overhaul environment functions

Rewrite environment access functions to slim down code, fix bugs and
avoid invoking undefined behavior.

* avoid using int-typed iterators where size_t would be correct;
* use strncmp instead of memcmp consistently;
* tighten prologues by invoking __strchrnul;
* handle NULL environ.

putenv:
* handle "=value" input via unsetenv too (will return -1/EINVAL);
* rewrite and simplify __putenv; fix the leak caused by failure to
  deallocate entry added by preceding setenv when called from putenv.

setenv:
* move management of libc-allocated entries to this translation unit,
  and use no-op weak symbols in putenv/unsetenv;

unsetenv:
* rewrite; this fixes UB caused by testing a free'd pointer against
  NULL on entry to subsequent loops.

Not changed:
Failure to extend allocation tracking array (previously __env_map, now
env_alloced) is ignored rather than causing to report -1/ENOMEM to the
caller; the worst-case consequence is leaking this allocation when it
is removed or replaced in a subsequent environment access.

Initially UB in unsetenv was reported by Alexander Cherepanov.
Using a weak alias to avoid pulling in malloc via unsetenv was
suggested by Rich Felker.

src/env/getenv.c

@@ -2,13 +2,14 @@
 #include <string.h>
 #include "libc.h"
 
+char *__strchrnul(const char *, int);
+
 char *getenv(const char *name)
 {
-	int i;
-	size_t l = strlen(name);
-	if (!__environ || !*name || strchr(name, '=')) return NULL;
-	for (i=0; __environ[i] && (strncmp(name, __environ[i], l)
-		|| __environ[i][l] != '='); i++);
-	if (__environ[i]) return __environ[i] + l+1;
-	return NULL;
+	size_t l = __strchrnul(name, '=') - name;
+	if (l && !name[l] && __environ)
+		for (char **e = __environ; *e; e++)
+			if (!strncmp(name, *e, l) && l[*e] == '=')
+				return *e + l+1;
+	return 0;
 }

src/env/putenv.c

@@ -1,58 +1,48 @@
 #include <stdlib.h>
 #include <string.h>
+#include "libc.h"
 
-extern char **__environ;
-char **__env_map;
+char *__strchrnul(const char *, int);
 
-int __putenv(char *s, int a)
-{
-	int i=0, j=0;
-	char *z = strchr(s, '=');
-	char **newenv = 0;
-	char **newmap = 0;
-	static char **oldenv;
+static void dummy(char *old, char *new) {}
+weak_alias(dummy, __env_rm_add);
 
-	if (!z) return unsetenv(s);
-	if (z==s) return -1;
-	for (; __environ[i] && memcmp(s, __environ[i], z-s+1); i++);
-	if (a) {
-		if (!__env_map) {
-			__env_map = calloc(2, sizeof(char *));
-			if (__env_map) __env_map[0] = s;
-		} else {
-			for (; __env_map[j] && __env_map[j] != __environ[i]; j++);
-			if (!__env_map[j]) {
-				newmap = realloc(__env_map, sizeof(char *)*(j+2));
-				if (newmap) {
-					__env_map = newmap;
-					__env_map[j] = s;
-					__env_map[j+1] = NULL;
-				}
-			} else {
-				free(__env_map[j]);
-				__env_map[j] = s;
+int __putenv(char *s, size_t l, char *r)
+{
+	size_t i=0;
+	if (__environ) {
+		for (char **e = __environ; *e; e++, i++)
+			if (!strncmp(s, *e, l+1)) {
+				char *tmp = *e;
+				*e = s;
+				__env_rm_add(tmp, r);
+				return 0;
 			}
-		}
 	}
-	if (!__environ[i]) {
-		newenv = malloc(sizeof(char *)*(i+2));
-		if (!newenv) {
-			if (a && __env_map) __env_map[j] = 0;
-			return -1;
-		}
-		memcpy(newenv, __environ, sizeof(char *)*i);
-		newenv[i] = s;
-		newenv[i+1] = 0;
-		__environ = newenv;
+	static char **oldenv;
+	char **newenv;
+	if (__environ == oldenv) {
+		newenv = realloc(oldenv, sizeof *newenv * (i+2));
+		if (!newenv) goto oom;
+	} else {
+		newenv = malloc(sizeof *newenv * (i+2));
+		if (!newenv) goto oom;
+		if (i) memcpy(newenv, __environ, sizeof *newenv * i);
 		free(oldenv);
-		oldenv = __environ;
 	}
-
-	__environ[i] = s;
+	newenv[i] = s;
+	newenv[i+1] = 0;
+	__environ = oldenv = newenv;
+	if (r) __env_rm_add(0, r);
 	return 0;
+oom:
+	free(r);
+	return -1;
 }
 
