[PATCH 3.16 076/305] MIPS: math-emu: Fix jalr emulation when rd == $0

3.16.37-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Paul Burton <redacted>

commit ab4a92e66741b35ca12f8497896bafbe579c28a1 upstream.

When emulating a jalr instruction with rd == $0, the code in
isBranchInstr was incorrectly writing to GPR $0 which should actually
always remain zeroed. This would lead to any further instructions
emulated which use $0 operating on a bogus value until the task is next
context switched, at which point the value of $0 in the task context
would be restored to the correct zero by a store in SAVE_SOME. Fix this
by not writing to rd if it is $0.

Fixes: 102cedc32a6e ("MIPS: microMIPS: Floating point support.")
Signed-off-by: Paul Burton <redacted>
Cc: Maciej W. Rozycki <redacted>
Cc: James Hogan <redacted>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/13160/
Signed-off-by: Ralf Baechle <redacted>
Signed-off-by: Ben Hutchings <redacted>
---
 arch/mips/math-emu/cp1emu.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

--- a/arch/mips/math-emu/cp1emu.c
+++ b/arch/mips/math-emu/cp1emu.c

@@ -443,9 +443,11 @@ static int isBranchInstr(struct pt_regs
 	case spec_op:
 		switch (insn.r_format.func) {
 		case jalr_op:
-			regs->regs[insn.r_format.rd] =
-				regs->cp0_epc + dec_insn.pc_inc +
-				dec_insn.next_pc_inc;
+			if (insn.r_format.rd != 0) {
+				regs->regs[insn.r_format.rd] =
+					regs->cp0_epc + dec_insn.pc_inc +
+					dec_insn.next_pc_inc;
+			}
 			/* Fall through */
 		case jr_op:
 			*contpc = regs->regs[insn.r_format.rs];