flock_no_ptrace.c

#define _GNU_SOURCE 1
#define _POSIX_SOURCE 2

#include <fcntl.h>
#include <unistd.h>
#include <assert.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/fcntl.h>
#include <sys/syscall.h>
#include <string.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/ptrace.h>
#include <stdio.h>

#define FILENAME "foo.txt"

int main(void) {
    int fd, err, i, status;
    pid_t pid;
    size_t pagesize = sysconf(_SC_PAGESIZE);
    fd = open(FILENAME, O_CREAT | O_EXCL | O_RDWR, 0600);
    assert(fd >= 0);

    unlink(FILENAME);

    /* Write a page's worth of data. */
    for (i = 0; i < pagesize / sizeof(i); ++i) {
        ssize_t nwritten = write(fd, &i, sizeof(i));
        assert(nwritten == sizeof(i));
    }

    struct flock64 lock = {
      .l_type = F_WRLCK, .l_whence = SEEK_SET, .l_start = 0, .l_len = pagesize
    };

    lock.l_type = F_WRLCK;
    err = fcntl(fd, F_SETLK64, &lock);
    assert(0 == err);

    if (0 == (pid = fork())) {
/*        ptrace(PTRACE_TRACEME, 0, 0, 0);
        raise(SIGSTOP); */

        signal(SIGPWR, SIG_IGN);

        lock.l_type = F_RDLCK;
        err = fcntl(fd, F_GETLK64, &lock);
        assert(0 == err);

        lock.l_type = F_RDLCK;
        lock.l_pid = 0;
        err = fcntl(fd, F_SETLKW64, &lock);
        assert(0 == err);

        lock.l_type = F_UNLCK;
        fcntl(fd, F_SETLK64, &lock);

        return 0;
    }

/*    assert(pid == waitpid(pid, &status, __WALL));
    assert(WIFSTOPPED(status) && WSTOPSIG(status) == SIGSTOP);
    ptrace(PTRACE_CONT, pid, 0, 0); */

    usleep(500000);
    kill(pid, 30);
/*    assert(pid == waitpid(pid, &status, __WALL)); */
    usleep(500000);
/*    ptrace(PTRACE_SYSCALL, pid, 0, 0);
    assert(pid == waitpid(pid, &status, __WALL)); */

    lock.l_type = F_UNLCK;
    fcntl(fd, F_SETLK64, &lock);

/*    ptrace(PTRACE_CONT, pid, 0, 0); */

    waitpid(pid, &status, 0);
    assert(WIFEXITED(status) && 0 == WEXITSTATUS(status));

    printf("Success!\n");

    return 0;
}