q1.c

// gcc -Wall -Wextra -Wpedantic -nostdlib -fno-stack-protector -static -Oz -s q1.c
#include <stddef.h>
#include <stdint.h>
#include <sys/types.h>


// https://github.com/bminor/musl/blob/master/arch/x86_64/syscall_arch.h
static inline int64_t __syscall1(int64_t n, int64_t a1)
{
    int64_t ret;
    __asm__ volatile ("syscall" : "=a"(ret) : "a"(n), "D"(a1) : "rcx", "r11", "memory");
    return ret;
}

static inline int64_t __syscall2(int64_t n, int64_t a1, int64_t a2)
{
    int64_t ret;
    __asm__ volatile ("syscall" : "=a"(ret) : "a"(n), "D"(a1), "S"(a2)
                      : "rcx", "r11", "memory");
    return ret;
}

static inline int64_t __syscall3(int64_t n, int64_t a1, int64_t a2, int64_t a3)
{
    int64_t ret;
    __asm__ volatile ("syscall" : "=a"(ret) : "a"(n), "D"(a1), "S"(a2),
                      "d"(a3) : "rcx", "r11", "memory");
    return ret;
}


// https://github.com/bminor/musl/blob/master/include/fcntl.h
#define O_RDONLY  0
#define O_WRONLY  1
#define O_RDWR    2


// https://github.com/bminor/musl/blob/master/arch/x86_64/bits/syscall.h.in
#define SYS_read   0
#define SYS_write  1
#define SYS_open   2
#define SYS_exit_group  231


static int open(const char *filename, int flags)
{
    return (int)__syscall2(SYS_open, (int64_t)filename, flags);
}

static ssize_t read(int fd, void *buf, size_t count)
{
    return (ssize_t)__syscall3(SYS_read, fd, (int64_t)buf, count);
}

static ssize_t write(int fd, const void *buf, size_t count)
{
    return (ssize_t)__syscall3(SYS_write, fd, (int64_t)buf, count);
}

static void _exit(int status)
{
    __syscall1(SYS_exit_group, (int64_t)status);
}


int main(void)
{
    int fd = open("flag", O_RDONLY);
    if (fd < 0) {
        return 1;
    }

    char buf[4096];
    ssize_t length;
    if ((length = read(fd, buf, 4096)) < 0) {
        return 2;
    }

    if (write(1, buf, length) < length) {
        return 3;
    }
    return 0;
}

void _start(void)
{
    _exit(main());
}