Exception cause SIGABRT

Overriding the `to_s` method of an exception and raise it from a sandboxed mruby evaluation result in a `abort()` call from `mruby`. This results with the whole ruby process terminating. Tested on [4cd4dfc855f0cce18b1ee2f318927c13edb20d14](https://github.com/Shopify/mruby-engine/tree/4cd4dfc855f0cce18b1ee2f318927c13edb20d14) # POC ``` # poc.rb class A < Exception def to_s end end raise A.new ``` `$ bin/sandbox poc.rb` # Crash Stacktrace: ``` Thread 1 (Thread 0x7fe2b0992700 (LWP 26764)): #0 0x00007fe2aff8e428 in __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:54 resultvar = 0 pid = 26764 selftid = 26764 #1 0x00007fe2aff9002a in __GI_abort () at abort.c:89 save_stage = 2 act = {__sigaction_handler = {sa_handler = 0x7fe2aadaf4e0, sa_sigaction = 0x7fe2aadaf4e0}, sa_mask = {__val = {140611505812704, 0, 140611505991664, 0, 140611527335906, 140611505812704, 140611527442253, 0, 0, 140611505812704, 140611505843808, 0, 0, 140611505812704, 140611505888960, 0}}, sa_flags = 0, sa_restorer = 0x7fe2aadb6e60} sigs = {__val = {32, 0 <repeats 15 times>}} #2 0x00007fe2ac234bbc in mrb_exc_raise (mrb=mrb@entry=0x7fe2aadaf4e0, exc=...) at /home/isra17/devel/mruby-engine/ext/mruby_engine/mruby/src/error.c:295 No locals. #3 0x00007fe2ac234f66 in mrb_raisef (mrb=mrb@entry=0x7fe2aadaf4e0, c=0x7fe2aadb6920, fmt=fmt@entry=0x7fe2ac2a3888 "%S cannot be converted to %S by #%S") at /home/isra17/devel/mruby-engine/ext/mruby_engine/mruby/src/error.c:371 args = <error reading variable args (Attempt to dereference a generic pointer.)> mesg = <optimized out> #4 0x00007fe2ac24c784 in mrb_convert_type (mrb=mrb@entry=0x7fe2aadaf4e0, val=val@entry=..., type=type@entry=MRB_TT_STRING, tname=tname@entry=0x7fe2ac2a23d9 "String", method=method@entry=0x7fe2ac29fdba "to_s") at /home/isra17/devel/mruby-engine/ext/mruby_engine/mruby/src/object.c:340 v = <optimized out> #5 0x00007fe2ac23a3ae in mrb_str_to_str (mrb=mrb@entry=0x7fe2aadaf4e0, str=...) at /home/isra17/devel/mruby-engine/ext/mruby_engine/mruby/src/string.c:1016 s = <optimized out> #6 0x00007fe2ac23d338 in mrb_string_value_cstr (mrb=0x7fe2aadaf4e0, ptr=ptr@entry=0x7ffefeff51e0) at /home/isra17/devel/mruby-engine/ext/mruby_engine/mruby/src/string.c:2222 str = <optimized out> ps = <optimized out> len = <optimized out> p = <optimized out> #7 0x00007fe2ac21e4ba in me_mruby_engine_get_exception (self=self@entry=0x7fe2aadaf3e0) at ../../../../ext/mruby_engine/mruby_engine.c:106 host_backtrace = 22683040 backtrace = {value = {p = 0x7fe2aadb7370, {i_flag = 0, i = 70305752922552}, {sym_flag = 112, sym = 32738}, bp = 0x7fe2aadb7370, fp = 0x7fe2aadb7370, vp = 0x7fe2aadb7370}, w = 140611505845104} class_name_obj = {value = {p = 0x7fe2aadb7280, {i_flag = 0, i = 70305752922432}, {sym_flag = 128, sym = 32738}, bp = 0x7fe2aadb7280, fp = 0x7fe2aadb7280, vp = 0x7fe2aadb7280}, w = 140611505844864} class_name = 0x7fe2aadb7280 message = {value = {p = 0x7fe2aadb7430, {i_flag = 0, i = 70305752922648}, {sym_flag = 48, sym = 32738}, bp = 0x7fe2aadb7430, fp = 0x7fe2aadb7430, vp = 0x7fe2aadb7430}, w = 140611505845296} err = <optimized out> #8 0x00007fe2ac21c04c in me_mruby_engine_eval (self=self@entry=0x7fe2aadaf3e0, proc=<optimized out>, err=err@entry=0x7ffefeff53d0) at ../../../../ext/mruby_engine/eval_monitored.c:227 