Re: Debugging malloc crash in gdb

[David Allsopp <david@tarides.com>]

On Tue, 18 Oct 2022 at 20:09, Jon Turney wrote:
>
> On 18/10/2022 11:35, David Allsopp wrote:
> > I'm wondering if I may be able to have some pointers for debugging what
> > seems to be an unexpected interaction between mmap/mprotect/munmap and
> > malloc with the OCaml runtime.
> >
> > At the moment, I know that we crash in malloc, so my main question is how to
> > go further in gdb. I installed the cygwin-debuginfo package, but all I'm
> > getting is:
>
> Firstly, if the crash is inside the cygwin DLL, you must follow the
> advice in [1], and use 'set cygwin-exceptions on' to tell gdb to stop on
> an exception inside cygwin itself.
>
> [1] https://cygwin.com/faq.html#faq.programming.debugging-cygwin
>
>
> > /cygdrive/d/a/scallywag/gdb/gdb-11.2-1.x86_64/src/gdb-11.2/gdb/infrun.c:2550
> > : internal-error: void resume_1(gdb_signal): Assertion
> > `pc_in_thread_step_range (pc, tp)' failed.

I'm not sure now which combination of stepping directly into the
malloc call, adding set cygwin-exceptions on or switching to gdb 12.1,
but either way I was able to get to an invalid memory access in
mmap_alloc in malloc.cc. At this point, p was a pointer to the start
of the 256M block which had been passed to munmap.

What I then noticed from that is a bug in our code - the mmap'd region
was actually 256M+64K but the size passed to munmap was 256M... so the
munmap call was not releasing the entire block. Fixing that on the
OCaml side fixes the error completely - I don't know whether what we
were seeing before counts as a bug in Cygwin's allocator?

Many thanks!


David