Problem with Perl script and image maps

[Rob Parsons]

In case anyone is interested, I did solve this mystery.

After spending a couple of hours with gdb and examining the source, I
tracked the Segmentation fault down to a realloc() instruction in the
im_iprintf() function located in the mapimagemap.c file. This function
appends new output onto the original string, allocating memory as
necessary to accommodate the growth of the output. The SIGSEGV was
occuring when memory needed to be reallocated, indicating that something
was writing into non-allocated space.

The source contains a compiler directive that checks for a #DEFINE that
indicates the availability of a vsnprintf() command on the local platform.
If it's not available, then the vsprintf() command is used instead. The
difference between these two commands is that vsnprintf() has a safety
stop that will end processing at a given number of bytes. vsprintf() has
no such mechanism. It took me a while to find because I knew that my
Debian Linux platform had vsnprintf(). Unfortunately, the
Mapserver 'configure' script does not seem to contain a way to properly
detect vsnprintf() and therefore the vsprintf() command was being used
instead. This command has no safety stop and there was nothing in the code
to prevent it from writing a few bytes past the end of allocated memory.
In this state, executing this code on any platform that doesn't have
vsprintf() will result in SIGSEGVs at best and corrupted memory at worst.

This problem affects not only Mapscript code, but SHP2IMG and others as
well. Fortunately there is an easy fix. If you know for sure that your
platform has vsnprintf() available, then you can SET and EXPORT "CFLAGS=-
DHAVE_VSNPRINTF" before invoking the 'configure' script. Alternately, or
if you don't know whether vsnprintf() is available, you can edit the
configure.in file and add the following line in the appropriate place:

AC_CHECK_FUNC(vsnprintf,      STRINGS="-DHAVE_VSNPRINTF $STRINGS")

After editing the file, run the autoconf utility to build a
new 'configure' script. Either of those solutions should fix this problem
on affected platforms.

- Rob Parsons