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/* This file is part of genrc
Copyryght (C) 2018 Sergey Poznyakoff
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
*/
#include "genrc.h"
#include <pwd.h>
#include <grp.h>

static void
addgid(gid_t **pgv, size_t *pgc, size_t *pgi, gid_t gid)
{
	gid_t *gv = *pgv;
	size_t gc = *pgc;
	size_t gi = *pgi;

	if (gi == gc) 
		gv = x2nrealloc(gv, &gc, sizeof(gv[0]));

	gv[gi++] = gid;
	*pgv = gv;
	*pgc = gc;
	*pgi = gi;
}

static int
member(gid_t *gv, size_t gc, gid_t gid)
{
	size_t i;

	for (i = 0; i < gc; i++)
		if (gv[i] == gid)
			return 1;
	return 0;
}

static size_t
get_user_groups(const char *user, gid_t **pgv, size_t *pgc)
{
	struct group *gr;
	size_t gi = 0;
	
	setgrent();
	while ((gr = getgrent())) {
		char **p;
		for (p = gr->gr_mem; *p; p++)
			if (strcmp(*p, user) == 0)
				addgid(pgv, pgc, &gi, gr->gr_gid);
	}
	endgrent();
	return gi;
}

static gid_t
strtogid(char const *str)
{
	struct group *gr;
	
	if (str[0] == '+') {
		char *end;
		unsigned long n;

		errno = 0;
		n = strtoul(str, &end, 10);
		if (errno || *end) {
			genrc_error("invalid group name %s", str);
			exit(1);
		}

		gr = getgrgid(n);
	} else 
		gr = getgrnam(str);

	if (!gr) {
		genrc_error("%s: no such group", str);
		exit(1);
	}

	return gr->gr_gid;
}

void
runas(void)
{
	struct passwd *pw;
	uid_t uid;
	gid_t gid;
	gid_t *gv;
	size_t gc, gn;
	char const *runas_user;
	char const *runas_groups;

	runas_user = getenv("GENRC_USER");
	runas_groups = getenv("GENRC_GROUP");
	
	if (!(runas_user || runas_groups))
		return;
	if (getuid() != 0) {
		genrc_error("not root: can't switch to user privileges");
		exit(1);
	}

	if (!runas_user) {
		pw = getpwuid(0);
		runas_user = "root";
	} else if (runas_user[0] == '+') {
		char *end;
		unsigned long n;

		errno = 0;
		n = strtoul(runas_user + 1, &end, 10);
		if (errno || *end) {
			genrc_error("invalid user name %s", runas_user);
			exit(1);
		}

		pw = getpwuid(n);
	} else
		pw = getpwnam(runas_user);

	if (!pw) {
		genrc_error("%s: no such user", runas_user);
		exit(1);
	}
	runas_user = pw->pw_name;
		
	uid = pw->pw_uid;
	gid = pw->pw_gid;
	
	gv = NULL;
	gc = 0;
	
	if (runas_groups && runas_groups[0]) {
		struct wordsplit ws;
		size_t i;
		
		ws.ws_delim = ",";
		ws.ws_error = genrc_error;
		if (wordsplit(runas_groups, &ws, 
			      WRDSF_NOCMD
			      | WRDSF_NOVAR
			      | WRDSF_DELIM
			      | WRDSF_ENOMEMABRT
			      | WRDSF_SHOWERR
			      | WRDSF_ERROR))
			exit(1);

		if (ws.ws_wordc == 0) {
			genrc_error("bad group list: '%s'", runas_groups);
			exit(1);
		}

		gid = strtogid(ws.ws_wordv[0]);
		for (i = 1; i < ws.ws_wordc; i++)
			addgid(&gv, &gc, &gn, strtogid(ws.ws_wordv[i]));

		wordsplit_free(&ws);
	}

	if (gc == 0) {
		gn = get_user_groups(runas_user, &gv, &gc);
		if (!member(gv, gn, gid))
			addgid(&gv, &gc, &gn, gid);
	}
	
	/* Reset group permissions */
	if (setgroups(gn, gv)) {
		system_error(errno, "setgroups");
		exit(1);
	}
	free(gv);
	
	if (gid) {
		/* Switch to the user's gid. */
		if (setgid(gid)) {
			system_error(errno, "setgid(%lu) failed",
				     (unsigned long) gid);
			exit(1);
		}
	}
	
	/* Now reset uid */
	if (uid) {
		if (setuid(uid)) {
			system_error(errno, "setuid(%lu) failed: %s",
				     (unsigned long) uid);
			exit(1);
		}
	}
}	

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