Chapter 2, "Overflowing Buffers on the Stack," page 27

Compiling the supplied shellcode.c seems to work, but it doesn't with the indicated command line.

todb@mazikeen:~/dev/sc$ gcc shellcode.c -o shellcode
todb@mazikeen:~/dev/sc$ ./shellcode
todb@mazikeen:~/dev/sc$ echo $SHELL
/bin/bash
todb@mazikeen:~/dev/sc$ 

Compiling with the preferred stack boundary set will make the supplied shellcode usable:

todb@mazikeen:~/dev/sc$ gcc -mpreferred-stack-boundary=2 shellcode.c -o shellcode
todb@mazikeen:~/dev/sc$ ./shellcode
$ echo $SHELL

Note, in the first place, all we did was return -- we weren't in a new spawned shell at all (or else we would see the bare sh prompt instead of my decorated bash prompt).

This post talks a little about what preferred-stack-boundary actually does (he quotes, apparently, from here); since this is shellcode, I assume the default boundary of 4 is causing the opcodes to get misaligned when it's left off of the gcc command line.