netdev
[Top] [All Lists]

Re: [IPX]: Fix checksum computation.

To: "David S. Miller" <davem@xxxxxxxxxx>
Subject: Re: [IPX]: Fix checksum computation.
From: Mark Huth <mhuth@xxxxxxxxxx>
Date: Fri, 31 Oct 2003 16:29:45 -0700
Cc: Joe Perches <joe@xxxxxxxxxxx>, netdev@xxxxxxxxxxx
References: <200310312006.h9VK62Hh005910@xxxxxxxxxxxxxxx> <1067635446.11564.92.camel@xxxxxxxxxxxxxxxxxxxxx> <20031031132331.35a9aaca.davem@xxxxxxxxxx>
Sender: netdev-bounce@xxxxxxxxxxx
User-agent: Mozilla/5.0 (Windows; U; Windows NT 5.0; en-US; rv:1.0.1) Gecko/20020823 Netscape/7.0


David S. Miller wrote:

On Fri, 31 Oct 2003 13:24:06 -0800
Joe Perches <joe@xxxxxxxxxxx> wrote:

Why is this a "fix"?  Performance?
It seems more like someone's idea of code neatening.

IPC checksums were being miscomputed in the original code,
we're as mystified as you are as to why it is that:

        if (sum & 0x10000) {
                sum++;
                sum &= 0xffff;
        }

works while:

        sum = ((sym >> 16) + sum) & 0xffff;

does not.  The theory was that it might be some x86 gcc bug,
but looking at the assembler diff Arnaldo Carvalho de Melo
(the appletalk maintainer) showed me between the before and
after:

Nah, they are different algorithms, as the assembler code clearly demonstrates. The above snippet is incomplete, with the crucial multiply by 2 or the shift left omitted. The assembler code reveals this crucial piece of information.


        xorl    %eax, %eax
-       decl    %ecx
        movb    (%ebx), %al
-       incl    %ebx
        addl    %eax, %edx
        addl    %edx, %edx
-       movl    %edx, %eax
-       shrl    $16, %eax
-       addl    %edx, %eax

The previous instruction adds 0, 1, or 2 to the checksum accumulation. That's because in the case where the byte added to the accumulation (addl %eax, %edx) causes the 16 bit to set, then when multiplied by 2 (addl %edx, %edx) the 17 bit is set and the 16 bit is clear.

-       movzwl  %ax,%edx
+       testl   $65536, %edx
+       je      .L982

This version adds one to the accumulation iff bit 16 is set following the multiply. The results are clearly different. This latter version would be a correct implementation of feedback shiftregister algorithm, assuming that is what is being computed instead of a simple checksum. Not knowing the specification for the algorithm offhand, I can't say which is correct.

They will, however, _sometimes_ give the same results. That probably explains the rest of the comments in this thread.

+       incl    %edx
+       andl    $65535, %edx
+.L982:
+       decl    %ecx
+       incl    %ebx
        cmpl    $-1, %ecx

we still can't see what's wrong.

He did confirm that the change in question makes IPX compute checksums
correctly.






<Prev in Thread] Current Thread [Next in Thread>