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From: Paul LeoNerd Evans <leonerd@leonerd.org.uk>
To: Andi Kleen <andi@firstfloor.org>, netdev@vger.kernel.org
Cc: David Miller <davem@davemloft.net>
Subject: Re: RFC: New BGF 'LOOP' instruction
Date: Tue, 3 Aug 2010 15:34:42 +0100	[thread overview]
Message-ID: <20100803143442.GW11110@cel.leo> (raw)
In-Reply-To: <20100803141110.GT11110@cel.leo>

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On Tue, Aug 03, 2010 at 03:11:10PM +0100, Paul LeoNerd Evans wrote:
>   switch(hdrtype)
>   {
>      case 1:
>        length = someconst; break;
>      case 2:
>        length = someotherconst; break;
>      case 3:
>        length = b[someoffset]; break;
>      ...
>   }

Of course, I completely forgot about finding also the offset of the
'next header' from the current header. That results in some code which,
in C, would look like:

  int hdrtype = b[IPv6_nexthdr];

  int x = size_of_IPv6_header;

  while(hdrtype != hdr_wanted) {
       int len;
       switch(hdrtype) {
         case 1:
           len = someconst;
           hdrtype = someotherconst;
           break;
         case 2:
           len = b[x+someoffs];
           hdrtype = b[x+someotheroffs];
           break;
         /* other IPv6 header types here */
       }

       x += len;
  }

You can't compile that idea into BPF without using a scratch memory
cell, because you can't have both 'len' and 'hdrtype' live in A at the
same time, nor can you atomically x += b[x+someoffs].

In short, much much easier if the C code did this part...

-- 
Paul "LeoNerd" Evans

leonerd@leonerd.org.uk
ICQ# 4135350       |  Registered Linux# 179460
http://www.leonerd.org.uk/

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      reply	other threads:[~2010-08-03 14:34 UTC|newest]

Thread overview: 26+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2010-08-02 11:03 RFC: New BGF 'LOOP' instruction Paul LeoNerd Evans
2010-08-02 11:13 ` RFC: New BPF " Paul LeoNerd Evans
2010-08-02 20:16 ` RFC: New BGF " Hagen Paul Pfeifer
2010-08-03  5:18   ` David Miller
2010-08-03  7:07     ` Paul LeoNerd Evans
2010-08-03  7:19       ` David Miller
2010-08-03  9:10         ` Hagen Paul Pfeifer
2010-08-03 13:40           ` Paul LeoNerd Evans
2010-08-03  9:03     ` Hagen Paul Pfeifer
2010-08-03  7:18   ` RFC: New BPF " Paul LeoNerd Evans
2010-08-03  5:13 ` RFC: New BGF " David Miller
2010-08-03  7:04   ` Paul LeoNerd Evans
2010-08-03  7:18     ` David Miller
2010-08-03 12:58       ` Andi Kleen
2010-08-03 13:07         ` David Miller
2010-08-03 13:34           ` RFC: New BPF " Paul LeoNerd Evans
2010-08-03 13:42             ` Paul LeoNerd Evans
2010-08-03 14:09             ` Rémi Denis-Courmont
2010-08-03 14:13               ` Paul LeoNerd Evans
2010-08-03 14:16                 ` Rémi Denis-Courmont
2010-08-03 14:19                   ` Paul LeoNerd Evans
2010-08-03 15:17                     ` Rémi Denis-Courmont
2010-08-03 15:27                       ` Paul LeoNerd Evans
2010-08-03 14:05           ` RFC: New BGF " Andi Kleen
2010-08-03 14:11             ` Paul LeoNerd Evans
2010-08-03 14:34               ` Paul LeoNerd Evans [this message]

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