System calls

• every process starts with 3 files open: stdin, stdout, stderr
• steps:

• what has to happen to print hello world to stdout?

  • build process:

    

  • iteration 1

    #include <stdio.h>
        int main(int argc, char **argv) {
        printf("Hello World!\n");
            return 0;
        }
    

  • iteration 2

    #include <unistd.h>
    #define STDOUT 1
    int main(int argc, char **argv) {
        char msg[] = "Hello World!\n";
        write(STDOUT, msg, sizeof(msg));
        return 0;
    }
    

  • iteration 3

    #define _GNU_SOURCE
    #include <sys/syscall.h>
    #define STDOUT 1
    int main(int argc, char **argv) {
        char msg[] = "Hello World!\n”;
        int nr = SYS_write;
        syscall(nr, STDOUT, msg, sizeof(msg));
        return 0;
    }
    

• syscall diagram

  

• syscall (x86 Linux) is triggered by instruction (like 0x80):

  • privilege level changed to kernel mode
  • program counter set to specific location
  • arguments passed in registers:
    • rax <- syscall number
    • ebx, ecdx, edx, esi, edi, ebp <- arguments
    • stack <- more arguments
  • x86-64 supports legacy int 0x80, new instruction syscall
    • rax <- syscall number (different from 32bit)
    • rdi, rsi, rdx, r10, r8, r9 <- arguments

• hello world without glibc – manual system calls, in-line assembly:

  ssize_t write(int fd, const void *buf, size_t nbytes) {
          ssize_t ret;
          asm volatile
              (
                  /* request syscall to OS (can also be ‘int $0x80’) */
                  “syscall”
  
                  /* return result in %eax */
                  : "=a" (ret)
  
                  /* __NR_write (1) into same place as operand 0, fd into %rdi, buffer into %rsi, length into %rdx */
  
                  : "0" (__NR_write), "D"(fd), "S"(buf), "d"(nbytes)
  
                  /*  modified cc, registers %rcx and %r11, and memory */
                  : "cc", "rcx", "r11", "memory"
              );
          return ret;
      }
  

  • actual objdump of this program