Detect if the application has cleared the environ variable by setting

the first value (environ[0]) to NULL.  This is in addition to the
current detection of environ being replaced, which includes being set to
NULL.  Without this fix, the environment is not truly wiped, but appears
to be by getenv() until an *env() call is made to alter the enviroment.

This change is necessary to support those applications that use this
method for clearing environ such as Dovecot and Postfix.  Applications
such as Sendmail and the base system's env replace environ (already
detected).  While neither of these methods are defined by SUSv3, it is
best to support them due to historic reasons and in lieu of a clean,
defined method.

Add extra units tests for clearing environ using four different methods:
1. Set environ to NULL pointer.
2. Set environ[0] to NULL pointer.
3. Set environ to calloc()'d NULL-terminated array.
4. Set environ to static NULL-terminated array.

Noticed by:	Timo Sirainen

MFC after:	3 days

1 files changed, 12 insertions, 7 deletions

diff --git a/lib/libc/stdlib/getenv.c b/lib/libc/stdlib/getenv.c
index 74595b2..0f84082 100644
--- a/lib/libc/stdlib/getenv.c
+++ b/lib/libc/stdlib/getenv.c
@@ -1,5 +1,5 @@
 /*-
- * Copyright (c) 2007 Sean C. Farley <scf@FreeBSD.org>
+ * Copyright (c) 2007-2008 Sean C. Farley <scf@FreeBSD.org>
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -431,11 +431,13 @@ getenv(const char *name)
 
 	/*
 	 * Find environment variable via environ if no changes have been made
-	 * via a *env() call or environ has been replaced by a running program,
-	 * otherwise, use the rebuilt environment.
+	 * via a *env() call or environ has been replaced or cleared by a
+	 * running program, otherwise, use the rebuilt environment.
 	 */
 	if (envVars == NULL || environ != intEnviron)
 		return (__findenv_environ(name, nameLen));
+	else if (environ[0] == NULL)
+		return (NULL);
 	else {
 		envNdx = envVarsTotal - 1;
 		return (__findenv(name, nameLen, &envNdx, true));
@@ -525,8 +527,8 @@ __setenv(const char *name, size_t nameLen, const char *value, int overwrite)
 
 /*
  * If the program attempts to replace the array of environment variables
- * (environ) environ, then deactivate all variables and merge in the new list
- * from environ.
+ * (environ) environ or sets the first varible to NULL, then deactivate all
+ * variables and merge in the new list from environ.
  */
 static int
 __merge_environ(void)
@@ -534,8 +536,11 @@ __merge_environ(void)
 	char **env;
 	char *equals;
 
-	/* environ has been replaced.  clean up everything. */
-	if (envVarsTotal > 0 && environ != intEnviron) {
+	/*
+	 * Internally-built environ has been replaced or cleared.  clean up
+	 * everything.
+	 */
+	if (envVarsTotal > 0 && (environ != intEnviron || environ[0] == NULL)) {
 		/* Deactivate all environment variables. */
 		if (envActive > 0) {
 			origEnviron = NULL;