realpath: reduce stack usage from 3*PATH_MAX (12k) to 1*PATH_MAX (4k).

reduction is achieved by direct use of user-supplied PATH_MAX sized
buffer for result (without intermediate copy) and changes
in copy_buf[] usage - now it is used for both "source" pathname
and link name (it works because they have to be less than PATH_MAX combined,
otherwise we return NULL).

libc/stdlib/realpath.c

@@ -43,20 +43,23 @@ libc_hidden_proto(getcwd)
 #define MAX_READLINKS 32
 
 #ifdef __STDC__
-char *realpath(const char *path, char resolved_path[])
+char *realpath(const char *path, char got_path[])
 #else
-char *realpath(path, resolved_path)
+char *realpath(path, got_path)
 const char *path;
-char resolved_path[];
+char got_path[];
 #endif
 {
 	char copy_path[PATH_MAX];
-	char link_path[PATH_MAX];
-	char got_path[PATH_MAX];
-	char *new_path = got_path;
+	/* use user supplied buffer directly - reduces stack usage */
+	/* char got_path[PATH_MAX]; */
 	char *max_path;
+	char *new_path;
+	size_t path_len;
 	int readlinks = 0;
-	int n;
+#ifdef S_IFLNK
+	int link_len;
+#endif
 
 	if (path == NULL) {
 		__set_errno(EINVAL);
@@ -67,17 +70,20 @@ char resolved_path[];
 		return NULL;
 	}
 	/* Make a copy of the source path since we may need to modify it. */
-	if (strlen(path) >= PATH_MAX - 2) {
+	path_len = strlen(path);
+	if (path_len >= PATH_MAX - 2) {
 		__set_errno(ENAMETOOLONG);
 		return NULL;
 	}
-	strcpy(copy_path, path);
-	path = copy_path;
-	max_path = copy_path + PATH_MAX - 2;
-	/* If it's a relative pathname use getcwd for starters. */
+	/* Copy so that path is at the end of copy_path[] */
+	strcpy(copy_path + (PATH_MAX-1) - path_len, path);
+	path = copy_path + (PATH_MAX-1) - path_len;
+	max_path = got_path + PATH_MAX - 2; /* points to last non-NUL char */
+	new_path = got_path;
 	if (*path != '/') {
-		/* Ohoo... */
-		getcwd(new_path, PATH_MAX - 1);
+		/* If it's a relative pathname use getcwd for starters. */
+		if (!getcwd(new_path, PATH_MAX - 1))
+			return NULL;
 		new_path += strlen(new_path);
 		if (new_path[-1] != '/')
 			*new_path++ = '/';
@@ -112,7 +118,7 @@ char resolved_path[];
 		}
 		/* Safely copy the next pathname component. */
 		while (*path != '\0' && *path != '/') {
-			if (path > max_path) {
+			if (new_path > max_path) {
 				__set_errno(ENAMETOOLONG);
 				return NULL;
 			}
@@ -124,35 +130,32 @@ char resolved_path[];
 			__set_errno(ELOOP);
 			return NULL;
 		}
-		/* See if latest pathname component is a symlink. */
+		path_len = strlen(path);
+		/* See if last (so far) pathname component is a symlink. */
 		*new_path = '\0';
-		n = readlink(got_path, link_path, PATH_MAX - 1);
-		if (n < 0) {
+		link_len = readlink(got_path, copy_path, PATH_MAX - 1);
+		if (link_len < 0) {
 			/* EINVAL means the file exists but isn't a symlink. */
 			if (errno != EINVAL) {
-				/* Make sure it's null terminated. */
-				*new_path = '\0';
-				strcpy(resolved_path, got_path);
 				return NULL;
 			}
 		} else {
+			/* Safe sex check. */
+			if (path_len + link_len >= PATH_MAX - 2) {
+				__set_errno(ENAMETOOLONG);
+				return NULL;
+			}
 			/* Note: readlink doesn't add the null byte. */
-			link_path[n] = '\0';
-			if (*link_path == '/')
+			/* copy_path[link_len] = '\0'; - we don't need it too */
+			if (*copy_path == '/')
 				/* Start over for an absolute symlink. */
 				new_path = got_path;
 			else
 				/* Otherwise back up over this component. */
 				while (*(--new_path) != '/');
-			/* Safe sex check. */
-			if (strlen(path) + n >= PATH_MAX - 2) {
-				__set_errno(ENAMETOOLONG);
-				return NULL;
-			}
-			/* Insert symlink contents into path. */
-			strcat(link_path, path);
-			strcpy(copy_path, link_path);
-			path = copy_path;
+			/* Prepend symlink contents to path. */
+			memmove(copy_path + (PATH_MAX-1) - link_len - path_len, copy_path, link_len);
+			path = copy_path + (PATH_MAX-1) - link_len - path_len;
 		}
 #endif							/* S_IFLNK */
 		*new_path++ = '/';
@@ -162,6 +165,5 @@ char resolved_path[];
 		new_path--;
 	/* Make sure it's null terminated. */
 	*new_path = '\0';
-	strcpy(resolved_path, got_path);
-	return resolved_path;
+	return got_path;
 }