MIPS: math-emu: Make microMIPS branch delay slot emulation work

Complement commit 102cedc32a6e ("MIPS: microMIPS: Floating point
support.") which introduced microMIPS FPU emulation, but did not adjust
the encoding of the BREAK instruction used to terminate the branch delay
slot emulation frame.  Consequently the execution of any such frame is
indeterminate and, depending on CPU configuration, will result in random
code execution or an offending program being terminated with SIGILL.

This is because the regular MIPS BREAK instruction is encoded with the 0
major and the 0xd minor opcode, however in the microMIPS instruction set
this major/minor opcode pair denotes an encoding reserved for the DSP
ASE.  Instead the microMIPS BREAK instruction is encoded with the 0
major and the 0x7 minor opcode.

Use the correct BREAK encoding for microMIPS FPU emulation then.

Signed-off-by: Maciej W. Rozycki <macro@imgtec.com>
Cc: Aurelien Jarno <aurelien@aurel32.net>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/12174/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/include/asm/fpu_emulator.h

@@ -79,7 +79,7 @@ int mm_isBranchInstr(struct pt_regs *regs, struct mm_decoded_insn dec_insn,
 /*
  * Break instruction with special math emu break code set
  */
-#define BREAK_MATH (0x0000000d | (BRK_MEMU << 16))
+#define BREAK_MATH(micromips) (((micromips) ? 0x7 : 0xd) | (BRK_MEMU << 16))
 
 #define SIGNALLING_NAN 0x7ff800007ff80000LL
 

arch/mips/include/asm/mips-r2-to-r6-emul.h

@@ -52,7 +52,7 @@ do {									\
 	__this_cpu_inc(mipsr2emustats.M);				\
 	err = __get_user(nir, (u32 __user *)regs->cp0_epc);		\
 	if (!err) {							\
-		if (nir == BREAK_MATH)					\
+		if (nir == BREAK_MATH(0))				\
 			__this_cpu_inc(mipsr2bdemustats.M);		\
 	}								\
 	preempt_enable();						\

arch/mips/math-emu/dsemul.c

@@ -38,6 +38,7 @@ struct emuframe {
  */
 int mips_dsemul(struct pt_regs *regs, mips_instruction ir, unsigned long cpc)
 {
+	mips_instruction break_math;
 	struct emuframe __user *fr;
 	int err;
 
@@ -65,6 +66,7 @@ int mips_dsemul(struct pt_regs *regs, mips_instruction ir, unsigned long cpc)
 	 * branches, but gives us a cleaner interface to the exception
 	 * handler (single entry point).
 	 */
+	break_math = BREAK_MATH(get_isa16_mode(regs->cp0_epc));
 
 	/* Ensure that the two instructions are in the same cache line */
 	fr = (struct emuframe __user *)
@@ -79,13 +81,13 @@ int mips_dsemul(struct pt_regs *regs, mips_instruction ir, unsigned long cpc)
 				 (u16 __user *)(&fr->emul));
 		err |= __put_user(ir & 0xffff,
 				  (u16 __user *)((long)(&fr->emul) + 2));
-		err |= __put_user(BREAK_MATH >> 16,
+		err |= __put_user(break_math >> 16,
 				  (u16 __user *)(&fr->badinst));
-		err |= __put_user(BREAK_MATH & 0xffff,
+		err |= __put_user(break_math & 0xffff,
 				  (u16 __user *)((long)(&fr->badinst) + 2));
 	} else {
 		err = __put_user(ir, &fr->emul);
-		err |= __put_user((mips_instruction)BREAK_MATH, &fr->badinst);
+		err |= __put_user(break_math, &fr->badinst);
 	}
 
 	err |= __put_user((mips_instruction)BD_COOKIE, &fr->cookie);
@@ -139,7 +141,8 @@ int do_dsemulret(struct pt_regs *xcp)
 	}
 	err |= __get_user(cookie, &fr->cookie);
 
-	if (unlikely(err || (insn != BREAK_MATH) || (cookie != BD_COOKIE))) {
+	if (unlikely(err || insn != BREAK_MATH(get_isa16_mode(xcp->cp0_epc)) ||
+		     cookie != BD_COOKIE)) {
 		MIPS_FPU_EMU_INC_STATS(errors);
 		return 0;
 	}