pxelinux: cleaner test for OACK trailing null bytes

Instead of looking for a string of null bytes at the end of the OACK
string, simply abort parsing if we run into a null byte where an
option is expected; either we are seeing junk at the end of the
packet, or we are hopelessly confused about how to make sense of the
rest of the packet -- in either case, ignoring is the "liberal"
option.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>

diff --git a/core/pxelinux.asm b/core/pxelinux.asm
index de1b10c..cbfae52 100644 (file)
--- a/core/pxelinux.asm
+++ b/core/pxelinux.asm
@@ -1105,16 +1105,16 @@ searchdir:
                ;  SI -> first byte of options; [E]CX -> byte count
 .parse_oack:
                jcxz .done_pkt                  ; No options acked
-.get_opt_name:
-               ; Some TFTP servers have junk NUL bytes at the end of the packet.
-               ; If all that is left is NUL, then consider the packet processed.
-               mov di,si
-               push cx
-               xor ax,ax
-               repz scasb
-               pop cx
-               jz .done_pkt
+               ; If we find an option which starts with a NUL byte,
+               ; (a null option), we're either seeing garbage that some
+               ; TFTP servers add to the end of the packet, or we have
+               ; no clue how to parse the rest of the packet (what is
+               ; an option name and what is a value?)  In either case,
+               ; discard the rest.
+               cmp byte [si],0
+               je .done_pkt
 
+.get_opt_name:
                mov di,si
                mov bx,si
 .opt_name_loop:        lodsb