MIPS: r2-on-r6-emu: Fix BLEZL and BGTZL identification

Fix the problem of inaccurate identification of instructions BLEZL and
BGTZL in R2 emulation code by making sure all necessary encoding
specifications are met.

Previously, certain R6 instructions could be identified as BLEZL or
BGTZL. R2 emulation routine didn't take into account that both BLEZL
and BGTZL instructions require their rt field (bits 20 to 16 of
instruction encoding) to be 0, and that, at same time, if the value in
that field is not 0, the encoding may represent a legitimate MIPS R6
instruction.

This means that a problem could occur after emulation optimization,
when emulation routine tried to pipeline emulation, picked up a next
candidate, and subsequently misrecognized an R6 instruction as BLEZL
or BGTZL.

It should be said that for single pass strategy, the problem does not
happen because CPU doesn't trap on branch-compacts which share opcode
space with BLEZL/BGTZL (but have rt field != 0, of course).
Signed-off-by: Leonid Yegoshin <leonid.yegoshin@imgtec.com>
Signed-off-by: Miodrag Dinic <miodrag.dinic@imgtech.com>
Signed-off-by: Aleksandar Markovic <aleksandar.markovic@imgtech.com>
Reported-by: Douglas Leung <douglas.leung@imgtec.com>
Reviewed-by: Paul Burton <paul.burton@imgtec.com>
Cc: james.hogan@imgtec.com
Cc: petar.jovanovic@imgtec.com
Cc: goran.ferenc@imgtec.com
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/15456/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/kernel/mips-r2-to-r6-emul.c

@@ -1096,10 +1096,20 @@ int mipsr2_decoder(struct pt_regs *regs, u32 inst, unsigned long *fcr31)
 		}
 		break;
 
-	case beql_op:
-	case bnel_op:
 	case blezl_op:
 	case bgtzl_op:
+		/*
+		 * For BLEZL and BGTZL, rt field must be set to 0. If this
+		 * is not the case, this may be an encoding of a MIPS R6
+		 * instruction, so return to CPU execution if this occurs
+		 */
+		if (MIPSInst_RT(inst)) {
+			err = SIGILL;
+			break;
+		}
+		/* fall through */
+	case beql_op:
+	case bnel_op:
 		if (delay_slot(regs)) {
 			err = SIGILL;
 			break;