Abcmidi and the Mayhem Bugs

When I managed to adopt the abcmidi package in Debian a while back, I had contacted Seymour Shlien (the upstream maintainer) about the four Mayhem bugs (here’s an example of one of them). Unfortunately, neither Seymour or I at the time could work out what was going on.

Although the Mayhem Team had provided a script that always confirmed the crash when a certain invalid command option was passed to the abcmidi commands, the core dump provided was from an i386 (32 bit) processor, and I am running an AMD 64 bit processor. This meant that gdb did not give a traceback at all, even when I went to the effort of downloading the source code and building the package again without stripping the debugging symbols.

Recently, I noticed that Seymour had released a new version of abcmidi, and I decided to tackle the problems again whilst packaging this new version. To make things easier for me, I added a -dbg package to abcmidi so that the debugging symbols would be available for gdb to provide a useful backtrace.

But even with this, I found that gdb was not dumping the core when the script was run to produce the crash. So I learnt a little bit more about debugging. Why was there no dump of the core? Running $ ulimit -c gave the output “0”, which meant – no core-dump! Following the advice on this very useful website (linux core dumps), I set things up so that the above command gave the output “unlimited”. Normally it is advisable that you put a limit on the core dumps to avoid your machine’s hard disk filling up, but I was doing this on an expendable Virtual Machine.

This time when I ran the script, the core was dumped! So when I loaded the core-dump into gdb with $ gdb usr/bin/abcmatch core, I was able to: > set logging on, then type > bt full, and > quit, and found a text file with the full backtrace that I emailed to Seymour for each binary command that had the bug.

Very quickly, Seymour fixed the bugs and released a new version. I packaged it straight away and Tobias Frost sponsored it. The package is still waiting in the NEW queue because of the new -dbg package, but hopefully it will be released into experimental soon and the bugs will be automatically closed.

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