linux - Loading raw code from C program -

i'm writing program loads , executes code file. got problem: "write" syscall not work. code loads , executes, not display text on screen.

program loads code:

#include < stdio.h > #include < stdlib.h >  int main(int argc,char* argv[]) {     unsigned int f_size = 0;     unsigned char* code_buf = null;     void (*func_call)(void) = null;      if(argc < 2)      {         printf("usage: %s <file>\n",argv[0]);         return 1;     }      file* fp = fopen(argv[1],"rb");     if(!fp)     {         printf("error while opening file: %s\n",argv[1]);         return 1;     }      unsigned int fsize = 0;     fseek(fp,0,seek_end);     fsize = ftell(fp);     fseek(fp,0,seek_set);     if(fsize < 4)     {         printf("code size must > 4 bytes\n");         return 1;     }      code_buf = (unsigned char*) malloc(sizeof(unsigned char)*fsize);     if(fread(code_buf,fsize,1,fp)<1)     {         printf("error while reading file: %s\n",argv[1]);         free(code_buf);         return 1;     }     func_call = (void (*)(void)) code_buf;      printf("[exec] binary loaded\n"            "\tfirst 2 bytes: 0x%x 0x%x\n"            "\tlast 2 bytes: 0x%x 0x%x\n",            code_buf[0],code_buf[1],            code_buf[fsize-2],code_buf[fsize-1]);     printf("[exec] starting code...\n");     (*func_call)();     printf("[exec] code executed!\n");      free(code_buf);      return 0; } 

code trying execute program (test.s):

.text     movl    $4, %eax     movl    $1, %ebx     movl    $str, %ecx     movl    $5, %edx     int     $0x80     jmp end     str:         .string "test\n"     end:     ret 

here how compile it:

 gcc -c test.s  objcopy -o binary test.o test.bin 

solved, @christoph

there working code:

.text     call start     str:         .string "test\n"     start:     movl    $4, %eax     movl    $1, %ebx     pop     %ecx     movl    $5, %edx     int     $0x80     ret 

your approach can't work: shellcode must position-independant, code refers absolute address str. unconditional jump can either relative or absolute: make sure relative verison (opcodes eb , e9 on x86).

see the technique of writing portable shell code more information.