FFI: Fix pragma push stack limit check and throw on overflow.

Reported by Sergey Kaplun.

(cherry-picked from commit 433d7e8)

`cp->packstack` is the array of size `CPARSE_MAX_PACKSTACK` (7). Before
the patch, `cp->curpack` is checked to be less than
`CPARSE_MAX_PACKSTACK`, but then `cp->packstack` is accessed at
`cp->curpack + 1`, which is out of bounds, so `cp->curpack` value is
overwritten.

This patch fixes a condition and also adds the error throw when counter
is overflow (instead of rewriting a top `cp->packstack` value).

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

Resolves tarantool/tarantool#9339
Part of tarantool/tarantool#9145

src/lj_cparse.c

@@ -1777,9 +1777,11 @@ static void cp_pragma(CPState *cp, BCLine pragmaline)
     cp_check(cp, '(');
     if (cp->tok == CTOK_IDENT) {
       if (cp_str_is(cp->str, "push")) {
-	if (cp->curpack < CPARSE_MAX_PACKSTACK) {
+	if (cp->curpack < CPARSE_MAX_PACKSTACK-1) {
 	  cp->packstack[cp->curpack+1] = cp->packstack[cp->curpack];
 	  cp->curpack++;
+	} else {
+	  cp_errmsg(cp, cp->tok, LJ_ERR_XLEVELS);
 	}
       } else if (cp_str_is(cp->str, "pop")) {
 	if (cp->curpack > 0) cp->curpack--;

test/tarantool-tests/lj-1114-ffi-pragma-pack.test.lua

@@ -0,0 +1,44 @@
+local tap = require('tap')
+
+-- Test file to demonstrate LuaJIT incorrect parsing of `#pragma`
+-- directive via FFI.
+-- See also: https://github.com/LuaJIT/LuaJIT/issues/1114.
+
+local test = tap.test('lj-1114-ffi-pragma-pack')
+local ffi = require 'ffi'
+
+test:plan(2)
+
+-- `cp->packstack` is the array of size `CPARSE_MAX_PACKSTACK`
+-- (7). Before the patch, `cp->curpack` is checked to be less than
+-- `CPARSE_MAX_PACKSTACK`, but then `cp->packstack` is accessed at
+-- `cp->curpack + 1`, which is out of bounds, so `cp->curpack`
+-- value is overwritten.
+-- As a result, the incorrect pack value (1) is chosen after pop.
+-- After the patch, the error is thrown in the case of overflow
+-- (instead of rewriting the top pack slot value), so we use the
+-- wrapper to catch the error.
+local function ffi_cdef_wp()
+  ffi.cdef[[
+    #pragma pack(push, 1)
+    #pragma pack(push, 1)
+    #pragma pack(push, 1)
+    #pragma pack(push, 1)
+    #pragma pack(push, 8)
+    #pragma pack(push, 8)
+    #pragma pack(push, 8)
+    #pragma pack(pop)
+    struct aligned_struct {uint64_t a; uint8_t b;};
+  ]]
+
+  -- Got 9 in case of buffer overflow.
+  return ffi.sizeof(ffi.new('struct aligned_struct'))
+end
+
+local err, msg = pcall(ffi_cdef_wp)
+
+test:ok(not err, 'the error is thrown when couner overflows')
+test:like(msg, 'chunk has too many syntax levels',
+          'the error message is correct')
+
+test:done(true)