FFI: Fix dangling reference to CType in carith_checkarg().

Reported by Sergey Kaplun.

(cherry-picked from commit db944b2)

During of an arithmetic operation with a cdata function object and some
cdata value in `carith_checkarg()`, reallocation of `cts->tab` in
`lj_ctype_intern()` may occur. In that case, the reference to the first
`CType` object (`ca->ct[0]`) becomes invalid. This patch saves the
`CTypeID` of this object and gets its `CType` again after possible
reallocation.

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

Part of tarantool/tarantool#9145

src/lj_carith.c

@@ -44,9 +44,13 @@ static int carith_checkarg(lua_State *L, CTState *cts, CDArith *ca)
 	p = (uint8_t *)cdata_getptr(p, ct->size);
 	if (ctype_isref(ct->info)) ct = ctype_rawchild(cts, ct);
       } else if (ctype_isfunc(ct->info)) {
+	CTypeID id0 = i ? ctype_typeid(cts, ca->ct[0]) : 0;
 	p = (uint8_t *)*(void **)p;
 	ct = ctype_get(cts,
 	  lj_ctype_intern(cts, CTINFO(CT_PTR, CTALIGN_PTR|id), CTSIZE_PTR));
+	if (i) {  /* cts->tab may have been reallocated. */
+	  ca->ct[0] = ctype_get(cts, id0);
+	}
       }
       if (ctype_isenum(ct->info)) ct = ctype_child(cts, ct);
       ca->ct[i] = ct;

test/tarantool-tests/lj-1108-fix-dangling-reference-to-ctype.test.lua

@@ -0,0 +1,67 @@
+local tap = require('tap')
+local ffi = require('ffi')
+local test = tap.test('lj-1108-fix-dangling-reference-to-ctype'):skipcond({
+  -- luacheck: no global
+  ['Impossible to predict the value of cts->top'] = _TARANTOOL,
+})
+
+test:plan(1)
+
+-- This test demonstrates LuaJIT's incorrect behaviour when the
+-- reallocation of `cts->tab` strikes during the arithmetic cdata
+-- metamethod.
+-- The test fails under ASAN.
+
+-- XXX: Just some C functions to be casted. There is no need to
+-- declare their prototypes correctly.
+ffi.cdef[[
+  int malloc(void);
+  int fprintf(void);
+  int printf(void);
+  int memset(void);
+  int memcpy(void);
+  int memmove(void);
+  int getppid(void);
+]]
+
+local cfunc_type = ffi.metatype(ffi.typeof('struct {int a;}'), {
+  -- Just some metatable with reloaded arithmetic operator.
+  __add = function(o1, _) return o1 end
+})
+-- Just some cdata with metamethod.
+local test_value = cfunc_type(1)
+
+-- XXX: structure to set `cts->top` to 112.
+local _ = ffi.new('struct {int a; long b; float c; double d;}', 0)
+
+-- Anchor table to prevent cdata objects from being collected.
+local anchor = {}
+-- Each call to this function grows `cts->top` by 3.
+local function save_new_func(func)
+  anchor[#anchor + 1] = ffi.cast('void (*)(void)', func)
+end
+
+save_new_func(ffi.C.fprintf) -- `cts->top` = 112
+save_new_func(ffi.C.printf)  -- `cts->top` = 115
+save_new_func(ffi.C.memset)  -- `cts->top` = 118
+save_new_func(ffi.C.memcpy)  -- `cts->top` = 121
+save_new_func(ffi.C.malloc)  -- `cts->top` = 124
+
+-- Assertions to check the `cts->top` value and step between
+-- calls.
+assert(ffi.typeinfo(124), 'cts->top >= 124')
+assert(not ffi.typeinfo(125), 'cts->top < 125')
+
+save_new_func(ffi.C.memmove) -- `cts->top` = 127
+
+assert(ffi.typeinfo(127), 'cts->top >= 127')
+assert(not ffi.typeinfo(128), 'cts->top < 128')
+
+-- Just check cdata arith metamethod. The function argument should
+-- be the second because dangling reference is the CType of the
+-- first argumnent.
+_ = test_value + ffi.C.getppid
+
+test:ok(true, 'no heap-use-after-free in carith_checkarg')
+
+test:done(true)