Buffer Overflow Experiment

Hi List,
I was wondering if someone has already done this.
I'm testing a small program written in C, especially coded for testing 
buffer overflow.
In source code, I have assigned char buffer[512]; I compile with gcc and 
run on bash.

As soon as I pass "512" characters as argument("A"x512--being precise), 
the program gives " Segmentation fault (core dumped )" -->as desired.

1] Using GDB debugger, it has been verified that the extended 
instruction pointer EIP has been overwritten, as expected. 
(EIP-->0x41414141)
2] But, on passing "513" chars. argument, the EIP becomes 0x0 -->WHY
3]On passing 520 chars. argument,  EIP takes 'some' value and EBP 
becomes 0x41414141
4]Thereafter, every increment by 1 --> in input characters causes EIP to 
remain the same as that 'some' value and EBP to take various incremental 
values.
every time, for several unit increments

[I am using:  2.6.21-1.3194.fc7 kernel,  i686, Disabled VA randomization]

Any views that help me understand why EIP becomes 0, are welcome.


Thanks.

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