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Message-ID: <20050102043741.9207.qmail@web41408.mail.yahoo.com>
Date: Sat, 1 Jan 2005 20:37:41 -0800 (PST)
From: cranium2003 <cranium2003@yahoo.com>
Subject: ip_fast_csum usage
To: linux-kernel@vger.kernel.org
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Sender: linux-kernel-owner@vger.kernel.org
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Lines: 65

Hello all,
          In linux kernel source code of IP layer i
found that calculation of IP header checksum requires
 ip_fast_csum function to be called with IP header as
unsigned char string parameter.
 I want to know what will happen if i replace that IP
header parameter with my own unsigned char *data
variable which is align to 4 octet boundry.

	iph->check = ip_fast_csum((unsigned char *) iph,
iph->ihl);

the ip_fast_csum defined in kernel source is as
static inline unsigned short ip_fast_csum(unsigned
char * iph,
					  unsigned int ihl)
{
	unsigned int sum;

	__asm__ __volatile__(
	    "movl (%1), %0	;\n"
	    "subl $4, %2	;\n"
	    "jbe 2f		;\n"
	    "addl 4(%1), %0	;\n"
	    "adcl 8(%1), %0	;\n"
	    "adcl 12(%1), %0	;\n"
"1:	    adcl 16(%1), %0	;\n"
	    "lea 4(%1), %1	;\n"
	    "decl %2		;\n"
	    "jne 1b		;\n"
	    "adcl $0, %0	;\n"
	    "movl %0, %2	;\n"
	    "shrl $16, %0	;\n"
	    "addw %w2, %w0	;\n"
	    "adcl $0, %0	;\n"
	    "notl %0		;\n"
"2:				;\n"
	/* Since the input registers which are loaded with
iph and ipl
	   are modified, we must also specify them as
outputs, or gcc
	   will assume they contain their original values. */
	: "=r" (sum), "=r" (iph), "=r" (ihl)
	: "1" (iph), "2" (ihl)
	: "memory");
	return(sum);
}

I am just experimenting with this function from kernel
module to check how it works?
Thanks in advance.
regards,
cranium.


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