Fix reported_len in Lua framewark when creating tvb from range.

This bug affects Lua plugin dissectors for encapsulation protocols like
GRE.  Typically the dissector creates a range for the payload packet, then
calls the next dissector with a tvb derived from the range, using
TvbRange_tvb().  The original version calls
tvb_new_subset_length_caplen() using the remaining capture length for the
reported_len argument.  The fix passes -1 as the reported length, and
tvb_new_subset_length_caplen() calculates the new reported_len as required.

The bug only affects large packets captured with a snaplen and
truncated, then decoded with a Lua plugin for the encapsulation header.

Here's the typical bug symptom, gleaned from tshark decode of
an encapsulated IP payload:

        [Expert Info (Error/Protocol): IPv4 total length exceeds packet length (114 bytes)]
            [IPv4 total length exceeds packet length (114 bytes)]

Closes #15655.

1 files changed, 2 insertions, 1 deletions

diff --git a/epan/wslua/wslua_tvb.c b/epan/wslua/wslua_tvb.c
index 400f6d19d4..2e4a45dab5 100644
--- a/epan/wslua/wslua_tvb.c
+++ b/epan/wslua/wslua_tvb.c
@@ -399,7 +399,8 @@ WSLUA_METHOD TvbRange_tvb(lua_State *L) {
         tvb = (Tvb)g_malloc(sizeof(struct _wslua_tvb));
         tvb->expired = FALSE;
         tvb->need_free = FALSE;
-        tvb->ws_tvb = tvb_new_subset_length_caplen(tvbr->tvb->ws_tvb,tvbr->offset,tvbr->len, tvbr->len);
+        // -1 means recalculate the reported_len based on the new offset
+        tvb->ws_tvb = tvb_new_subset_length_caplen(tvbr->tvb->ws_tvb,tvbr->offset,tvbr->len,-1);
         return push_wsluaTvb(L, tvb);
     } else {
         luaL_error(L,"Out Of Bounds");