-protection=return -mshstk
   %
   
  What do you think ?  Is the patch OK for the mainline ?

Cheers
  Nick

gcc/ChangeLog
2018-02-05  Nick Clifton  <nickc@redhat.com>

	PR 84145
	* config/i386/i386.c (ix86_option_override_internal): Rework
	checks for -fcf-protection and -mibt/-mshstk.

Index: gcc/config/i386/i386.c
===================================================================
--- gcc/config/i386/i386.c	(revision 257389)
+++ gcc/config/i386/i386.c	(working copy)
@@ -4915,30 +4915,43 @@
   /* Do not support control flow instrumentation if CET is not enabled.  */
   if (opts->x_flag_cf_protection != CF_NONE)
     {
-      if (!(TARGET_IBT_P (opts->x_ix86_isa_flags2)
-	    || TARGET_SHSTK_P (opts->x_ix86_isa_flags)))
+      switch (flag_cf_protection)
 	{
-	  if (flag_cf_protection == CF_FULL)
+	case CF_NONE:
+	  break;
+	case CF_BRANCH:
+	  if (! TARGET_IBT_P (opts->x_ix86_isa_flags2))
 	    {
-	      error ("%<-fcf-protection=full%> requires CET support "
-		     "on this target. Use -mcet or one of -mibt, "
-		     "-mshstk options to enable CET");
+	      error ("%<-fcf-protection=branch%> requires CET support "
+		     "on this target. Use -mcet or -mibt to enable CET");
+	      flag_cf_protection = CF_NONE;
+	      return false;
 	    }
-	  else if (flag_cf_protection == CF_BRANCH)
+	  break;
+	case CF_RETURN:
+	  if (! TARGET_SHSTK_P (opts->x_ix86_isa_flags))
 	    {
-	      error ("%<-fcf-protection=branch%> requires CET support "
-		     "on this target. Use -mcet or one of -mibt, "
-		     "-mshstk options to enable CET");
+	      error ("%<-fcf-protection=return%> requires CET support "
+		     "on this target. Use -mcet or -mshstk to enable CET");
+	      flag_cf_protection = CF_NONE;
+	      return false;
 	    }
-	  else if (flag_cf_protection == CF_RETURN)
+	  break;
+	case CF_FULL:
+	  if (   ! TARGET_IBT_P (opts->x_ix86_isa_flags2)
+		 || ! TARGET_SHSTK_P (opts->x_ix86_isa_flags))
 	    {
-	      error ("%<-fcf-protection=return%> requires CET support "
-		     "on this target. Use -mcet or one of -mibt, "
+	      error ("%<-fcf-protection=full%> requires CET support "
+		     "on this target. Use -mcet or both of -mibt and "
 		     "-mshstk options to enable CET");
+	      flag_cf_protection = CF_NONE;
+	      return false;
 	    }
-	  flag_cf_protection = CF_NONE;
-	  return false;
+	  break;
+	default:
+	  gcc_unreachable ();
 	}
+
       opts->x_flag_cf_protection =
 	(cf_protection_level) (opts->x_flag_cf_protection | CF_SET);
     }