[cfe-dev] Sanitizer test failures when switching to libc++ by default

Thu Feb 11 00:32:52 PST 2016

From: Alexey Samsonov [mailto:vonosmas at gmail.com] 
Sent: Thursday, February 11, 2016 1:07 AM
To: Hahnfeld, Jonas
Cc: cfe-dev at lists.llvm.org
Subject: Re: [cfe-dev] Sanitizer test failures when switching to libc++ by default

On Wed, Feb 10, 2016 at 7:30 AM, Hahnfeld, Jonas <Hahnfeld at itc.rwth-aachen.de> wrote:

From: cfe-dev [mailto:cfe-dev-bounces at lists.llvm.org] On Behalf Of Hahnfeld, Jonas via cfe-dev
Sent: Wednesday, February 10, 2016 7:43 AM
To: Alexey Samsonov
Cc: cfe-dev at lists.llvm.org

Subject: Re: [cfe-dev] Sanitizer test failures when switching to libc++ by default

From: Alexey Samsonov [mailto:vonosmas at gmail.com] 
Sent: Wednesday, February 10, 2016 1:54 AM
To: Hahnfeld, Jonas
Cc: Kostya Serebryany; cfe-dev at lists.llvm.org
Subject: Re: [cfe-dev] Sanitizer test failures when switching to libc++ by default

On Sat, Feb 6, 2016 at 1:33 AM, Hahnfeld, Jonas <Hahnfeld at itc.rwth-aachen.de> wrote:

From: Alexey Samsonov [mailto:vonosmas at gmail.com] 
Sent: Friday, February 05, 2016 11:05 PM
To: Kostya Serebryany
Cc: Hahnfeld, Jonas; cfe-dev at lists.llvm.org
Subject: Re: [cfe-dev] Sanitizer test failures when switching to libc++ by default

On Fri, Feb 5, 2016 at 11:11 AM, Kostya Serebryany via cfe-dev <cfe-dev at lists.llvm.org> wrote:

On Fri, Feb 5, 2016 at 6:21 AM, Hahnfeld, Jonas via cfe-dev <cfe-dev at lists.llvm.org> wrote:

Hi all,

I'm currently looking into (optionally) switching the default C++ stdlib to
libc++ (see http://reviews.llvm.org/D15920 which hasn't received a review so
far)
I've resolved all regression test failures in clang which are now passing
cleanly.

I'm now investigating the failing sanitizer tests in compiler-rt, here's
what I got so far:
 - two tests use -static-libstdc++ which obviously needs a -stdlib=libstdc++
(asan/TestCases/throw_invoke_test.cc and tsan/static_init6.cc)

Makes sense. 

I wonder if we can also use -static-libc++ for these tests

At least in my build there is not static version of libc++… I think this isn’t built by default but I haven’t yet searched for an opt-in.

 - tsan builds an instrumented version of libc++ and adds their headers.
This leads to errors with default C headers which are #include_next'd in
libc++. These #include_next then find the headers of the "normal" libc++
which in turn instantly return because of the header guards. Therefore basic
functions like printf aren't defined.
Solution: Pass -nostdinc++ to disable the headers of the "normal" libc++

emm, probably. 

Just to clarify: do you want to pass it to TSan lit tests? How would you ensure that

we'll still pick up C++ standard library headers when tests are run with Clang configured

to use libstdc++?

I’m planning on passing it in test/tsan/lit.cfg around line 57. Those flags are only applied if an instrumented version of libcxx is available and there is an explicit definition of the header include path, so this should work.

I see. Yes, probably -nostdinc++ would work in this case.

 - lsan completely fails with libc++, I had to generally pass
-stdlib=libstdc++ - is this known and possibly still work in progress?

This is not known to me. What are the symptoms? 

“LeakSanitizer has encountered a fatal error” on all C++ tests. This is fixed when building the tests with “-stdlib=libstdc++”, everything else still built with libc++…

I also can’t debug with gdb because the errors seems to not occur inside a debugger :-( is there any special trick with the sanitizers inside a debugger that I have not found yet?

LSan uses ptrace to stop the program and list all its threads, the same as GDB. So, LSan is sadly special in this case. You can try to gain more insight into what's happening by running the binary

with smth. like LSAN_OPTIONS=verbosity=1:log_pointers=1:log_threads=1 (apart from LSAN_OPTIONS specified in each lit test case).

Thanks for the hint, I will try and see whether I can get some reliable information…

--

Ok, so here are my findings so far:

verbosity=1 revealed the output “Tracer caught signal 11: […]”

So I disabled the signal handler and the core-file told me that it died when dereferencing the first TLS address in “ScanRangeForPointers()” in lib/lsan/lsan_common.cc.

(this also explains the behavior within gdb: It then can’t attach to the thread and therefore doesn’t  crash on scanning TLS)

For me it looks like “ThreadSelf()” in lib/sanitizer_common/sanitizer_linux_libcdep.cc reports a wrong address…

I can reproduce this with a small C++ program that only contains an empty main: It crashes with –fsanitize=leak –stdlib=libc++ and works fine with –stdlib=libstdc++.

... but if there's only main() then you don't have threads except for the main thread. Or does LSan crash when trying to access TLS of a main thread?

That's weird anyway. It may be the case that libc++ initializers use TLS differently than libstdc++ ones.

This seems to be the case… Are there any shared library constructors in libc++ or libc++abi that aren’t in libstdc++?

(CC’ing Marshall Clow and Eric Fiselier, maybe they can help…)

Information on my system: x86-64, Scientific Linux 6.6 -> glibc 2.12

It seems to work with libc++ on CentOS 7, but then why is libstdc++ also working on SL6?

I don't know. Is there a difference in what ThreadSelf() and (in particular) ThreadDescriptorSize() report on these platforms? This code is really fragile, and it can easily fail for certain versions or forks of glibc.

ThreadSelf() will report different addresses on each run, both for libc++ and libstdc++.

ThreadDescriptorSize() “correctly” returns 2288 for this 64-bit glibc 2.12 – how can I test if this actually correct on my system?

Greetings,
Jonas

--
Jonas Hahnfeld, MATSE-Auszubildender

IT Center
Group: High Performance Computing
Division: Computational Science and Engineering
RWTH Aachen University
Seffenter Weg 23
D 52074  Aachen (Germany)
Hahnfeld at itc.rwth-aachen.de
www.itc.rwth-aachen.de

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-- 

Alexey Samsonov
vonosmas at gmail.com

-- 

Alexey Samsonov
vonosmas at gmail.com

-- 

Alexey Samsonov
vonosmas at gmail.com

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