[PATCH 4.9 55/60] MIPS: Fix bnezc/jialc return address calculation

[Greg Kroah-Hartman]

4.9-stable review patch.  If anyone has any objections, please let me know.

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

From: Paul Burton <paul.burton@xxxxxxxxxx>

commit 1a73d9310e093fc3adffba4d0a67b9fab2ee3f63 upstream.

The code handling the pop76 opcode (ie. bnezc & jialc instructions) in
__compute_return_epc_for_insn() needs to set the value of $31 in the
jialc case, which is encoded with rs = 0. However its check to
differentiate bnezc (rs != 0) from jialc (rs = 0) was unfortunately
backwards, meaning that if we emulate a bnezc instruction we clobber $31
& if we emulate a jialc instruction it actually behaves like a jic
instruction.

Fix this by inverting the check of rs to match the way the instructions
are actually encoded.

Signed-off-by: Paul Burton <paul.burton@xxxxxxxxxx>
Fixes: 28d6f93d201d ("MIPS: Emulate the new MIPS R6 BNEZC and JIALC instructions")
Cc: linux-mips@xxxxxxxxxxxxxx
Patchwork: https://patchwork.linux-mips.org/patch/16178/
Signed-off-by: Ralf Baechle <ralf@xxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 arch/mips/kernel/branch.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

--- a/arch/mips/kernel/branch.c
+++ b/arch/mips/kernel/branch.c
@@ -804,8 +804,10 @@ int __compute_return_epc_for_insn(struct
 			break;
 		}
 		/* Compact branch: BNEZC || JIALC */
-		if (insn.i_format.rs)
+		if (!insn.i_format.rs) {
+			/* JIALC: set $31/ra */
 			regs->regs[31] = epc + 4;
+		}
 		regs->cp0_epc += 8;
 		break;
 #endif