+ return pop(fd, buf, size, ptr - buf + 1);
+}
+
+/*
+ * Decode an octal-escaped and space-terminated path of the form used by
+ * /proc/mounts. May be used to decode a path in-place. "out" must be at least
+ * as large as the input. The output is always null-terminated. "len" gets the
+ * length of the output, excluding the trailing null. Returns 0 if a full path
+ * was successfully decoded, otherwise an error.
+ */
+static int decode_path(const char *in, char *out, size_t *len)
+{
+ char *first = out;
+ int c;
+ int i;
+ int ret = -EINVAL;
+ while (1) {
+ switch (*in) {
+ case '\0':
+ goto out;
+
+ case ' ':
+ ret = 0;
+ goto out;
+
+ case '\\':
+ in++;
+ c = 0;
+ for (i = 0; i < 3; i++) {
+ if (*in < '0' || *in > '7')
+ goto out;
+ c = (c << 3) | (*in++ - '0');
+ }
+ *(unsigned char *)out++ = (unsigned char) c;
+ break;
+
+ default:
+ *out++ = *in++;
+ break;
+ }
+ }
+
+out:
+ *out = '\0';
+ *len = out - first;
+ return ret;
+}
+
+/*
+ * Computes the length of s when encoded with three-digit octal escape sequences
+ * for the characters in chars.
+ */
+static size_t octal_encoded_length(const char *s, const char *chars)
+{
+ size_t len = strlen(s);
+ while ((s = strpbrk(s, chars)) != NULL) {
+ len += 3;
+ s++;
+ }
+
+ return len;
+}
+
+enum {
+ OUTCOME_NOTHING_MOUNTED,
+ OUTCOME_TMPFS_MOUNT,
+ OUTCOME_NON_TMPFS_MOUNT,
+};
+
+/* Read a line of /proc/mounts data looking for a tmpfs mount at "path". */
+static int read_mount(int fd, char *buf, size_t bufsize, const char *path,
+ int *outcome)
+{
+ int found;
+ int match;
+ char *space;
+ size_t len;
+
+ enum {
+ MATCH_NONE,
+ MATCH_EXACT,
+ MATCH_PARENT,
+ };
+
+ found = next(fd, buf, bufsize, ' ');
+ if (found != 1)
+ return found;