ftp: keep parsing after a truncated line's LF

When a command or reply runs past the configured maximum length, the
parser keeps what fits, reports it truncated, and discards the remainder
of that line as it arrives. Only the remainder should go, but the parser
was discarding the whole stream slice that the line's LF landed in.

A slice holding "<remainder>\r\nQUIT\r\n" carries a complete QUIT behind
the remainder, and a response slice carries whatever reply comes next. The
recovery branch zeroed the remaining length and returned an error, which
ended the caller's parse loop, so those lines were discarded along with
the remainder they followed. They never became transactions, so nothing
inspected them and nothing logged them, which means a command can be
hidden by placing it after an over-long one.

Consume through the first LF, clear the truncation state there, and pick
up the next line in the same slice. The scan becomes a loop so the
remainder goes through the ordinary path, including when it is over-long
itself; the run-out-of-input check becomes the loop condition.

Raised in review of the 8859 fix.

Issue: 8859

src/app-layer-ftp.c

@@ -281,44 +281,45 @@ static AppLayerResult FTPGetLineForDirection(
 {
     SCEnter();
 
-    /* we have run out of input */
-    if (input->len <= 0)
-        return APP_LAYER_ERROR;
+    while (input->len > 0) {
+        const uint8_t *lf_idx = memchr(input->buf + input->consumed, 0x0a, input->len);
+
+        if (lf_idx == NULL) {
+            if (!(*current_line_truncated) && (uint32_t)input->len >= ftp_max_line_len) {
+                *current_line_truncated = true;
+                line->buf = input->buf + input->consumed;
+                line->len = ftp_max_line_len;
+                line->delim_len = 0;
+                /* No caller reads consumed after this; advance it so the cursor
+                 * still describes the slice it was handed. */
+                input->consumed += input->len;
+                input->len = 0;
+                SCReturnStruct(APP_LAYER_OK);
+            }
+            SCReturnStruct(APP_LAYER_INCOMPLETE(input->consumed, input->len + 1));
+        }
 
-    const uint8_t *lf_idx = memchr(input->buf + input->consumed, 0x0a, input->len);
+        const uint32_t o_consumed = input->consumed;
+        input->consumed = (uint32_t)(lf_idx - input->buf + 1);
+        const uint32_t line_len = (uint32_t)(input->consumed - o_consumed);
+        input->len -= (int32_t)line_len;
+        DEBUG_VALIDATE_BUG_ON((input->consumed + input->len) != input->orig_len);
 
-    if (lf_idx == NULL) {
-        if (!(*current_line_truncated) && (uint32_t)input->len >= ftp_max_line_len) {
-            *current_line_truncated = true;
-            line->buf = input->buf + input->consumed;
-            line->len = ftp_max_line_len;
-            line->delim_len = 0;
-            /* No caller reads consumed after this; advance it so the cursor
-             * still describes the slice it was handed. */
-            input->consumed += input->len;
-            input->len = 0;
-            SCReturnStruct(APP_LAYER_OK);
+        if (*current_line_truncated) {
+            /* The tail of an over-long line ends at this LF. Discard just that
+             * tail -- anything after the LF is a line of its own and still has
+             * to be parsed. */
+            *current_line_truncated = false;
+            continue;
         }
-        SCReturnStruct(APP_LAYER_INCOMPLETE(input->consumed, input->len + 1));
-    } else if (*current_line_truncated) {
-        // Whatever came in with first LF should also get discarded
-        *current_line_truncated = false;
-        line->len = 0;
-        line->delim_len = 0;
-        input->len = 0;
-        SCReturnStruct(APP_LAYER_ERROR);
-    } else {
+
+        line->lf_found = true;
+        line->buf = input->buf + o_consumed;
+        line->len = line_len;
         // There could be one chunk of command data that has LF but post the line limit
         // e.g. input_len = 5077
         //      lf_idx = 5010
         //      max_line_len = 4096
-        uint32_t o_consumed = input->consumed;
-        input->consumed = (uint32_t)(lf_idx - input->buf + 1);
-        line->len = input->consumed - o_consumed;
-        input->len -= line->len;
-        line->lf_found = true;
-        DEBUG_VALIDATE_BUG_ON((input->consumed + input->len) != input->orig_len);
-        line->buf = input->buf + o_consumed;
         if (line->len >= ftp_max_line_len) {
             *current_line_truncated = true;
             line->len = ftp_max_line_len;
@@ -333,6 +334,9 @@ static AppLayerResult FTPGetLineForDirection(
         }
         SCReturnStruct(APP_LAYER_OK);
     }
+
+    /* we have run out of input */
+    return APP_LAYER_ERROR;
 }
 
 /**