Entries tagged make

Since my last example of fixing a bug received some interesting feedback (although I notice no upload of the package in question ..) we'll have another go.

Looking over my ~/.bash_history file one command I use multiple times a day is make. Happily GNU make has at least one interesting bug open:

• #419190 - make: double free or corruption

I verified this bug by saving the Makefile in the report and running make:

skx@gold:~$ make
make: file.c:84: lookup_file: Assertion `*name != '\0'' failed.
Aborted

(OK so this isn't a segfault; but an assertion failure is just as bad. Honest!)

So I downloaded the source to make, and rebuilt it. This left me with a binary with debugging symbols. The execution was much more interesting this time round:

skx@gold:~$ ./make
*** glibc detected ***
  /home/skx/./make: double free or corruption (fasttop): 0x00000000006327b0 ***
======= Backtrace: =========
/lib/libc.so.6[0x2b273dbdd8a8]
/lib/libc.so.6(cfree+0x76)[0x2b273dbdf9b6]
/home/skx/./make[0x4120a5]
/home/skx/./make[0x4068ee]
/home/skx/./make[0x406fb2]
...
[snip mucho texto]

And once I'd allowed core-file creation ("ulimit -c 9999999") I found I had a core file to help debugging.

Running the unstripped version under gdb showed this:

(gdb) up
#5  0x00000000004120a5 in multi_glob (chain=0x1c, size=40) at read.c:3106
3106			    free (memname);

So it seems likely that this free is causing the abort. There are two simple things to do here:

• Comment out the free() call - to see if the crash goes away (!)
• Understand the code to see why this pointer might be causing us pain.

To get started I did the first of these: Commenting out the free() call did indeed fix the problem, or at least mask it (at the cost of a memory leak):

skx@gold:~$ ./make
make: *** No rule to make target `Erreur_Lexicale.o', needed by `compilateur'.  Stop.

So, now we need to go back to read.c and see why that free was causing problems.

The function containing the free() is "multi_glob". It has scary pointer magic in it, and it took me a lot of tracing to determine the source of the bug. In short we need to change this:

free (memname);

To this:

free (memname);
memname = 0;

Otherwise the memory is freed multiple times, (once each time through the loop in that function. See the source for details).

Patch mailed.

Tags: bugs, debian, make, public bugfixing | 5 comments

I've uploaded a new make package to fix the memory corruption bug which I recently tracked down, with kind permission from the maintainer.

I've also been working on a Debian Planet filtering/exclusion system. I've put together a (working) online demo, and I think I could probably inject it via greasemonkey without too many problems. (I'm a little reluctant to install that addon, because I suspect the security implications are severe).

Still it was a nice hack, and actually reminds me that I like javascript these days. The demo will probably disappear in a week or two, but otherwise works as expected - just a couple of GUI issues to solve.

(As with the Debian Planet search this isn't tied to our install, and could work on any PlanetPlanet installation.)

Maybe it isn't the friendliest of ideas, but I think it is a good one regardless.

ObQuote: Last King of Scotland

Tags: make, planet-debian, planet-planet | 1 comment

There are some tools that we use daily, whether we realize it or not, that are unduly ugly. Over time you learn to use them and you forget just how hard they are to learn, and you take it for granted.

Today I had to guide somebody through using procmail, and I'd forgotten how annoying it is.

In brief I use procmail in three ways, each of which I had to document:

• Run a command, given a new email, and replace the original email with the output of that command.
• Run a command, silently. Just for fun.
• Match a regular expression on a header-field, and file accordingly.
  • Later extended to matching regexps on multiple headers. ("AND" + "OR" )

There are some projects that are too entrenched to ever be replaced ("make", I'm looking at you), but procmail? I reckon there's a chance a replacement would be useful, quickly.

Then again, maybe I'm biased.

Tags: make, procmail | 11 comments