wslua: fix crash when a LUA error is raised in TRY block

The dissect_tcp_pdus function in LUA is passed two LUA functions that
get the PDU length and the dissect a PDU. When one of these functions
fail, a longjmp is made to the the caller of lua_pcall.

This is no problem for the PDU length function, but the PDU dissect
function is wrapped in a TRY/CATCH/ENDTRY block which also uses longjmp
and need to be fully executed. Without doing so, LUA exceptions will
crash on a weird location (except_pop).

Fix the crash by not using luaL_error, but throw dissector errors which
properly breaks out of the tcp_dissect_pdus C function and then convert
it to a LUA error such that the dissector can handle it.

Test with `tshark -X lua_script:crash.lua -r ssl.pcap`:

    trivial_proto = Proto("trivial", "Trivial Protocol")
    function dissect_foo(tvb, pinfo, tree)
        error("triggering a LUA error");
    end
    function get_pdu_len(tvb, pinfo, tree) return 5; end
    function trivial_proto.dissector(tvb, pinfo, tree)
        dissect_tcp_pdus(tvb, tree, 5, get_pdu_len, dissect_foo)
    end
    tcp_table = DissectorTable.get("tcp.port")
    tcp_table:add(443, trivial_proto)

It should not crash and will print this:

    Lua Error: dissect_tcp_pdus dissect_func: [string "crash.lua"]:3: triggering a LUA error

Change-Id: Ibd079cc5eb3a2e4d2e62ea49a512fa2cc8e561ea
Reviewed-on: https://code.wireshark.org/review/10685
Petri-Dish: Peter Wu <peter@lekensteyn.nl>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Peter Wu <peter@lekensteyn.nl>
Reviewed-by: Evan Huus <eapache@gmail.com>

epan/exceptions.h

@@ -372,6 +372,10 @@
 		except_throw(XCEPT_GROUP_WIRESHARK, (x), (y)); \
 } G_STMT_END
 
+/* Throws a formatted message, its memory is cleared after catching it. */
+#define THROW_FORMATTED(x, ...) \
+	except_throwf(XCEPT_GROUP_WIRESHARK, (x), __VA_ARGS__)
+
 #define GET_MESSAGE			except_message(exc)
 
 #define RETHROW                                     \

epan/wslua/wslua.h

@@ -654,6 +654,26 @@ extern int C##_register(lua_State* L); \
 extern gboolean is##C(lua_State* L,int i); \
 extern C shift##C(lua_State* L,int i)
 
+
+/* Throws a Wireshark exception, catchable via normal exceptions.h routines. */
+#define THROW_LUA_ERROR(...) \
+    THROW_FORMATTED(DissectorError, __VA_ARGS__)
+
+/* Catches any Wireshark exceptions in code and convert it into a LUA error.
+ * Normal restrictions for TRY/CATCH apply, in particular, do not return! */
+#define WRAP_NON_LUA_EXCEPTIONS(code) \
+{ \
+    volatile gboolean has_error = FALSE; \
+    TRY { \
+        code \
+    } CATCH_ALL { \
+        lua_pushstring(L, GET_MESSAGE);  \
+        has_error = TRUE; \
+    } ENDTRY; \
+    if (has_error) { lua_error(L); } \
+}
+
+
 extern packet_info* lua_pinfo;
 extern TreeItem lua_tree;
 extern tvbuff_t* lua_tvb;

epan/wslua/wslua_proto.c

@@ -30,6 +30,7 @@
 
 #include "wslua.h"
 #include <epan/dissectors/packet-tcp.h>
+#include <epan/exceptions.h>
 
 /* WSLUA_MODULE Proto Functions for new protocols and dissectors
 
@@ -761,6 +762,7 @@ static int
 wslua_dissect_tcp_dissector(tvbuff_t *tvb, packet_info *pinfo,
                             proto_tree *tree, void *data)
 {
+    /* WARNING: called from a TRY block, do not call luaL_error! */
     func_saver_t* fs = (func_saver_t*)data;
     lua_State* L = fs->state;
     int consumed_bytes = 0;
@@ -776,7 +778,7 @@ wslua_dissect_tcp_dissector(tvbuff_t *tvb, packet_info *pinfo,
         push_TreeItem(L, tree, (proto_item*)tree);
 
         if  ( lua_pcall(L,3,1,0) ) {
-            luaL_error(L, "Lua Error dissect_tcp_pdus dissect_func: %s", lua_tostring(L,-1));
+            THROW_LUA_ERROR("dissect_tcp_pdus dissect_func: %s", lua_tostring(L, -1));
         } else {
             /* if the Lua dissector reported the consumed bytes, pass it to our caller */
             if (lua_isnumber(L, -1)) {
@@ -787,7 +789,7 @@ wslua_dissect_tcp_dissector(tvbuff_t *tvb, packet_info *pinfo,
         }
 
     } else {
-        luaL_error(L,"Lua Error dissect_tcp_pdus: did not find the dissect_func dissector");
+        REPORT_DISSECTOR_BUG("dissect_tcp_pdus: did not find the dissect_func dissector");
     }
 
     return consumed_bytes;
@@ -857,9 +859,11 @@ WSLUA_FUNCTION wslua_dissect_tcp_pdus(lua_State* L) {
            destroy them before they get invoked */
         g_ptr_array_add(outstanding_FuncSavers, fs);
 
-        tcp_dissect_pdus(tvb->ws_tvb, lua_pinfo, ti->tree, proto_desegment,
-                         fixed_len, wslua_dissect_tcp_get_pdu_len,
-                         wslua_dissect_tcp_dissector, (void*)fs);
+        WRAP_NON_LUA_EXCEPTIONS(
+            tcp_dissect_pdus(tvb->ws_tvb, lua_pinfo, ti->tree, proto_desegment,
+                             fixed_len, wslua_dissect_tcp_get_pdu_len,
+                             wslua_dissect_tcp_dissector, (void*)fs);
+        )
     } else {
         luaL_error(L,"The third and fourth arguments need to be Lua functions");
     }