+/**
+ * Make path appear as if a chroot had occurred:
+ *
+ * @li 1. remove leading "/" (or replace with "." if at end)
+ *
+ * @li 2. remove leading ".." components (except those allowed by @p reldir)
+ *
+ * @li 3. delete any other "<dir>/.." (recursively)
+ *
+ * Can only shrink paths, so sanitizes in place.
+ *
+ * While we're at it, remove double slashes and "." components like
+ * clean_fname() does, but DON'T remove a trailing slash because that
+ * is sometimes significant on command line arguments.
+ *
+ * If @p reldir is non-null, it is a sanitized directory that the path will be
+ * relative to, so allow as many ".." at the beginning of the path as
+ * there are components in reldir. This is used for symbolic link targets.
+ * If reldir is non-null and the path began with "/", to be completely like
+ * a chroot we should add in depth levels of ".." at the beginning of the
+ * path, but that would blow the assumption that the path doesn't grow and
+ * it is not likely to end up being a valid symlink anyway, so just do
+ * the normal removal of the leading "/" instead.
+ *
+ * Contributed by Dave Dykstra <dwd@bell-labs.com>
+ */
+void sanitize_path(char *p, char *reldir)
+{
+ char *start, *sanp;
+ int depth = 0;
+ int allowdotdot = 0;
+
+ if (reldir) {
+ int new_component = 1;
+ while (*reldir) {
+ if (*reldir++ == '/')
+ new_component = 1;
+ else if (new_component) {
+ new_component = 0;
+ depth++;
+ }
+ }
+ }
+ start = p;
+ sanp = p;
+ while (*p == '/') {
+ /* remove leading slashes */
+ p++;
+ }
+ while (*p != '\0') {
+ /* this loop iterates once per filename component in p.
+ * both p (and sanp if the original had a slash) should
+ * always be left pointing after a slash
+ */
+ if (*p == '.' && (p[1] == '/' || p[1] == '\0')) {
+ /* skip "." component */
+ while (*++p == '/') {
+ /* skip following slashes */
+ ;
+ }
+ continue;
+ }
+ allowdotdot = 0;
+ if (*p == '.' && p[1] == '.' && (p[2] == '/' || p[2] == '\0')) {
+ /* ".." component followed by slash or end */
+ if (depth > 0 && sanp == start) {
+ /* allow depth levels of .. at the beginning */
+ --depth;
+ allowdotdot = 1;
+ } else {
+ p += 2;
+ if (*p == '/')
+ p++;
+ if (sanp != start) {
+ /* back up sanp one level */
+ --sanp; /* now pointing at slash */
+ while (sanp > start && sanp[-1] != '/') {
+ /* skip back up to slash */
+ sanp--;
+ }
+ }
+ continue;
+ }
+ }
+ while (1) {
+ /* copy one component through next slash */
+ *sanp++ = *p++;
+ if (*p == '\0' || p[-1] == '/') {
+ while (*p == '/') {
+ /* skip multiple slashes */
+ p++;
+ }
+ break;
+ }
+ }
+ if (allowdotdot) {
+ /* move the virtual beginning to leave the .. alone */
+ start = sanp;
+ }
+ }
+ if (sanp == start && !allowdotdot) {
+ /* ended up with nothing, so put in "." component */
+ /*
+ * note that the !allowdotdot doesn't prevent this from
+ * happening in all allowed ".." situations, but I didn't
+ * think it was worth putting in an extra variable to ensure
+ * it since an extra "." won't hurt in those situations.
+ */
+ *sanp++ = '.';
+ }
+ *sanp = '\0';
+}
+
+/* Works much like sanitize_path(), with these differences: (1) a new buffer
+ * is allocated for the sanitized path rather than modifying it in-place; (2)
+ * a leading slash gets transformed into the rootdir value (which can be empty
+ * or NULL if you just want the slash to get dropped); (3) no "reldir" can be
+ * specified. */
+char *alloc_sanitize_path(const char *path, const char *rootdir)
+{
+ char *buf;
+ int rlen, plen = strlen(path);
+
+ if (*path == '/' && rootdir) {
+ rlen = strlen(rootdir);
+ if (rlen == 1)
+ path++;
+ } else
+ rlen = 0;
+ if (!(buf = new_array(char, rlen + plen + 1)))
+ out_of_memory("alloc_sanitize_path");
+ if (rlen)
+ memcpy(buf, rootdir, rlen);
+ memcpy(buf + rlen, path, plen + 1);
+
+ if (rlen > 1)
+ rlen++;
+ sanitize_path(buf + rlen, NULL);
+ if (rlen && buf[rlen] == '.' && buf[rlen+1] == '\0') {
+ if (rlen > 1)
+ rlen--;
+ buf[rlen] = '\0';
+ }
+
+ return buf;
+}