[3.19.y-ckt stable] Patch "MIPS: math-emu: Fix jalr emulation when rd == $0" has been added to the 3.19.y-ckt tree

Kamal Mostafa kamal at canonical.com
Wed Jul 6 21:00:51 UTC 2016

This is a note to let you know that I have just added a patch titled

    MIPS: math-emu: Fix jalr emulation when rd == $0

to the linux-3.19.y-queue branch of the 3.19.y-ckt extended stable tree 
which can be found at:


This patch is scheduled to be released in version 3.19.8-ckt23.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.19.y-ckt tree, see



>From bc1487c9109b97c514886bc17da86e0b045dd417 Mon Sep 17 00:00:00 2001
From: Paul Burton <paul.burton at imgtec.com>
Date: Thu, 21 Apr 2016 14:04:55 +0100
Subject: MIPS: math-emu: Fix jalr emulation when rd == $0

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 <paul.burton at imgtec.com>
Cc: Maciej W. Rozycki <macro at imgtec.com>
Cc: James Hogan <james.hogan at imgtec.com>
Cc: linux-mips at linux-mips.org
Cc: linux-kernel at vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/13160/
Signed-off-by: Ralf Baechle <ralf at linux-mips.org>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
 arch/mips/math-emu/cp1emu.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/arch/mips/math-emu/cp1emu.c b/arch/mips/math-emu/cp1emu.c
index 9dfcd7f..862bc86 100644
--- a/arch/mips/math-emu/cp1emu.c
+++ b/arch/mips/math-emu/cp1emu.c
@@ -443,9 +443,11 @@ static int isBranchInstr(struct pt_regs *regs, struct mm_decoded_insn dec_insn,
 	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];

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