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 -- Problem reports: https://cygwin.com/problems.html FAQ: https://cygwin.com/faq/ Documentation: https://cygwin.com/docs.html Unsubscribe info: https://cygwin.com/ml/#unsubscribe-simple
