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Date: Tue, 18 Oct 2022 20:08:48 +0100
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Subject: Re: Debugging malloc crash in gdb
To: David Allsopp <David.Allsopp@cl.cam.ac.uk>,
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From: Jon Turney <jon.turney@dronecode.org.uk>
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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.

This looks similar to the gdb crash reported [2], which I just don't 
have any time to look into.

[2] https://cygwin.com/pipermail/cygwin/2022-June/251714.html

I'd suggest reporting this as directed in 
https://www.sourceware.org/gdb/bugs/

(Note that self-service account creation is disabled on the sourceware 
bugzilla, due to spam problems, so you need to mail overseers as 
directed there, to request a Sourceware Bugzilla account.)

> The reproduction case is below (it's the OCaml runtime, so it's not exactly
> minimal, but it seems to be very repeatable to get gdb to the position of
> the crash).
> 
[...]
> 
> Any assistance to debug this further hugely appreciated!

It might be worth exploring if this gdb crash is seen in older versions 
of gcc, or with older gcc...