Make sure we don't set the snapend before the beginning of the packet.

If a caller attempts to set it after the current snapend, just silently
ignore the attempt.

If they try to set it before the beginning of the packet, report it as a
bug and quit dissection.  Add a new setjmp() return value meaning "bug"
rather than "truncated packet", add an "nd_bug_longjmp()" routine for
reporting bugs where we should quit dissecting, and use that in this
case.

1 files changed, 16 insertions, 1 deletions

diff --git a/netdissect.h b/netdissect.h
index e2b77860..605a9c9b 100644
--- a/netdissect.h
+++ b/netdissect.h
@@ -193,7 +193,8 @@ struct netdissect_saved_packet_info {
 };
 
 /* 'val' value(s) for longjmp */
-#define ND_TRUNCATED 1
+#define ND_TRUNCATED 1          /* packet data too short */
+#define ND_BUG       2          /* bug of some sort */
 
 struct netdissect_options {
   int ndo_bflag;		/* print 4 byte ASes in ASDOT notation */
@@ -283,6 +284,20 @@ nd_trunc_longjmp(netdissect_options *ndo)
 #endif /* _AIX */
 }
 
+static inline NORETURN void
+nd_bug_longjmp(netdissect_options *ndo)
+{
+	longjmp(ndo->ndo_early_end, ND_BUG);
+#ifdef _AIX
+	/*
+	 * In AIX <setjmp.h> decorates longjmp() with "#pragma leaves", which tells
+	 * XL C that the function is noreturn, but GCC remains unaware of that and
+	 * yields a "'noreturn' function does return" warning.
+	 */
+	ND_UNREACHABLE
+#endif /* _AIX */
+}
+
 #define PT_VAT		1	/* Visual Audio Tool */
 #define PT_WB		2	/* distributed White Board */
 #define PT_RPC		3	/* Remote Procedure Call */