gistfile1.c

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

// NOTE: double precision floats count as two arguments
// givenArgc - count of arguments generated function should accept
// largeStruct - true if the function returns a struct larger than 4bytes
// func - function to wrap
// partialArgc - count of arguments passed by the trampoline to func
// ... - The partial arguments to pass
//
// trampoline(n, 0, func, m, p1, ..., pm) returns a function f such that
// when f(g1, ..., gn) is called it calls func(p1, ..., pm, g1, ..., gn)
void *trampoline(int givenArgc, int largeStruct, void *func, int partialArgc, ...){
  size_t size = 15 + 7*givenArgc + 8*partialArgc + (largeStruct ? 7 : 0);
  uint8_t *buff = malloc(size);
  
  size_t i = 0;
  // mov %esp,%ecx - copy stack pointer
  buff[i++] = 0x89;
  buff[i++] = 0xe1;
  // sub $0x10,%esp - extend stack
  buff[i++] = 0x83;
  buff[i++] = 0xec;
  buff[i++] = (partialArgc + 0xf)&(~0xf);
  
  if(largeStruct){
    // mov 0x4(%ecx),%eax - copy struct return address
    buff[i++] = 0x8b;
    buff[i++] = 0x41;
    buff[i++] = 0x04;
    // mov %eax,0x4(%esp) - write struct return address
    buff[i++] = 0x89;
    buff[i++] = 0x44;
    buff[i++] = 0x24;
    buff[i++] = 0x04;
  }
  
  // mov (%ecx),%eax - copy ret
  buff[i++] = 0x8b;
  buff[i++] = 0x01;
  // mov %eax,(%esp) - write ret
  buff[i++] = 0x89;
  buff[i++] = 0x04;
  buff[i++] = 0x24;
  
  int offset = (largeStruct ? 2 : 1);
  for(int index=0; index<givenArgc; index++){
    // mov 0x4(%ecx),%eax - copy param
    buff[i++] = 0x8b;
    buff[i++] = 0x41;
    buff[i++] = (index + offset)*4;
    // mov %eax,0xc(%esp) - write param
    buff[i++] = 0x89;
    buff[i++] = 0x44;
    buff[i++] = 0x24;
    buff[i++] = (index + partialArgc + offset)*4;
  }
  
  va_list params;
  va_start(params, partialArgc);
  for(int index=0; index<partialArgc; index++){
    uint32_t param = va_arg(params, uint32_t);
     // movl param,0x8(%esp) - write closure 1
    buff[i++] = 0xc7;
    buff[i++] = 0x44;
    buff[i++] = 0x24;
    buff[i++] = (index + offset)*4;
    buff[i++] = param>>0  & 0xff;
    buff[i++] = param>>8  & 0xff;
    buff[i++] = param>>16 & 0xff;
    buff[i++] = param>>24 & 0xff;
  }
  va_end(params);
  
  uint32_t rel = (uint32_t)func - (uint32_t)buff - size;
   // jmp to func
  buff[i++] = 0xe9;
  buff[i++] = rel>>0  & 0xff;
  buff[i++] = rel>>8  & 0xff;
  buff[i++] = rel>>16 & 0xff;
  buff[i++] = rel>>24 & 0xff;
  
  return buff;
}

void func(char a, short b, int c, float d){
	printf("%d %d %d %f\n", a, b, c, d);
}

int add(int a, int b){return a+b;}

typedef struct packshort {short a, b;} packshort;
packshort makePackshort(short a, short b){return (packshort){a, b};}

typedef struct packint {int a, b, c;} packint;
packint makePackint(int a, int b, int c){return (packint){a, b, c};}

double doubleadd(double a, double b){return a+b;}

int main(void){
  
	void (*f1)(float) = trampoline(1, 0, func, 3, 1, 2, 3);
	f1(4.0);
	
	int (*f2)(void) = trampoline(0, 0, add, 2, 5, 6);
	printf("%d\n", f2());
	
	packshort (*f3)(short b) = trampoline(1, 0, makePackshort, 1, 8);
	packshort ps = f3(9);
	printf("%d %d\n", ps.a, ps.b);

	packint (*f4)(int b) = trampoline(1, 1, makePackint, 2, 7, 8);
	packint pi = f4(9);
	printf("%d %d %d\n", pi.a, pi.b, pi.c);
	
	// doubles count as two args!
	double (*f5)(double b) = trampoline(2, 0, doubleadd, 2, 5.0);
	printf("%f\n", f5(6.0));

	return 0;	
}