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SubjectRe: CLONE_NEWNS and bind mounts to make "chroot" jail
Quoting Leibowitz, Michael (michael.leibowitz@intel.com):
> If I understand correctly, the following should accomplish what I'm
> looking for. However, pivot_root gives me EBUSY. I played around with
> moving the mount --bind /jail /jail to before the unshared, as well as
> making old_root a bind mount to itself. However, pivot_root always
> seems to fail. Is there something obvious that I'm doing wrong? The

Yes, you
cd /jail
mount --bind /jail /jail
pivot_root . old_root

but . is now mounted over.

-serge

> following is my test code (error checking has been removed for clarity,
> except for pivot_root).
>
> char *newargv[]= { "sh", NULL };
>
> chdir("/jail");
> unshare(CLONE_NEWNS));
> mount("/jail", "/jail", NULL, MS_BIND, NULL));
> mount("/bin", "bin", NULL, MS_BIND, NULL));
> mount("/usr", "usr", NULL, MS_BIND, NULL));
> mount("/lib", "lib", NULL, MS_BIND, NULL));
> if (pivot_root(".", "old_root")) perror("pivot_root . old_root");
> exec("./bash-static"); /* copied to /jail prior to running */
>
> Thanks.
>
> >Serge replies:
> [snip...snip]
> >Try a few more things. Since you had entered /jail, you can view '/'
> by
> >looking at .. . But if you look at /, you dereference your
> >task->fsroot. You never changed that, so it points to the original
> >mount. If however you 'ls ..', you should see your 'jail' directory.
> >However it won't have the /bin and /lib mounted because you didn't
> > mount --rbind /jail /
> >What you really want to do is
> > mount --bind /jail /jail
> >to make sure it's a mountpoint, then set up the new /jail using bind
> >mounts like you're doing (and likely some rbinds in some places), then
> >use pivot_root() to change your root. Then umount2("/old_root",
> >MNT_DETACH).
> >
> >-serge
>
> --
> Michael Leibowitz
> Software Engineer, UMG
> Intel Corporation
> michael.leibowitz at intel.com
>
>
> --
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