Why does this program cause a segmentation fault?

Question

Please somebody give me a explanation of how this program cause segmentation fault and how can i exploit it?

#include<iostream>
using namespace std;

void a();
void b();

void a()
{
        cout<<"Hello World\n";
        b();
}
void b()
{
        cout<<"Hello World\n";
        a();
}

int main()
{
        a();
        return 0;
}

i just want to know how this program cause overflow on memory level — r00tk1ll3r, Apr 21 '20 at 10:21
You cause a stack overflow since you put an infinite number of calls on the stack. — Ted Lyngmo, Apr 21 '20 at 10:21
A recursive function is a function that calls itself during its execution. This enables the function to repeat itself several times, outputting the result and the end of each iteration. but here two functions call one and each other — r00tk1ll3r, Apr 21 '20 at 10:21
Every function call you do places the program counter etc. on the stack and gives the newly called function a context. When the function returns, the program counter is pop:ed from the stack. This will never happen in your program. You just put more and more things on the stack until it explodes. — Ted Lyngmo, Apr 21 '20 at 10:23
@TedLyngmo this is the answer i expect please can you explain bit more — r00tk1ll3r, Apr 21 '20 at 10:24
I'm supposed to be working right now so I don't have time to write a proper answer but I'm sure someone else can do a write-up. :-) — Ted Lyngmo, Apr 21 '20 at 10:26
@r00tk1ll3r you need to look up ebp, esp ,eip in order to understand fully. — RaHuL, Apr 21 '20 at 10:31

score 2 · Accepted Answer · answered Apr 21 '20 at 10:38

2

The situation you describe is known as "infinite recursion", and is described in this post.

In the mentioned post, main() is calling main(), calling main(), ...

In your case, a() is calling b(), calling a(), calling b(), ...

The mentioned post shows the effect of this on the stack and gives an idea on how this is handled in the memory of your computer (using assembly).

answered Apr 21 '20 at 10:38

Dominique

16,450
15
56
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mm now i understand and thanks to RaHuL to remind me that epb and esp and thanks to Dominique – r00tk1ll3r Apr 21 '20 at 10:42

score 1 · Answer 2 · answered Apr 21 '20 at 10:53

What you see is the effect stack overflow that happens due to the infinite recursion. While this is in theory should just print "Hello World" infinitely, the usage of a() and b() forces to store the "return address" on the stack. Typical implementations use a fixed size stack and thus it recursive calls eventually results in overflow of call stack.

Note that the "stack" is common terminology and isn't specified in the standard nor is it required to be used.

But if you optimise your code, it could simply be turned into an infinite loop without the use of functions or recursion. For example, your code can be transformed into:

#include<iostream>

int main()
{
    while (true)
        std::cout<<"Hello World\n";
}

This is allowed and valid under the "as-if" rule of C++ standard. In this case, you are not going to encounter stack overflow or segfault.

Compiling & running your code with:

g++ -Wall -Wextra -O3 test.cpp

doesn't, in fact, result in segfault.

So what you see isn't guaranteed by the standard.

score 0 · Answer 3 · answered Apr 21 '20 at 10:45

0

The program you write leads to an Infinite loop. In main function you call a(). a() call b() and again in b function, a() is called. So it leads to Segmentation Fault.

answered Apr 21 '20 at 10:45

Sina Raoufi

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2
6

Why does this program cause a segmentation fault?

3 Answers3