Strcat stack smashing behaviour

Question

When running the following intentional stack smashing code, strcat copies the value of source exactly ten times.

#include <stdio.h>
#include <stdlib.h>

int main() {
    char a[16];
    char b[16];
    char c[32];

    strcpy(a, "abcdefghijklmnop");
    printf("a = %s\nb = %s\nc = %s\n\n", a, b, c);

    strcpy(b, "ABCDEFGHIJKLMNOP");
    printf("a = %s\nb = %s\nc = %s\n\n", a, b, c);

    strcpy(c, b);
    printf("a = %s\nb = %s\nc = %s\n\n", a, b, c);

    strcat(c, b);
    printf("a = %s\nb = %s\nc = %s\n\n", a, b, c);

    return 0;
}

Output:

a = abcdefghijklmnop b = c =

a = abcdefghijklmnopABCDEFGHIJKLMNOP b = ABCDEFGHIJKLMNOP c =

a = abcdefghijklmnopABCDEFGHIJKLMNOPABCDEFGHIJKLMNOP b = ABCDEFGHIJKLMNOPABCDEFGHIJKLMNOP c = ABCDEFGHIJKLMNOP

a = abcdefghijklmnopABCDEFGHIJKLMNOPABCDEFGHIJKLMNOPABCDEFGHIJKLMNOPABCDEFGHIJKLMNOPABCDEFGHIJKLMNOPABCDEFGHIJKLMNOPABCDEFGHIJKLMNOPABCDEFGHIJKLMNOPABCDEFGHIJKLMNOPABCDEFGHIJKLMNOPABCDEFGHIJKLMNOPABCDEFGHIJKLMNOP b = ABCDEFGHIJKLMNOPABCDEFGHIJKLMNOPABCDEFGHIJKLMNOPABCDEFGHIJKLMNOPABCDEFGHIJKLMNOPABCDEFGHIJKLMNOPABCDEFGHIJKLMNOPABCDEFGHIJKLMNOPABCDEFGHIJKLMNOPABCDEFGHIJKLMNOPABCDEFGHIJKLMNOPABCDEFGHIJKLMNOP c = ABCDEFGHIJKLMNOPABCDEFGHIJKLMNOPABCDEFGHIJKLMNOPABCDEFGHIJKLMNOPABCDEFGHIJKLMNOPABCDEFGHIJKLMNOPABCDEFGHIJKLMNOPABCDEFGHIJKLMNOPABCDEFGHIJKLMNOPABCDEFGHIJKLMNOPABCDEFGHIJKLMNOP

** stack smashing detected *: ./strcpytest terminated

Building parameters:

gcc -O0 -g3 -Wall -c -fmessage-length=0

The code is run on a x86_64 architecture.

Why does it only concatenate ten times?

Answer 1

The behaviour of strcpy() and strcat() is undefined for overlapping strings. Both of your writes to c[] are therefore suspect, and rather than just testing smashing the stack, you're also testing your compiler's treatment of this undefined behaviour.

I'd expect the strcpy(c, b) line to fail, but the implementation must somehow get b's length before it overwrites the trailing zero at the start of c. This could happen if it copies from the last byte to the first, for example.

strcat(c, b) may be implemented in a more straightforward manner. Maybe ten times the data is enough to hit some limit that terminates it.

If you want to just test corrupting the stack, don't use these methods. Instead just use one array, and write past the end of it with a loop, e.g. "for (i = 0; i < 1000000; i++) c[i] = 'h';"