Accomplishment of porting ptrace(2) test scenarios

March 10, 2020 posted by Kamil Rytarowski

This month I have finished porting ptrace(2) tests from other Operating Systems. I have determined which test scenarios were missing, compared to FreeBSD and Linux, and integrated them into the ATF framework. I have skipped some of the tests as the interesting behavior was already covered in existing tests (sometimes indirectly) or tools (like picotrace), or the NetBSD kernel exhibits different behavior.

As my work is reaching the end, I was trying to clean up the state with other projects.

ptrace(2) ATF tests

I have determined which test scenarios were missing and integrated them. Certain tests like wrapping FreeBSD specific pdfork(2) call were omitted as not applicable.

There are few new tests that are marked as expected failure for corner cases that are scheduled for fixing in future.

I have also worked on SIGCHLD-based debugging and analysis of its behavior. I have found out that SA_NOCLDWAIT behaves suspiciously. This flag passed to sigaction(2) is an extension. If set, the system will not create a zombie when the child exits, but the child process will be automatically waited for. The same effect can be achieved by setting the signal handler for SIGCHLD to SIG_IGN. Currently it behaves differently under a debugger as the child process is never collected and is waiting for parent to collect it. According to my research this behavior is unexpected. A potential fix might not be difficult in the kernel, but due to time constraints I have decided to add an ATF tests for this scenario, mark it as failed and include a comment deferring this case into future.

I have also refactored the remaining threaded tests, switching them from low-level LWP API to pthread(3) one.

Other changes

I was working on finishing projects that were left behind.

GDB and qemu upstreaming

I'm working on upstreaming NVMM support to mainline QEMU. This process is still ongoing.

I am slowly reducing the patchset against the GDB repository.

jemalloc changes

The jemalloc allocator is a general purpose malloc(3) implementation that emphasizes fragmentation avoidance and scalable concurrency support. It's the default allocator in the NetBSD Operating System since 2007.

There are a few workarounds that make jemalloc compatible with NetBSD internals and I was trying to remove them. Unfortunately, the allocator tries to initialize itself too early using a C++-like constructor and intercepts the first malloc(3). The is done before initializing libpthread, and the pthread startup code uses malloc() when registering pthread_atfork(3) callbacks. In order to make it work, we allow premature usage of the libpthread functionality. I was trying to correct this, but I've introduced slight regressions in corner cases. They are hard to debug as the allocator is corrupted internally and randomly misbehaves (hangs, occasional crashes). I've discussed with the upstream developers about addressing this properly, but as reproducing the setup needs familiarity with the process of development NetBSD, we are still working on it.

Meanwhile, I have managed to correct known Undefined Behavior issues in jemalloc and address all known issues working together with upstream.


I received write access to the syzkaller GitHub repository. I also helped to get Kernel MSan (unauthorized memory access) operational on the syzbot node.

Miscellaneous changes

I helped with the libc++ upgrade that was done by Michal Gorny (but still not merged into mainline). As part of this work we gained a support for errno codes for POSIX robust mutexes.

I have implemented missing DT_GNU_HASH support as specified by GNU and LLVM linkers. This code was based on the implementation from three other major BSDs.

The micro-UBSan implementation gained support for alignment_assumptions. A number of UBSan reports were addressed.

Plan for the next and the last milestone

Upstream gdbserver support and address as many remaining bugs as the time will permit.

This work was sponsored by The NetBSD Foundation.

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