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help debugging a segfault


From: Sam Halliday
Subject: help debugging a segfault
Date: Mon, 17 Oct 2016 13:16:33 -0700 (PDT)
User-agent: G2/1.0

Hi all,

Despite being very stable since I installed it when released, Emacs 25.1 has 
started to segfault on me this week in a hard-to-reproduce manner.

I use ArchLinux and I recompiled with debugging symbols to obtain a detailed 
core dump / crash log.

But this is all I got when running with gdb until it crashed:

=============================
Thread 1 "emacs" received signal SIGSEGV, Segmentation fault.
re_search_2 (address@hidden <searchbufs+2912>, address@hidden <error: Cannot 
access memory at address 0x597b790>, address@hidden, 
    address@hidden <error: Cannot access memory at address 0x597b790>, 
size2=65565, startpos=646, address@hidden, range=509, 
    regs=0xc2c8d0 <search_regs>, stop=1155) at regex.c:4464
4464                  int len = BYTES_BY_CHAR_HEAD (*p);
=============================


When this happens I have just typed `M-x ensime` and it seems like emacs is 
crashing when loading the .ensime file, which is an s-expression file that is 
loaded as data in 
https://github.com/ensime/ensime-emacs/blob/master/ensime-config.el#L153-L168 
(actually if anybody knows of a more efficient way to load the file, I'd be 
keen to update our code, I'm a maintainer).

The exact file that it is apparently performing the re_search within is 
https://gist.github.com/fommil/d906918819cd2632e9864842e1d59b57


Could anybody please shed some light on what could be going wrong? I also have 
a core dump file but I have no idea what to do with it, I've never used gdb in 
anger.


Almost certainly an ArchLinux system update occurred somewhere around the time 
that this happened. Emacs is the only app that suffers and I get exactly the 
same behaviour on my laptop, so I'm ruling out a memory defect.

This is almost certainly a problem with my OS... is anybody else seeing this?

I may crosspost on ArchLinux and reference back here.

Best regards,
Sam


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