shared: dns-name - introduce dns_label_unescape_suffix()

Intended to be called repeatedly, and returns then successive unescaped labels
from the most to the least significant (left to right).

This is slightly inefficient as it scans the string three times (two would be
sufficient): once to find the end of the string, once to find the beginning
of each label and lastly once to do the actual unescaping. The latter two
could be done in one go, but that seemed unnecessarily convoluted.

3 files changed, 104 insertions, 0 deletions

diff --git a/src/shared/dns-domain.c b/src/shared/dns-domain.c
index 20a44ce4e1..8a472fbcb4 100644
--- a/src/shared/dns-domain.c
+++ b/src/shared/dns-domain.c
@@ -114,6 +114,68 @@ int dns_label_unescape(const char **name, char *dest, size_t sz) {
         return r;
 }
 
+/* @label_terminal: terminal character of a label, updated to point to the terminal character of
+ *                  the previous label (always skipping one dot) or to NULL if there are no more
+ *                  labels. */
+int dns_label_unescape_suffix(const char *name, const char **label_terminal, char *dest, size_t sz) {
+        const char *terminal;
+        int r;
+
+        assert(name);
+        assert(label_terminal);
+        assert(dest);
+
+        /* no more labels */
+        if (!*label_terminal) {
+                if (sz >= 1)
+                        *dest = 0;
+
+                return 0;
+        }
+
+        assert(**label_terminal == '.' || **label_terminal == 0);
+
+        /* skip current terminal character */
+        terminal = *label_terminal - 1;
+
+        /* point name to the last label, and terminal to the preceding terminal symbol (or make it a NULL pointer) */
+        for (;;) {
+                if (terminal < name) {
+                        /* reached the first label, so indicate that there are no more */
+                        terminal = NULL;
+                        break;
+                }
+
+                /* find the start of the last label */
+                if (*terminal == '.') {
+                        const char *y;
+                        unsigned slashes = 0;
+
+                        for (y = terminal - 1; y >= name && *y == '\\'; y--)
+                                slashes ++;
+
+                        if (slashes % 2 == 0) {
+                                /* the '.' was not escaped */
+                                name = terminal + 1;
+                                break;
+                        } else {
+                                terminal = y;
+                                continue;
+                        }
+                }
+
+                terminal --;
+        }
+
+        r = dns_label_unescape(&name, dest, sz);
+        if (r < 0)
+                return r;
+
+        *label_terminal = terminal;
+
+        return r;
+}
+
 int dns_label_escape(const char *p, size_t l, char **ret) {
         _cleanup_free_ char *s = NULL;
         char *q;
diff --git a/src/shared/dns-domain.h b/src/shared/dns-domain.h
index 00caf5d700..5728ce34bb 100644
--- a/src/shared/dns-domain.h
+++ b/src/shared/dns-domain.h
@@ -29,6 +29,7 @@
 #define DNS_NAME_MAX 255
 
 int dns_label_unescape(const char **name, char *dest, size_t sz);
+int dns_label_unescape_suffix(const char *name, const char **label_end, char *dest, size_t sz);
 int dns_label_escape(const char *p, size_t l, char **ret);
 
 int dns_label_apply_idna(const char *encoded, size_t encoded_size, char *decoded, size_t decoded_max);
diff --git a/src/test/test-dns-domain.c b/src/test/test-dns-domain.c
index 527cdd3b54..9a5a2a1c23 100644
--- a/src/test/test-dns-domain.c
+++ b/src/test/test-dns-domain.c
@@ -50,6 +50,46 @@ static void test_dns_label_unescape(void) {
         test_dns_label_unescape_one("foobar.", "foobar", 20, 6);
 }
 
+static void test_dns_label_unescape_suffix_one(const char *what, const char *expect1, const char *expect2, size_t buffer_sz, int ret1, int ret2) {
+        char buffer[buffer_sz];
+        const char *label;
+        int r;
+
+        label = what + strlen(what);
+
+        r = dns_label_unescape_suffix(what, &label, buffer, buffer_sz);
+        assert_se(r == ret1);
+        if (r >= 0)
+                assert_se(streq(buffer, expect1));
+
+        r = dns_label_unescape_suffix(what, &label, buffer, buffer_sz);
+        assert_se(r == ret2);
+        if (r >= 0)
+                assert_se(streq(buffer, expect2));
+}
+
+static void test_dns_label_unescape_suffix(void) {
+        test_dns_label_unescape_suffix_one("hallo", "hallo", "", 6, 5, 0);
+        test_dns_label_unescape_suffix_one("hallo", "hallo", "", 4, -ENOSPC, -ENOSPC);
+        test_dns_label_unescape_suffix_one("", "", "", 10, 0, 0);
+        test_dns_label_unescape_suffix_one("hallo\\.foobar", "hallo.foobar", "", 20, 12, 0);
+        test_dns_label_unescape_suffix_one("hallo.foobar", "foobar", "hallo", 10, 6, 5);
+        test_dns_label_unescape_suffix_one("hallo.foobar\n", "foobar", "foobar", 20, -EINVAL, -EINVAL);
+        test_dns_label_unescape_suffix_one("hallo\\", "hallo", "hallo", 20, -EINVAL, -EINVAL);
+        test_dns_label_unescape_suffix_one("hallo\\032 ", "hallo  ", "", 20, 7, 0);
+        test_dns_label_unescape_suffix_one(".", "", "", 20, 0, 0);
+        test_dns_label_unescape_suffix_one("..", "", "", 20, 0, 0);
+        test_dns_label_unescape_suffix_one(".foobar", "foobar", "", 20, 6, -EINVAL);
+        test_dns_label_unescape_suffix_one("foobar.", "", "foobar", 20, 0, 6);
+        test_dns_label_unescape_suffix_one("foo\\\\bar", "foo\\bar", "", 20, 7, 0);
+        test_dns_label_unescape_suffix_one("foo.bar", "bar", "foo", 20, 3, 3);
+        test_dns_label_unescape_suffix_one("foo..bar", "bar", "", 20, 3, -EINVAL);
+        test_dns_label_unescape_suffix_one("foo...bar", "bar", "", 20, 3, -EINVAL);
+        test_dns_label_unescape_suffix_one("foo\\.bar", "foo.bar", "", 20, 7, 0);
+        test_dns_label_unescape_suffix_one("foo\\\\.bar", "bar", "foo\\", 20, 3, 4);
+        test_dns_label_unescape_suffix_one("foo\\\\\\.bar", "foo\\.bar", "", 20, 8, 0);
+}
+
 static void test_dns_label_escape_one(const char *what, size_t l, const char *expect, int ret) {
         _cleanup_free_ char *t = NULL;
         int r;
@@ -180,6 +220,7 @@ static void test_dns_name_reverse(void) {
 int main(int argc, char *argv[]) {
 
         test_dns_label_unescape();
+        test_dns_label_unescape_suffix();
         test_dns_label_escape();
         test_dns_name_normalize();
         test_dns_name_equal();