Question

The operating system is Ubuntu 18.04

Download and compile hello.ce and syscall-hello.com. Compile them statically and dynamically. How do the library and system calls produced by them compare (as shown by ltrace and strace)?

hello.c

#include <stdio.h>

int main(int argc, char *argv[]) {

        printf("Hello world!\n");

        return 0;
}

syscall-hello.c

#include <unistd.h>
#include <sys/syscall.h>

char *buf = "Hello world!\n";

int main(int argc, char *argv) {
        size_t result;

        /* "man 2 write" to see arguments to write syscall */
        result = syscall(SYS_write, 1, buf, 13);

        return (int) result;
}

Answer 1

A machine instruction that:

– Increases the CPU privilege level and

– Passes the control to a predefined place in the kernel.

● Arguments are (typically) passed in CPU registers.

● Instructions: – x86: int 0x80, sysenter, syscall

– MIPS: syscall

– ARM: swi

hello.c

#include <stdio.h>

int main (int argc, char *argv[])

{

printf(“Hello world\n”);

}

Let's look how to do it without libc write system call

– Documentation: man 2 write

– ssize_t write(int fd, const void *buf, size_t count);

– Three arguments

– startup symbol

hello:

.ascii "Hello world\n"

.global _start

_start:

mov $4,%eax # write

mov $1,%ebx # stdout

mov $hello,%ecx # ptr to data

  mov $12,%edx # length of data

int $0x80

Compile: gcc -m32 -nostdlib -o hello1 hello1.s

Run: ./hello1

Why it ends with segmentation fault? Disassemble the binary: objdump -d hello1

hello:

.ascii "Hello world\n"

.global _start

_start:

mov mov mov mov	$4,%eax $1,%ebx $hello,%ecx $12,%edx	# # # #	write stdout ptr to data length of the
int	$0x80
mov	$1,%eax	#	exit
mov int	$0,%ebx $0x80	#	exit code

We need to tell the OS that we are about to finish – with exit syscall.

Inspect the syscalls invoked by the program:

strace ./hello > /dev/null