Segfault in on Ubuntu 14.04 32-bit

[Zachary Dremann]

I'm running on Ubuntu 14.04, 32 bit.

I have a custom g++ 10.1.0 as g++-10 installed in /usr/local, built from source with no modifications or customizations

I cloned from master (commit 96936a7446164424d8f3f50430d83ee21f63f6a2), and ran `autoreconf -fi`, then `./configure --without-openssl --with-pic LDFLAGS='-L/usr/local/lib -Wl,-rpath,/usr/local/lib' CXX=/usr/local/bin/g++-10`, followed by `make`, and `make check`.

`VERBOSE=1 make check` fails with (truncated to show interesting stuff):

[ TEST ] kj/async-test.c++:860: start a fiber

[ PASS ] kj/async-test.c++:860: start a fiber (112 μs)

[ TEST ] kj/async-test.c++:881: fiber promise chaining

[ PASS ] kj/async-test.c++:881: fiber promise chaining (9 μs)

[ TEST ] kj/async-test.c++:904: throw from a fiber

/bin/bash: line 5: 22656 Segmentation fault      (core dumped) ${dir}$tst

FAIL: capnp-test

Randomly testing backwards-compatibility scenarios with seed: 1604520183

PASS: capnp-evolution-test

PASS: src/capnp/compiler/capnp-test.sh

===========================================

1 of 3 tests failed

Please report to capn...@googlegroups.com

===========================================

Running `libtool --mode=execute gdb ./capnp-test`, and getting a backtrace at the time of the segfault gives the following:

#0  0xb793d71e in x86_fallback_frame_state (context=<optimized out>, context=<optimized out>, fs=0xb57565b0)

    at ./md-unwind-support.h:132

#1  uw_frame_state_for () at ../../../gcc-10.1.0/libgcc/unwind-dw2.c:1271

#2  0xb793ecb1 in _Unwind_Backtrace () at ../../../gcc-10.1.0/libgcc/unwind.inc:302

#3  0xb7876d75 in __GI___backtrace (array=0xb5756720, size=34) at ../sysdeps/i386/backtrace.c:126

#4  0xb7bb098a in kj::getStackTrace(kj::ArrayPtr<void*>, unsigned int) ()

   from /home/build/capnproto/c++/.libs/libkj-0.9-dev.so

#5  0xb7bb0b39 in kj::Exception::extendTrace(unsigned int) () from /home/build/capnproto/c++/.libs/libkj-0.9-dev.so

#6  0xb7bb0c62 in kj::throwRecoverableException(kj::Exception&&, unsigned int) ()

   from /home/build/capnproto/c++/.libs/libkj-0.9-dev.so

#7  0x0820011c in kj::Promise<void>::wait(kj::WaitScope&) ()

#8  0x081e1620 in kj::_::Fiber<kj::(anonymous namespace)::TestCase904::run()::{lambda(kj::WaitScope&)#1}>::runImpl(kj::WaitScope&) ()

#9  0xb7c36203 in kj::_::RunnableImpl<kj::_::FiberBase::run()::{lambda()#2}>::run() ()

   from /home/build/capnproto/c++/.libs/libkj-async-0.9-dev.so

#10 0xb7bb0ed2 in kj::_::runCatchingExceptions(kj::_::Runnable&) ()

   from /home/build/capnproto/c++/.libs/libkj-0.9-dev.so

#11 0xb7c3b9de in kj::_::FiberBase::run() () from /home/build/capnproto/c++/.libs/libkj-async-0.9-dev.so

#12 0xb7c3bdf7 in kj::_::FiberStack::run() () from /home/build/capnproto/c++/.libs/libkj-async-0.9-dev.so

#13 0xb7c42c09 in kj::_::FiberStack::StartRoutine::run(int, int) ()

   from /home/build/capnproto/c++/.libs/libkj-async-0.9-dev.so

#14 0xb77bbdeb in makecontext () at ../sysdeps/unix/sysv/linux/i386/makecontext.S:87

#15 0x00008c48 in ?? ()

#16 0x00000862 in ?? ()

#17 0x00000000 in ?? ()

[Kenton Varda]

Hi Zachary,

It looks like the segfault is happening in glibc's backtrace() function while gathering the stack trace for an exception. An exception is actually expected here (the test case tests throwing an exception). Since it happened specifically in fiber-related tests, my guess is that glibc's backtrace has a bug in which it is confused by the stack manipulation done by fibers.

14.04 is quite old, so I imagine you're using a pretty old glibc here? Maybe this is a glibc bug that was fixed in newer versions?

Note that fibers are new and completely optional feature of KJ. If you don't use them then you can probably ignore the test failure.

-Kenton

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[Zachary Dremann]

This looks related: https://lists.debian.org/debian-glibc/2014/01/msg00023.html

I rebuilt everything in the same way on an ubuntu 16.04 VM, and I don't get the segfault. I don't need to use fibers, so I'm okay to just avoid them.

[Kenton Varda]

Yep, that looks like a smoking gun to me.

-Kenton

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