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.

1 files changed, 28 insertions, 13 deletions

diff --git a/src/env/setenv.c b/src/env/setenv.c
index 76e8ee12..a7dd2b60 100644
--- a/src/env/setenv.c
+++ b/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);
 }