 int putenv(char *s)
 {
-	return __putenv(s, 0);
+	size_t l = __strchrnul(s, '=') - s;
+	if (!l || !s[l]) return unsetenv(s);
+	return __putenv(s, l, 0);
 }

src/env/setenv.c

@@ -2,29 +2,44 @@
 #include <string.h>
 #include <errno.h>
 
-int __putenv(char *s, int a);
+char *__strchrnul(const char *, int);
+int __putenv(char *, size_t, char *);
+
+void __env_rm_add(char *old, char *new)
+{
+	static char **env_alloced;
+	static size_t env_alloced_n;
+	for (size_t i=0; i < env_alloced_n; i++)
+		if (env_alloced[i] == old) {
+			env_alloced[i] = new;
+			free(old);
+			return;
+		} else if (!env_alloced[i] && new) {
+			env_alloced[i] = new;
+			new = 0;
+		}
+	if (!new) return;
+	char **t = realloc(env_alloced, sizeof *t * (env_alloced_n+1));
+	if (!t) return;
+	(env_alloced = t)[env_alloced_n++] = new;
+}
 
 int setenv(const char *var, const char *value, int overwrite)
 {
 	char *s;
-	int l1, l2;
+	size_t l1, l2;
 
-	if (!var || !*var || strchr(var, '=')) {
+	if (!var || !(l1 = __strchrnul(var, '=') - var) || var[l1]) {
 		errno = EINVAL;
 		return -1;
 	}
 	if (!overwrite && getenv(var)) return 0;
 
-	l1 = strlen(var);
 	l2 = strlen(value);
 	s = malloc(l1+l2+2);
-	if (s) {
-		memcpy(s, var, l1);
-		s[l1] = '=';
-		memcpy(s+l1+1, value, l2);
-		s[l1+l2+1] = 0;
-		if (!__putenv(s, 1)) return 0;
-	}
-	free(s);
-	return -1;
+	if (!s) return -1;
+	memcpy(s, var, l1);
+	s[l1] = '=';
+	memcpy(s+l1+1, value, l2+1);
+	return __putenv(s, l1, s);
 }

src/env/unsetenv.c

@@ -1,31 +1,30 @@
 #include <stdlib.h>
 #include <string.h>
 #include <errno.h>
+#include "libc.h"
 
-extern char **__environ;
-extern char **__env_map;
+char *__strchrnul(const char *, int);
+
+static void dummy(char *old, char *new) {}
+weak_alias(dummy, __env_rm_add);
 
 int unsetenv(const char *name)
 {
-	int i, j;
-	size_t l = strlen(name);
-
-	if (!*name || strchr(name, '=')) {
+	size_t l = __strchrnul(name, '=') - name;
+	if (!l || name[l]) {
 		errno = EINVAL;
 		return -1;
 	}
-again:
-	for (i=0; __environ[i] && (memcmp(name, __environ[i], l) || __environ[i][l] != '='); i++);
-	if (__environ[i]) {
-		if (__env_map) {
-			for (j=0; __env_map[j] && __env_map[j] != __environ[i]; j++);
-			free (__env_map[j]);
-			for (; __env_map[j]; j++)
-				__env_map[j] = __env_map[j+1];
-		}
-		for (; __environ[i]; i++)
-			__environ[i] = __environ[i+1];
-		goto again;
+	if (__environ) {
+		char **e = __environ, **eo = e;
+		for (; *e; e++)
+			if (!strncmp(name, *e, l) && l[*e] == '=')
+				__env_rm_add(*e, 0);
+			else if (eo != e)
+				*eo++ = *e;
+			else
+				eo++;
+		if (eo != e) *eo = 0;
 	}
 	return 0;
 }