linux-kernel - [PATCH 11/11] MIPS: math-emu: Fix jalr emulation when rd == $0

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Message-ID: <1461243895-30371-12-git-send-email-paul.burton@imgtec.com>
Date:	Thu, 21 Apr 2016 14:04:55 +0100
From:	Paul Burton <paul.burton@...tec.com>
To:	<linux-mips@...ux-mips.org>, Ralf Baechle <ralf@...ux-mips.org>
CC:	Paul Burton <paul.burton@...tec.com>,
	"stable # v3 . 10" <stable@...r.kernel.org>,
	"Maciej W. Rozycki" <macro@...tec.com>,
	<linux-kernel@...r.kernel.org>,
	James Hogan <james.hogan@...tec.com>,
	"Markos Chandras" <markos.chandras@...tec.com>
Subject: [PATCH 11/11] MIPS: math-emu: Fix jalr emulation when rd == $0

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.")
Cc: stable <stable@...r.kernel.org> # v3.10
Signed-off-by: Paul Burton <paul.burton@...tec.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 85dd174..d96e912 100644
--- a/arch/mips/math-emu/cp1emu.c
+++ b/arch/mips/math-emu/cp1emu.c
@@ -445,9 +445,11 @@ 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:
 			/* For R6, JR already emulated in jalr_op */
-- 
2.8.0