working permissions

fixed problem where only euid and egid were set so programs complained, now
us correctly sets e/ruid, e/rgid and groups list to match the wanted user

us.c

@@ -1,4 +1,5 @@
 #define _POSIX_C_SOURCE 200809L
+#define _DEFAULT_SOURCE
 
 #include <sys/types.h>
 #include <stdio.h>
@@ -7,11 +8,13 @@
 #include <errno.h>
 #include <pwd.h>
 #include <unistd.h>
+#include <limits.h>
+#include <grp.h>
 #include <security/pam_appl.h>
 #include <security/pam_misc.h>
 
 static void usage (void);
-static int perm_set (uid_t, gid_t);
+static int perm_set (const char *, const char *);
 
 // FIXME: misc_conv is a separate library, should stick to plain PAM
 static struct pam_conv conv = {misc_conv, NULL};
@@ -25,7 +28,7 @@ int main (int argc, char *argv[])
 	}
 
 	uid_t ruid = getuid();
-	gid_t rgid = getgid();
+//	gid_t rgid = getgid();
 	struct passwd *pw = getpwuid(ruid);
 	if (!pw) {
 		fprintf(stderr, "getpwid: %s\n", strerror(errno));
@@ -77,7 +80,7 @@ int main (int argc, char *argv[])
 
 	errno = 0;
 	/* Set permissions */
-	if (perm_set(0, 0) == -1) { // 0 = root
+	if (perm_set(NULL, NULL) == -1) { // 0 = root
 		fprintf(stderr, "perm_set: %s\n", strerror(errno));
 		goto fail_end;
 	}
@@ -86,11 +89,13 @@ int main (int argc, char *argv[])
 	if (execvp(*c_argv, c_argv) == -1) // execvp searches in path
 		fprintf(stderr, "execv: %s\n", strerror(errno));
 
-	/* If exec fails reset the permissions */
-	if (perm_set(ruid, rgid) == -1) { // 0 = root
+
+	/* We may no longer have permission to reset the permissions
+	// If exec fails reset the permissions
+	if (perm_set(pw->pw_name, NULL) == -1) { // 0 = root
 		fprintf(stderr, "perm_set: %s\n", strerror(errno));
 		goto fail_end;
-	}
+	} */
 
 	/* Cleanup and return */
 	fail_end:
@@ -115,23 +120,79 @@ static inline void usage (void)
 	printf("usage: us [command]\n");
 }
 
-static int perm_set (uid_t uid, gid_t gid)
+static int perm_set (const char	*user, const char *group)
 {
-	// FIXME: This doesn't change the uid and gid, it only changes the
-	// e(ffective)uid and egid keeping the original gid, uid and groups list
-	// to make it work like sudo, the uid gid and groups list must be all
-	// set to 0 (or the corresponding value for other users).
-	// An option could be to add an argument to keep the groups list,
-	// another to change only the euid/egid.
-	// For more information run:
-	// $ sudo id
-	// and check the output
-	int err = 0;
-	// FIXME: using setresuid() and setresgid() is preferred
-	if ((err = seteuid(uid)) == -1)
-		return err;
-	else if ((err = setegid(gid)) == -1)
-		return err;
-	// FIXME: to set the group list use initgroups()
-	return err;
+	if (!user)
+		user = "root";
+
+	struct passwd *pw;
+	struct group *gr;
+	uid_t uid;
+	gid_t gid;
+	long uid_l, gid_l;
+
+	errno = 0;
+	if (user[0] != '#') {
+		pw = getpwnam(user);
+	} else {
+		uid_l = strtol(&user[1], NULL, 10);
+		if (uid_l < 0 || errno) {
+			errno = errno ? errno : EINVAL;
+			return -1;
+		}
+		pw = getpwuid((uid_t)uid_l);
+	}
+
+	if (!pw) {
+		if (!errno)
+			errno = EINVAL;
+		return -1;
+	}
+
+	uid = pw->pw_uid;
+	gid = pw->pw_gid;
+
+	if (group) {
+		if (group[0] != '#') {
+			gr = getgrnam(group);
+		} else {
+			gid_l = strtol(&group[1], NULL, 10);
+			if (gid_l < 0 || errno) {
+				errno = errno ? errno : EINVAL;
+				return -1;
+			}
+			gr = getgrgid((gid_t)gid_l);
+		}
+
+		if (!gr) {
+			if (!errno)
+				errno = EINVAL;
+			return -1;
+		}
+		gid = gr->gr_gid;
+	}
+
+	/* Set permissions, setting group perms first because in the case of
+	 * dropping from higher permissions setting the uid first results in
+	 * an error */
+	int err;
+	/* Non POSIX but implemented in most systems anyways */
+	err = initgroups(pw->pw_name, pw->pw_gid);
+	if (err == -1) {
+		printf("initgroups failed\n");
+		return -1;
+	}
+
+	// FIXME: ideally when failing reset the permissions
+	if (setregid(gid, gid) == -1) {
+		printf("setregid failed\n");
+		return -1;
+	}
+	
+	if (setreuid(uid, uid) == -1) {
+		printf("setreuid failed\n");
+		return -1;
+	}
+
+	return 0;
 }