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>

1 files changed, 3 insertions, 1 deletions

diff --git a/arch/mips/kernel/branch.c b/arch/mips/kernel/branch.c
index b11facd11c9d..f702a459a830 100644
--- 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