Hi, Maxim,

thanks for review! Please see my comments below.

Fixes applied and force-pushed.

Hi, Sergey!
Thanks for the patch!
Please consider my comments below.
On Tue, Mar 26, 2024 at 06:29:11PM +0300, Sergey Bronnikov wrote:

From: Sergey Bronnikov <sergeyb@tarantool.org>

Reported by XmiliaH.

(cherry picked from commit 0e66fc96377853d898390f1a02723c54ec3a42f7)

It is possible to get an infinite loop in a function `snap_usedef`
when a `UCLO` makes a tight loop.

The description should include explanation for the cause of the issue
and should explain how it was resolved.

Added more details about patch to commit message:

Updated description:

    It is possible to get an infinite loop in a function `snap_usedef`
    when a `UCLO` makes a tight loop. This infinite loop could happen
    when `snap_usedef()` is called on trace exit processes UCLO
    bytecode instruction and this instruction attempts a jump with
    negative value. The patch fixes the problem by checking a number
    of slots in a jump argument and replace this value my `maxslot` if
    a value is negative.

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

Part of tarantool/tarantool#9595
---
Branch: https://github.com/tarantool/luajit/tree/ligurio/lj-736-prevent-loop-in-snap_usedef
Issues:
- https://github.com/LuaJIT/LuaJIT/issues/736
- https://github.com/tarantool/tarantool/issues/9595

 src/lj_snap.c                                 |  7 ++-
 .../lj-736-BC_UCLO-triggers-infinite-loop.lua | 59 +++++++++++++++++++
 2 files changed, 65 insertions(+), 1 deletion(-)
 create mode 100644 test/tarantool-tests/lj-736-BC_UCLO-triggers-infinite-loop.lua

diff --git a/src/lj_snap.c b/src/lj_snap.c
index 5a00b5cd..0710e1f0 100644
--- a/src/lj_snap.c
+++ b/src/lj_snap.c
@@ -252,7 +252,12 @@ static BCReg snap_usedef(jit_State *J, uint8_t *udf,
       BCReg minslot = bc_a(ins);
       if (op >= BC_FORI && op <= BC_JFORL) minslot += FORL_EXT;
       else if (op >= BC_ITERL && op <= BC_JITERL) minslot += bc_b(pc[-2])-1;
-      else if (op == BC_UCLO) { pc += bc_j(ins); break; }
+      else if (op == BC_UCLO) {
+	ptrdiff_t delta = bc_j(ins);
+	if (delta < 0) return maxslot;  /* Prevent loop. */
+	pc += delta;
+	break;
+      }
       for (s = minslot; s < maxslot; s++) DEF_SLOT(s);
       return minslot < maxslot ? minslot : maxslot;
       }
diff --git a/test/tarantool-tests/lj-736-BC_UCLO-triggers-infinite-loop.lua b/test/tarantool-tests/lj-736-BC_UCLO-triggers-infinite-loop.lua
new file mode 100644
index 00000000..28a2b61b
--- /dev/null
+++ b/test/tarantool-tests/lj-736-BC_UCLO-triggers-infinite-loop.lua
@@ -0,0 +1,59 @@
+local tap = require('tap')
+local test = tap.test('lj-736-BC_UCLO-triggers-infinite-loop'):skipcond({
+  ['Test requires JIT enabled'] = not jit.status(),
+})
+
+test:plan(1)
+
+-- Test reproduces an issue when BC_UCLO triggers an infinite loop.
+-- See details in https://github.com/LuaJIT/LuaJIT/issues/736.
+--
+-- Listing below demonstrates a problem -
+-- the bytecode UCLO on the line 13 makes a loop at 0013-0014:
+--
+-- - BYTECODE -- bc_uclo.lua:0-20
+-- 0001    KPRI     0   0
+-- 0002    FNEW     1   0      ; bc_uclo.lua:5
+-- 0003    KSHORT   2   1
+-- 0004    KSHORT   3   4
+-- 0005    KSHORT   4   1
+-- 0006    FORI     2 => 0011
+-- 0007 => ISNEN    5   0      ; 2
+-- 0008    JMP      6 => 0010
+-- 0009    UCLO     0 => 0012
+-- 0010 => FORL     2 => 0007
+-- 0011 => UCLO     0 => 0012
+-- 0012 => KPRI     0   0
+-- 0013    UCLO     0 => 0012
+-- 0014    FNEW     1   1      ; bc_uclo.lua:18
+-- 0015    UCLO     0 => 0016
+-- 0016 => RET0     0   1
+
+jit.opt.start('hotloop=1')
+
+do
+  local uv = 0
+  local w = function() return uv end -- luacheck: no unused
+  for i = 1, 2 do

Add a comment that we have two iterations only because we only
need to record the trace.

+    -- Infinite loop is here.
+    if i == 2 then
+      if i == 2 then
+        goto pass
+      end
+      goto unreachable
+    end
+  end
+end
+
+::unreachable::
+-- Lua chunk below is required for reproducing a bug.
+do
+  local uv = 0 -- luacheck: no unused
+  goto unreachable
+  local w = function() return uv end -- luacheck: ignore
+end
+
+::pass::

Please add a comment explaining why do we need a goto statement
and an unreachable code segment.

+
+test:ok(true, 'BC_UCLO does not trigger an infinite loop')
+os.exit(test:check() and 0 or 1)

The test executes without any failures for the x86 non-GC64
build before the patch. The exact command:
$ cmake .. -DCMAKE_BUILD_TYPE=Debug -DLUA_USE_ASSERT=ON -DLUA_USE_APICHECK=ON && make -j && make test

Fixed that. Now without patch test hangs and passed with applied patch.

--
2.34.1