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Jon, help please.
Can somebody explain why this code outputs what it does?
Code:
// tested with Core 2 Duo, Core 2 Quad and Xeon
// tested with gcc4.1.2 gcc4.4.3 and gcc4.6.1
// compile with: gcc -O0 -m64
#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
#include <setjmp.h>
jmp_buf p;
void (*q)();
const char *data =
"\x8b\x44\x24\x04\x8b\x5c\x24\x08"
"\x8b\x00\x8b\x1b\x31\xc3\x31\xd8"
"\x31\xc3\x8b\x4c\x24\x04\x89\x01"
"\x8b\x4c\x24\x08\x89\x19\xc3\x90"
"\x55\x89\xe5\x8b\x45\x04\xc9\xc3"
"\x55\x90\x90\x89\xe5\x90\x90\x90"
"\x8b\x45\x08\x89\x45\x04\xc9\xc3"
"\x60\x09\x0e\x13\x14\x01\x0c\x0c"
"\xc0\x07\x05\x0e\x14\x0f\x0f\x60"
"\x00\x67\x6f\x74\x6f\x20\x63\x6f"
"\x6e\x73\x69\x64\x65\x72\x65\x64"
"\x20\x68\x61\x72\x6d\x66\x75\x6c"
"\x6c\x00\x90\x90\x1c\x1b\x0a\x20";
int f(int x)
{
static int b = 0; static int s = 0;
int a = 0, t;
if (!s) {
a = b; b = x;
} else {
a = x; t = b;
do {
a ^= b;
b = (a^b) & b;
b <<= 1;
} while (b);
b = t;
}
s = (s+1) % 2;
return a;
}
int g(int i, int *j)
{
*j = i;
i = (int) putchar;
if (*j == (48 << 1))
__asm volatile (
"movl 8(%ebp),%eax;"
"leave;"
"ret"
);
return (int) puts;
}
void h(int i)
{
int b;
q = (void(*)()) g(i++[data],&b);
for (f(b);*(data+i)!=b;++i,f(b))
q(f(i[data])%0xff);
}
void sh(int s)
{
if (s == 010)
((void(*)())g(s,&s))("F");
longjmp(p,s);
}
int main(void)
{
int base, addr = 0xffffffff, offs = 16;
int a = 11, b = 32, i = 25;
int s = 8, t = 1, u = 4;
((void(*)()) data)(&a,&b);
((void(*)()) data)(&b,&t);
((void(*)()) data)(&t,&s);
addr ^= a;
a ^= addr;
addr ^= a;
base = ((int(*)())data+addr)();
if (a == -1)
goto over;
puts("A");
base = (1<<3) | ((f(addr) + f(offs)) & ~0xff);
h(base+addr+offs);
exit(0);
over:
signal(t,sh);signal(s,sh);signal(u,sh);
if (!(s = setjmp(p))) {
q = (void(*)()) g(0x30, &a);
q(data + a + i);
s = a / (b-1);
puts("B");
} else if (s == 0xb) {
puts("C");
((int(*)(int)) data+addr+(offs/2))(base);
} else {
puts("D");
*((int*) base+s) = 0xffffffff;
}
puts("E");
return 1;
}
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