[Tarantool-patches] [PATCH luajit] Fix recording of __concat metamethod.

[Maxim Kokryashkin via Tarantool-patches <tarantool-patches@dev.tarantool.org>]

From: Mike Pall <mike>

Reported by Elias Oelschner. Analyzed by XmiliaH.

(cherry-picked from commit 3ee3c9cfa988339f1bf3068530515e2a6fb179d2)

During the recording of `__concat` methametod, the `rec_mm_arith`
function overrides stack slots that are not restored for GC64
mode later after the call. This leads to a segmentation fault
later on. This patch fixes the issue by accounting for those
additional slots in the array that is used to restore stack
values.

Maxim Kokryashkin:
* added the description and the test for the problem

Part of tarantool/tarantool#9145
---
Branch: https://github.com/tarantool/luajit/tree/fckxorg/lj-839-concat-recording
PR: https://github.com/tarantool/tarantool/pull/9597
Issues: https://github.com/tarantool/tarantool/issues/9145
https://github.com/luajit/luajit/issues/839

 src/lj_record.c                               |  2 +-
 .../lj-839-concat-recording.test.lua          | 27 +++++++++++++++++++
 2 files changed, 28 insertions(+), 1 deletion(-)
 create mode 100644 test/tarantool-tests/lj-839-concat-recording.test.lua

diff --git a/src/lj_record.c b/src/lj_record.c
index a929b8aa..59549b03 100644
--- a/src/lj_record.c
+++ b/src/lj_record.c
@@ -1932,7 +1932,7 @@ static TRef rec_tnew(jit_State *J, uint32_t ah)
 static TRef rec_cat(jit_State *J, BCReg baseslot, BCReg topslot)
 {
   TRef *top = &J->base[topslot];
-  TValue savetv[5];
+  TValue savetv[5+LJ_FR2];
   BCReg s;
   RecordIndex ix;
   lj_assertJ(baseslot < topslot, "bad CAT arg");
diff --git a/test/tarantool-tests/lj-839-concat-recording.test.lua b/test/tarantool-tests/lj-839-concat-recording.test.lua
new file mode 100644
index 00000000..9ec0ed96
--- /dev/null
+++ b/test/tarantool-tests/lj-839-concat-recording.test.lua
@@ -0,0 +1,27 @@
+local tap = require('tap')
+local test = tap.test('lj-839-concat-recording'):skipcond({
+  ['Test requires JIT enabled'] = not jit.status(),
+})
+test:plan(1)
+
+-- Test file to demonstrate LuaJIT overriding stack slots without
+-- restoration during the recording of the concat metamethod.
+-- See also: https://github.com/LuaJIT/LuaJIT/issues/839.
+
+-- Setup value with the `__concat` metamethod.
+local v1 = setmetatable({}, {
+    __concat = function() return "" end,
+});
+
+jit.opt.start('hotloop=1')
+local result
+for _ = 1, 4 do
+  result = v1 .. ""  .. v1 ..  ""
+end
+
+-- There should be an empty string in case of success.
+-- Failure results in a segmentation fault.
+-- The issue is GC64-specific, yet it is still being tested for
+-- other builds.
+test:is(result, '', 'correct stack restoration')
+test:done(true)
--
2.43.0