Fix stack allocation after on-trace stack check.

(cherry picked from commit 204cee2)

It is possible that a snapshot topslot is less than the possible topslot
of the Lua stack. In that case, if the Lua stack overflows in
`lj_vmevent_prepare()`, the error is raised inside
`lj_vm_exit_handler()`, which has no corresponding DWARF eh_frame [1],
so it leads to the crash.

This patch fix-ups the topslot of the snapshot on trace exit to the
maximum possible one.

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

[1]: https://refspecs.linuxfoundation.org/LSB_3.0.0/LSB-Core-generic/LSB-Core-generic/ehframechpt.html

Part of tarantool/tarantool#10199

Reviewed-by: Sergey Bronnikov <[email protected]>
Reviewed-by: Maxim Kokryashkin <[email protected]>
Signed-off-by: Sergey Kaplun <[email protected]>
(cherry picked from commit 82820a6)

src/lj_trace.c

@@ -522,7 +522,11 @@ static void trace_stop(jit_State *J)
     lj_assertJ(J->parent != 0 && J->cur.root != 0, "not a side trace");
     lj_asm_patchexit(J, traceref(J, J->parent), J->exitno, J->cur.mcode);
     /* Avoid compiling a side trace twice (stack resizing uses parent exit). */
-    traceref(J, J->parent)->snap[J->exitno].count = SNAPCOUNT_DONE;
+    {
+      SnapShot *snap = &traceref(J, J->parent)->snap[J->exitno];
+      snap->count = SNAPCOUNT_DONE;
+      if (J->cur.topslot > snap->topslot) snap->topslot = J->cur.topslot;
+    }
     /* Add to side trace chain in root trace. */
     {
       GCtrace *root = traceref(J, J->cur.root);

test/tarantool-tests/fix-stack-alloc-on-trace-exit.test.lua

@@ -0,0 +1,53 @@
+local tap = require('tap')
+
+-- Test file to demonstrate incorrect Lua stack restoration on
+-- exit from trace by the stack overflow.
+
+local test = tap.test('fix-stack-alloc-on-trace-exit'):skipcond({
+  ['Test requires JIT enabled'] = not jit.status(),
+})
+
+local jit_dump = require('jit.dump')
+
+test:plan(2)
+
+-- Before the patch, it is possible that a snapshot topslot is
+-- less than the possible topslot of the Lua stack. In that case,
+-- if the Lua stack overflows in `lj_vmevent_prepare()`, the error
+-- is raised inside `lj_vm_exit_handler()`, which has no
+-- corresponding DWARF eh_frame, so it leads to the crash.
+
+-- Need for the stack growing in `lj_vmevent_prepare`.
+jit_dump.start('x', '/dev/null')
+
+-- Create a coroutine with a fixed stack size.
+local coro = coroutine.create(function()
+  jit.opt.start('hotloop=1', 'hotexit=1', 'callunroll=1')
+
+  -- `math.modf` recording is NYI.
+  -- Local `math_modf` simplifies `jit.dump()` output.
+  local math_modf = math.modf
+
+  local function trace(n)
+    n = n + 1
+    -- luacheck: ignore
+    -- Start a side trace here.
+    if n % 2 == 0 then end
+    -- Stop the recording of the side trace and a main trace,
+    -- stitching.
+    math_modf(1, 1)
+    -- Grow stack, avoid tail calls.
+    local unused = trace(n)
+    return unused
+  end
+
+  local n = 0
+  trace(n)
+end)
+
+local result, errmsg = coroutine.resume(coro)
+
+test:ok(not result, 'correct status and no crash')
+test:like(errmsg, 'stack overflow', 'correct error message')
+
+test:done(true)