MIPS: Send SIGILL for R6 branches in `__compute_return_epc_for_insn'

Fix:

* commit 8467ca01 ("MIPS: Emulate the new MIPS R6 branch compact
(BC) instruction"),

* commit 84fef630 ("MIPS: Emulate the new MIPS R6 BALC
instruction"),

* commit 69b9a2fd ("MIPS: Emulate the new MIPS R6 BEQZC and JIC
instructions"),

* commit 28d6f93d ("MIPS: Emulate the new MIPS R6 BNEZC and JIALC
instructions"),

* commit c893ce38 ("MIPS: Emulate the new MIPS R6 BOVC, BEQC and
BEQZALC instructions")

and send SIGILL rather than returning -SIGILL for R6 branch and jump
instructions.  Returning -SIGILL is never correct as the API defines
this function's result upon error to be -EFAULT and a signal actually
issued.

Fixes: 8467ca01 ("MIPS: Emulate the new MIPS R6 branch compact (BC) instruction")
Fixes: 84fef630 ("MIPS: Emulate the new MIPS R6 BALC instruction")
Fixes: 69b9a2fd ("MIPS: Emulate the new MIPS R6 BEQZC and JIC instructions")
Fixes: 28d6f93d ("MIPS: Emulate the new MIPS R6 BNEZC and JIALC instructions")
Fixes: c893ce38 ("MIPS: Emulate the new MIPS R6 BOVC, BEQC and BEQZALC instructions")
Signed-off-by: Maciej W. Rozycki <macro@imgtec.com>
Cc: James Hogan <james.hogan@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: stable@vger.kernel.org # 3.19+
Patchwork: https://patchwork.linux-mips.org/patch/16399/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/kernel/branch.c

@@ -771,35 +771,27 @@ int __compute_return_epc_for_insn(struct pt_regs *regs,
 #else
 	case bc6_op:
 		/* Only valid for MIPS R6 */
-		if (!cpu_has_mips_r6) {
-			ret = -SIGILL;
-			break;
-		}
+		if (!cpu_has_mips_r6)
+			goto sigill_r6;
 		regs->cp0_epc += 8;
 		break;
 	case balc6_op:
-		if (!cpu_has_mips_r6) {
-			ret = -SIGILL;
-			break;
-		}
+		if (!cpu_has_mips_r6)
+			goto sigill_r6;
 		/* Compact branch: BALC */
 		regs->regs[31] = epc + 4;
 		epc += 4 + (insn.i_format.simmediate << 2);
 		regs->cp0_epc = epc;
 		break;
 	case pop66_op:
-		if (!cpu_has_mips_r6) {
-			ret = -SIGILL;
-			break;
-		}
+		if (!cpu_has_mips_r6)
+			goto sigill_r6;
 		/* Compact branch: BEQZC || JIC */
 		regs->cp0_epc += 8;
 		break;
 	case pop76_op:
-		if (!cpu_has_mips_r6) {
-			ret = -SIGILL;
-			break;
-		}
+		if (!cpu_has_mips_r6)
+			goto sigill_r6;
 		/* Compact branch: BNEZC || JIALC */
 		if (!insn.i_format.rs) {
 			/* JIALC: set $31/ra */
@@ -811,10 +803,8 @@ int __compute_return_epc_for_insn(struct pt_regs *regs,
 	case pop10_op:
 	case pop30_op:
 		/* Only valid for MIPS R6 */
-		if (!cpu_has_mips_r6) {
-			ret = -SIGILL;
-			break;
-		}
+		if (!cpu_has_mips_r6)
+			goto sigill_r6;
 		/*
 		 * Compact branches:
 		 * bovc, beqc, beqzalc, bnvc, bnec, bnezlac
@@ -837,6 +827,11 @@ int __compute_return_epc_for_insn(struct pt_regs *regs,
 		current->comm);
 	force_sig(SIGILL, current);
 	return -EFAULT;
+sigill_r6:
+	pr_info("%s: R6 branch but no MIPSr6 ISA support - sending SIGILL.\n",
+		current->comm);
+	force_sig(SIGILL, current);
+	return -EFAULT;
 }
 EXPORT_SYMBOL_GPL(__compute_return_epc_for_insn);