dnsdist segfaults when using a luafunction which doesn't exist

[franklouwers]

• This is not a support question, I have read about opensource and will send support questions to the IRC channel, GitHub Discussions or the mailing list.
• I have read and understood the 'out in the open' support policy

• Program: dnsdist (haven't tested if it happens in any of the others)
• Issue type: Bug report

Short description

When using a non-existing function in a LuaAction(), dnsdist segfaults

Environment

• Operating system: Linux
• Software version: 1.9.8
• Software source: PowerDNS repository

Steps to reproduce

1. Load the following as crash_config.conf:

function debug(dq)
        local mystring = "world"
        errorlog(string.format("Hello %s", mystring))
        return DNSAction.None -- None, so process next rule
end

newServer({address="9.9.9.9"})

addAction(AllRule(), LuaAction(debug))

(Note the typo: errorlog() doesn't exist. errlog() does.)

2. Run dnsdist -C crash_config.conf
3. Run any dnsquery against it (eg: dig github.com @localhost)

Expected behaviour

Not crash

Actual behaviour

Segfaults

[rgacogne]

I don't think we should crash on this kind of configuration error, and I can indeed reproduce the issue:

(gdb) bt
#0  lua_rawgeti (L=0x7ffff7f9e380, idx=<optimized out>, n=<optimized out>) at /usr/src/debug/luajit/LuaJIT-a4f56a459a588ae768801074b46ba0adcfb49eb1/src/lj_api.c:844
#1  0x000055555580a4c2 in LuaContext::callRaw (state=0x7ffff7f9e380, functionAndArguments=..., outArguments=2) at ext/luawrapper/include/LuaContext.hpp:1454
#2  0x00005555558aa5c4 in LuaContext::call<std::tuple<int, boost::optional<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > >, DNSQuestion*> (state=0x7ffff7f9e380, toCall=...) at ext/luawrapper/include/LuaContext.hpp:1413
#3  LuaContext::LuaFunctionCaller<std::tuple<int, boost::optional<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > >(DNSQuestion*)>::operator() (this=0x555556008030, params#0=<optimized out>) at ext/luawrapper/include/LuaContext.hpp:1984
#4  std::__invoke_impl<std::tuple<int, boost::optional<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > >, LuaContext::LuaFunctionCaller<std::tuple<int, boost::optional<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > >(DNSQuestion*)>&, DNSQuestion*> (__f=...) at /usr/include/c++/14.2.1/bits/invoke.h:61
#5  std::__invoke_r<std::tuple<int, boost::optional<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > >, LuaContext::LuaFunctionCaller<std::tuple<int, boost::optional<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > >(DNSQuestion*)>&, DNSQuestion*> (__fn=...) at /usr/include/c++/14.2.1/bits/invoke.h:116
#6  std::_Function_handler<std::tuple<int, boost::optional<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > >(DNSQuestion*), LuaContext::LuaFunctionCaller<std::tuple<int, boost::optional<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > >(DNSQuestion*)> >::_M_invoke (__functor=..., __args#0=<optimized out>)
    at /usr/include/c++/14.2.1/bits/std_function.h:291
#7  0x0000555555898bdc in std::function<std::tuple<int, boost::optional<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > >(DNSQuestion*)>::operator() (this=0x555556011108, __args#0=<optimized out>) at /usr/include/c++/14.2.1/bits/std_function.h:591
#8  LuaAction::operator() (this=0x555556011100, dnsquestion=0x7fffb7ffd210, ruleresult=0x7fffb7ffd020) at dnsdist-lua-actions.cc:516
#9  0x0000555555c35d3c in applyRulesToQuery (holders=..., dq=..., now=...) at dnsdist.cc:1213
#10 processQuery (dq=..., holders=..., selectedBackend=std::shared_ptr<DownstreamState> (empty) = {...}) at dnsdist.cc:1675
#11 0x0000555555c38414 in processUDPQuery (cs=..., holders=..., msgh=<optimized out>, remote=..., dest=..., query=std::vector of length 48, capacity 4368 = {...}, responsesVect=0x0, queuedResponses=0x0, respIOV=0x0, respCBuf=0x0) at dnsdist.cc:1834
#12 0x0000555555c39e76 in operator() (__closure=0x7fffb7ffd5d0, param=...) at dnsdist.cc:2157
#13 udpClientThread (states=...) at dnsdist.cc:2174
#14 0x0000555555c3b170 in std::__invoke_impl<void, void (*)(std::vector<ClientState*, std::allocator<ClientState*> >), std::vector<ClientState*, std::allocator<ClientState*> > > (__f=<optimized out>) at /usr/include/c++/14.2.1/bits/invoke.h:61
#15 std::__invoke<void (*)(std::vector<ClientState*, std::allocator<ClientState*> >), std::vector<ClientState*, std::allocator<ClientState*> > > (__fn=<optimized out>) at /usr/include/c++/14.2.1/bits/invoke.h:96
#16 std::thread::_Invoker<std::tuple<void (*)(std::vector<ClientState*, std::allocator<ClientState*> >), std::vector<ClientState*, std::allocator<ClientState*> > > >::_M_invoke<0ul, 1ul> (this=<optimized out>) at /usr/include/c++/14.2.1/bits/std_thread.h:301
#17 std::thread::_Invoker<std::tuple<void (*)(std::vector<ClientState*, std::allocator<ClientState*> >), std::vector<ClientState*, std::allocator<ClientState*> > > >::operator() (this=<optimized out>) at /usr/include/c++/14.2.1/bits/std_thread.h:308
#18 std::thread::_State_impl<std::thread::_Invoker<std::tuple<void (*)(std::vector<ClientState*, std::allocator<ClientState*> >), std::vector<ClientState*, std::allocator<ClientState*> > > > >::_M_run (this=<optimized out>) at /usr/include/c++/14.2.1/bits/std_thread.h:253
#19 0x00007ffff64e1f74 in std::execute_native_thread_routine (__p=0x5555560172b0) at /usr/src/debug/gcc/gcc/libstdc++-v3/src/c++11/thread.cc:104
#20 0x00007ffff62a370a in start_thread (arg=<optimized out>) at pthread_create.c:448
#21 0x00007ffff6327aac in __GI___clone3 () at ../sysdeps/unix/sysv/linux/x86_64/clone3.S:78

I'm unsure whether I can actually fix it from DNSdist, though, but I'll look into it.

[rgacogne]

We crash in LuaWrapper indeed, while trying to access the traceback:

            // stack top: {error, traceback}
            lua_rawgeti(state, -1, 1); // stack top: {error, traceback}, er\
ror