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SubjectRE: CLONE_NEWNS and bind mounts to make "chroot" jail
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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
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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