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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