err_no = <optimized out> thread = 140611505809152 ru_then = {ru_utime = {tv_sec = 0, tv_usec = 296000}, ru_stime = {tv_sec = 0, tv_usec = 20000}, {ru_maxrss = 27092, __ru_maxrss_word = 27092}, {ru_ixrss = 0, __ru_ixrss_word = 0}, {ru_idrss = 0, __ru_idrss_word = 0}, {ru_isrss = 0, __ru_isrss_word = 0}, {ru_minflt = 4523, __ru_minflt_word = 4523}, {ru_majflt = 0, __ru_majflt_word = 0}, {ru_nswap = 0, __ru_nswap_word = 0}, {ru_inblock = 0, __ru_inblock_word = 0}, {ru_oublock = 0, __ru_oublock_word = 0}, {ru_msgsnd = 0, __ru_msgsnd_word = 0}, {ru_msgrcv = 0, __ru_msgrcv_word = 0}, {ru_nsignals = 0, __ru_nsignals_word = 0}, {ru_nvcsw = 10, __ru_nvcsw_word = 10}, {ru_nivcsw = 30, __ru_nivcsw_word = 30}} ru_now = {ru_utime = {tv_sec = 0, tv_usec = 296000}, ru_stime = {tv_sec = 0, tv_usec = 20000}, {ru_maxrss = 27160, __ru_maxrss_word = 27160}, {ru_ixrss = 0, __ru_ixrss_word = 0}, {ru_idrss = 0, __ru_idrss_word = 0}, {ru_isrss = 0, __ru_isrss_word = 0}, {ru_minflt = 4525, __ru_minflt_word = 4525}, {ru_majflt = 0, __ru_majflt_word = 0}, {ru_nswap = 0, __ru_nswap_word = 0}, {ru_inblock = 0, __ru_inblock_word = 0}, {ru_oublock = 0, __ru_oublock_word = 0}, {ru_msgsnd = 0, __ru_msgsnd_word = 0}, {ru_msgrcv = 0, __ru_msgrcv_word = 0}, {ru_nsignals = 0, __ru_nsignals_word = 0}, {ru_nvcsw = 11, __ru_nvcsw_word = 11}, {ru_nivcsw = 30, __ru_nivcsw_word = 30}} bypass_ctx = <optimized out> cid = -214394 wait_result = <optimized out> #9 0x00007fe2ac21cc61 in ext_mruby_engine_eval (rself=22683280, rpath=22683200, rsource=22683080) at ../../../../ext/mruby_engine/ext.c:199 err = 8 proc = <optimized out> #10 0x00007fe2b049a50b in ?? () from /usr/lib/x86_64-linux-gnu/libruby-2.3.so.2.3 No symbol table info available. #11 0x00007fe2b04a84a3 in ?? () from /usr/lib/x86_64-linux-gnu/libruby-2.3.so.2.3 No symbol table info available. #12 0x00007fe2b04a94d3 in ?? () from /usr/lib/x86_64-linux-gnu/libruby-2.3.so.2.3 No symbol table info available. #13 0x00007fe2b049e269 in ?? () from /usr/lib/x86_64-linux-gnu/libruby-2.3.so.2.3 No symbol table info available. #14 0x00007fe2b04a3142 in ?? () from /usr/lib/x86_64-linux-gnu/libruby-2.3.so.2.3 No symbol table info available. #15 0x00007fe2b0389cfd in ?? () from /usr/lib/x86_64-linux-gnu/libruby-2.3.so.2.3 No symbol table info available. #16 0x00007fe2b038b6fd in ruby_exec_node () from /usr/lib/x86_64-linux-gnu/libruby-2.3.so.2.3 No symbol table info available. #17 0x00007fe2b038d83e in ruby_run_node () from /usr/lib/x86_64-linux-gnu/libruby-2.3.so.2.3 No symbol table info available. #18 0x000000000040087b in ?? () No symbol table info available. #19 0x00007fe2aff79830 in __libc_start_main (main=0x400830, argc=3, argv=0x7ffefeff5e38, init=<optimized out>, fini=<optimized out>, rtld_fini=<optimized out>, stack_end=0x7ffefeff5e28) at ../csu/libc-start.c:291 result = <optimized out> unwind_buf = {cancel_jmp_buf = {{jmp_buf = {0, -1400422201668288379, 4196480, 140733176569392, 0, 0, 1399860906120036485, 1393279154571296901}, mask_was_saved = 0}}, priv = {pad = {0x0, 0x0, 0x7ffefeff5e58, 0x7fe2b09c7168}, data = {prev = 0x0, cleanup = 0x0, canceltype = -16818600}}} not_first_call = <optimized out> #20 0x00000000004008a9 in _start () No symbol table info available. Title: ruby2.3 crashed with SIGABRT in mrb_exc_raise() ``` Cheers!