[Tarantool-patches] [PATCH luajit] Fix bytecode dump unpatching.

[Sergey Kaplun via Tarantool-patches <tarantool-patches@dev.tarantool.org>]

From: Mike Pall <mike>

Reported by Christopher Oliver.

(cherry picked from commit 20ac817a747cf8cab044ae81b09c08d23e34342b)

When a compiled function with up-recursion RET bytecodes are patched to
JLOOP bytecode. During dump of those bytecodes they should be unpatched
to the original one. It is done by restoring the opcode by subtraction
the diff between JLOOP and ILOOP bytecodes. That gives the LOOP
bytecodes instead RET as expected.

This patch fixes the bytecode unpatching via copy the original start
instruction, that was patched.

Sergey Kaplun:
* added the description and the test for the problem

Part of tarantool/tarantool#6548
---

Branch: https://github.com/tarantool/luajit/tree/skaplun/gh-noticket-wrong-bc-ret
Tarantool branch: https://github.com/tarantool/tarantool/tree/skaplun/gh-noticket-wrong-bc-ret-full-ci
Related issue: https://github.com/tarantool/tarantool/issues/6548

 src/lj_bcwrite.c                              |  5 +----
 .../bc-jit-unpatching.test.lua                | 22 +++++++++++++++++++
 2 files changed, 23 insertions(+), 4 deletions(-)
 create mode 100644 test/tarantool-tests/bc-jit-unpatching.test.lua

diff --git a/src/lj_bcwrite.c b/src/lj_bcwrite.c
index 5e05caea..a86d6d00 100644
--- a/src/lj_bcwrite.c
+++ b/src/lj_bcwrite.c
@@ -219,10 +219,7 @@ static char *bcwrite_bytecode(BCWriteCtx *ctx, char *p, GCproto *pt)
 	q[LJ_ENDIAN_SELECT(0, 3)] = (uint8_t)(op-BC_IFORL+BC_FORL);
       } else if (op == BC_JFORL || op == BC_JITERL || op == BC_JLOOP) {
 	BCReg rd = q[LJ_ENDIAN_SELECT(2, 1)] + (q[LJ_ENDIAN_SELECT(3, 0)] << 8);
-	BCIns ins = traceref(J, rd)->startins;
-	q[LJ_ENDIAN_SELECT(0, 3)] = (uint8_t)(op-BC_JFORL+BC_FORL);
-	q[LJ_ENDIAN_SELECT(2, 1)] = bc_c(ins);
-	q[LJ_ENDIAN_SELECT(3, 0)] = bc_b(ins);
+	memcpy(q, &traceref(J, rd)->startins, 4);
       }
     }
   }
diff --git a/test/tarantool-tests/bc-jit-unpatching.test.lua b/test/tarantool-tests/bc-jit-unpatching.test.lua
new file mode 100644
index 00000000..9f9cb390
--- /dev/null
+++ b/test/tarantool-tests/bc-jit-unpatching.test.lua
@@ -0,0 +1,22 @@
+local tap = require('tap')
+local utils = require('utils')
+
+local test = tap.test('bc-jit-unpatching')
+test:plan(1)
+
+-- Function with up-recursion.
+local function f(n)
+  return n < 2 and n or f(n - 1) + f(n - 2)
+end
+
+local ret1bc = 'RET1%s*1%s*2'
+-- Check that this bytecode still persists.
+assert(utils.hasbc(loadstring(string.dump(f)), ret1bc))
+
+-- Compile function to get JLOOP bytecode in recursion.
+f(10)
+
+test:ok(utils.hasbc(loadstring(string.dump(f)), ret1bc),
+        'bytecode unpached correctly')
+
+os.exit(test:check() and 0 or 1)
-- 
2.34.1