MIPS: Fix bnezc/jialc return address calculation

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@imgtec.com>
Fixes: 28d6f93d201d ("MIPS: Emulate the new MIPS R6 BNEZC and JIALC instructions")
Cc: stable <stable@vger.kernel.org> # v4.0+
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/16178/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

diff --git a/arch/mips/kernel/branch.c b/arch/mips/kernel/branch.c
index b11facd11c9d05dfae733094359da713c948ad84..f702a459a830060f6aa77fe09eda28c03a3d4f4c 100644 (file)
--- a/arch/mips/kernel/branch.c
+++ b/arch/mips/kernel/branch.c
@@ -804,8 +804,10 @@ int __compute_return_epc_for_insn(struct pt_regs *regs,
                        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