[Solved] Mandatory lock so processes can't read a file

ConnorBehan · 2011-05-01 04:35:09

Yesterday I thought I would need to use a mandatory lock and decided to follow the steps on http://www.hackinglinuxexposed.com/arti … 30623.html. It is no longer of immediate concern but I am starting to wonder what I was doing wrong. Guided by http://www.unixguide.net/unix/programming/2.5.shtml I have the following code called "locker.c":

#include <stdio.h>
#include <fcntl.h>

struct flock *file_lock(short type, short whence) {
    static struct flock ret;
    
    ret.l_type = type;
    ret.l_start = 0;
    ret.l_whence = whence;
    ret.l_len = 0;
    ret.l_pid = getpid();

    return &ret;
}

int main(int argc, char **argv) {
    if (argc > 1) {
        int fd = open(argv[1], O_RDONLY);
        fcntl(fd, F_SETLKW, file_lock(F_WRLCK, SEEK_SET));
        while (1) {
            scanf("%c", NULL);
        }
    }
}

I first mount my ext2 /boot partition with mandatory locks enabled:

mount -oremount,mand /boot

I then enable mandatory locks on /boot/foo

chmod g+s,g-x /boot/foo

I then try to lock /boot/foo

locker /boot/foo

however while this program is running I can still open foo in a text editor and even save changes to it. Does anyone know how to fix this?

Last edited by ConnorBehan (2011-05-01 22:42:49)

tavianator · 2011-05-01 22:10:29

~~Returning the address of ret from the file_lock() function invokes undefined behaviour.~~ (Sorry, didn't see the "static".) As well, you should be checking the return value of fcntl().

Last edited by tavianator (2011-05-01 22:11:20)

ConnorBehan · 2011-05-01 22:42:07

Ah, thanks for reminding me that it returns a value! Yes, fcntl() was the function that failed and I think that's because it needs write access for a write lock. Changing O_RDONLY to O_WRONLY makes everything work.

Arch Linux

#1 2011-05-01 04:35:09

[Solved] Mandatory lock so processes can't read a file

#2 2011-05-01 22:10:29

Re: [Solved] Mandatory lock so processes can't read a file

#3 2011-05-01 22:42:07

Re: [Solved] Mandatory lock so processes can't read a file

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