Environment handling in ld.elf_so

[Joerg Sonnenberger <joerg%britannica.bec.de@localhost>]

Hi all,
attached patch drops the dependency of ld.elf_so on getenv. The access
and the removal of entries is explicitly done using an open coding.
Since the libc code for getenv has recently become a bit more involved,
this removes another non-trivial dependency.

Test by creating a new user and running
        LD_LIBRARY_PATH=/lib su test -c env
confirm that the restrictions for suid/sgid binaries work.

Joerg

Index: rtld.c
===================================================================
RCS file: /home/joerg/repo/netbsd/src/libexec/ld.elf_so/rtld.c,v
retrieving revision 1.132
diff -u -p -r1.132 rtld.c
--- rtld.c      29 Oct 2010 15:08:17 -0000      1.132
+++ rtld.c      9 Dec 2010 23:23:01 -0000
@@ -312,20 +312,21 @@ _rtld(Elf_Addr *sp, Elf_Addr relocbase)
                       *pAUX_phent, *pAUX_phnum, *pAUX_euid, *pAUX_egid,
                       *pAUX_ruid, *pAUX_rgid;
        const AuxInfo  *pAUX_pagesz;
-       char          **env;
+       char          **env, **oenvp;
        const AuxInfo  *aux;
        const AuxInfo  *auxp;
        Elf_Addr       *const osp = sp;
        bool            bind_now = 0;
-       const char     *ld_bind_now;
+       const char     *ld_bind_now, *ld_preload, *ld_library_path;
        const char    **argv;
        const char     *execname;
        long            argc;
        const char **real___progname;
        const Obj_Entry **real___mainprog_obj;
        char ***real_environ;
-#if defined(RTLD_DEBUG)
+#ifdef DEBUG
        int i = 0;
+       const char     *ld_debug;
 #endif
 
        /*
@@ -428,24 +429,64 @@ _rtld(Elf_Addr *sp, Elf_Addr relocbase)
            ((pAUX_egid ? (gid_t)pAUX_egid->a_v : getegid()) ==
            (pAUX_rgid ? (gid_t)pAUX_rgid->a_v : getgid()));
 
-       ld_bind_now = getenv("LD_BIND_NOW");
+#ifdef DEBUG
+       ld_debug = NULL;
+#endif
+       ld_bind_now = NULL;
+       ld_library_path = NULL;
+       ld_preload = NULL;
+       for (oenvp = env; *env != NULL; ++env) {
+               static const char bind_var[] = "LD_BIND_NOW=";
+               static const char debug_var[] =  "LD_DEBUG=";
+               static const char path_var[] = "LD_LIBRARY_PATH=";
+               static const char preload_var[] = "LD_PRELOAD=";
+#define LEN(x) (sizeof(x) - 1)
+
+               if ((*env)[0] != 'L' || (*env)[1] != 'D') {
+                       /*
+                        * Special case to skip most entries without
+                        * the more expensive calls to strncmp.
+                        */
+                       *oenvp++ = *env;
+               } else if (strncmp(*env, debug_var, LEN(debug_var)) == 0) {
+                       if (_rtld_trust) {
+#ifdef DEBUG
+                               ld_debug = *env + LEN(debug_var);
+#endif
+                               *oenvp++ = *env;
+                       }
+               } else if (strncmp(*env, bind_var, LEN(bind_var)) == 0) {
+                       ld_bind_now = *env + LEN(bind_var);
+               } else if (strncmp(*env, path_var, LEN(path_var)) == 0) {
+                       if (_rtld_trust) {
+                               ld_library_path = *env + LEN(path_var);
+                               *oenvp++ = *env;
+                       }
+               } else if (strncmp(*env, preload_var, LEN(preload_var)) == 0) {
+                       if (_rtld_trust) {
+                               ld_preload = *env + LEN(preload_var);
+                               *oenvp++ = *env;
+                       }
+               } else {
+                       *oenvp++ = *env;
+               }
+#undef LEN
+       }
+       *oenvp++ = NULL;
+
        if (ld_bind_now != NULL && *ld_bind_now != '\0')
                bind_now = true;
        if (_rtld_trust) {
 #ifdef DEBUG
-               const char     *ld_debug = getenv("LD_DEBUG");
 #ifdef RTLD_DEBUG
                debug = 0;
 #endif
                if (ld_debug != NULL && *ld_debug != '\0')
                        debug = 1;
 #endif
-               _rtld_add_paths(execname, &_rtld_paths,
-                   getenv("LD_LIBRARY_PATH"));
+               _rtld_add_paths(execname, &_rtld_paths, ld_library_path);
        } else {
                execname = NULL;
-               if (xunsetenv("LD_DEBUG") || xunsetenv("LD_LIBRARY_PATH"))
-                       _rtld_die();
        }
        _rtld_process_hints(execname, &_rtld_paths, &_rtld_xforms,
            _PATH_LD_HINTS);
@@ -512,17 +553,15 @@ _rtld(Elf_Addr *sp, Elf_Addr relocbase)
        _rtld_objmain->mainref = 1;
        _rtld_objlist_push_tail(&_rtld_list_main, _rtld_objmain);
 
-       if (_rtld_trust) {
+       if (ld_preload) {
                /*
                 * Pre-load user-specified objects after the main program
                 * but before any shared object dependencies.
                 */
                dbg(("preloading objects"));
-               if (_rtld_preload(getenv("LD_PRELOAD")) == -1)
-                       _rtld_die();
-       } else
-               if (xunsetenv("LD_PRELOAD"))
+               if (_rtld_preload(ld_preload) == -1)
                        _rtld_die();
+       }
 
        dbg(("loading needed objects"));
        if (_rtld_load_needed_objects(_rtld_objmain, RTLD_MAIN) == -1)