Use setresuid() and setresgid() where available

apt-pkg/contrib/fileutl.cc

@@ -2252,22 +2252,32 @@ bool DropPrivileges()							/*{{{*/
       return _error->Error("No user %s, can not drop rights", toUser.c_str());
 
    // Do not change the order here, it might break things
+   // Get rid of all our supplementary groups first
    if (setgroups(1, &pw->pw_gid))
       return _error->Errno("setgroups", "Failed to setgroups");
 
+   // Now change the group ids to the new user
+#ifdef HAVE_SETRESGID
+   if (setresgid(pw->pw_gid, pw->pw_gid, pw->pw_gid) != 0)
+      return _error->Errno("setresgid", "Failed to set new group ids");
+#else
    if (setegid(pw->pw_gid) != 0)
       return _error->Errno("setegid", "Failed to setegid");
 
    if (setgid(pw->pw_gid) != 0)
       return _error->Errno("setgid", "Failed to setgid");
+#endif
 
+   // Change the user ids to the new user
+#ifdef HAVE_SETRESUID
+   if (setresuid(pw->pw_uid, pw->pw_uid, pw->pw_uid) != 0)
+      return _error->Errno("setresuid", "Failed to set new user ids");
+#else
    if (setuid(pw->pw_uid) != 0)
       return _error->Errno("setuid", "Failed to setuid");
-
-   // the seteuid() is probably uneeded (at least thats what the linux
-   // man-page says about setuid(2)) but we cargo culted it anyway
    if (seteuid(pw->pw_uid) != 0)
       return _error->Errno("seteuid", "Failed to seteuid");
+#endif
 
    // Verify that the user has only a single group, and the correct one
    gid_t groups[1];

buildlib/config.h.in

@@ -28,9 +28,11 @@
 /* If there is no socklen_t, define this for the netdb shim */
 #undef NEED_SOCKLEN_T_DEFINE
 
-/* We need the getresuid() function */
+/* Check for getresuid() function and similar ones */
 #undef HAVE_GETRESUID
 #undef HAVE_GETRESGID
+#undef HAVE_SETRESUID
+#undef HAVE_SETRESGID
 
 /* Define to the size of the filesize containing structures */
 #undef _FILE_OFFSET_BITS

configure.ac

@@ -174,9 +174,11 @@ AC_EGREP_HEADER(h_errno, netdb.h, [AC_MSG_RESULT(normal)],
 
 
 dnl check for setuid checking function
-AC_CHECK_FUNCS(getresuid getresgid)
+AC_CHECK_FUNCS(getresuid getresgid setresuid setresgid)
 AC_SUBST(HAVE_GETRESUID)
 AC_SUBST(HAVE_GETRESGID)
+AC_SUBST(HAVE_SETRESUID)
+AC_SUBST(HAVE_SETRESGID)
 
 dnl Check for doxygen
 AC_PATH_PROG(DOXYGEN, doxygen)