super_chroot - replacement for chroot for building dists

[Rich Felker <dalias@aerifal.cx>]

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Attached is a very simplified proof-of-concept for a tool that will
allow building musl-based dists without the need for root privileges.
It needs Linux 3.8 or newer (with user namespaces), but nothing else,
and should work whether compiled against musl or glibc. I'm hoping
this will be able to be adopted by some (most) of the musl-based dists
as an alternative to the current enter_chroot approach, so that it
becomes possible to build on systems where root access is not
available or where policy would prohibit use of root for building.

Note that if you're using an early 3.8 series kernel, you should
probably upgrade, as the technique used in this tool provides a
trivial way to get root on many systems.

Rich

[-- Attachment #2: super_chroot.c --]
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#define _GNU_SOURCE
#include <sched.h>
#include <stdio.h>
#include <unistd.h>
#include <signal.h>
#include <stdlib.h>
#include <fcntl.h>
#include <sys/mount.h>

int main(int argc, char **argv)
{
	uid_t uid = getuid();
	uid_t gid = getgid();

	unshare(CLONE_NEWUSER|CLONE_NEWNS);

	int fd = open("/proc/self/uid_map", O_RDWR);
	dprintf(fd, "%u %u 1\n", 0, uid);
	close(fd);
	fd = open("/proc/self/gid_map", O_RDWR);
	dprintf(fd, "%u %u 1\n", 0, gid);
	close(fd);

	setgroups(0, 0);

	chdir(argv[1]);
	mount("/dev", "./dev", 0, MS_BIND|MS_REC, 0);
	mount("/proc", "./proc", 0, MS_BIND|MS_REC, 0);
	chroot(".");

	execv(argv[2], argv+2);
}