creation of sock

creation of sock

[Sourav Ghosh]

Hello,

I'm using linux 2.2.15 kernel on redhat.
I have added some variables (pointers) on "sock" data structure.
I was initializing them to NULL in sk_alloc() function.

But it seems some sock structures are allocated for TCP bypassing this
sk_alloc() and due to this my added pointers are not initialized to NULL
all the time.

Can anyone tell me which function is being called for generating sock
for TCP connections ( I guess for a aprticular TCP packet type, not for
all, as I'm getting into this problem intermittently, esp., when I try
access some specified website from my PC) ?

Thanks
--
Sourav

Re: creation of sock

[Jacob L E Blain Christen]

im a kernel newbie here so pardon "the blind leading the blind" ...

doing a quick search for all calls to sk_alloc in the entire kernel
sources
yields only one call that sets the "zero out the allocated struct"
boolean
to false and that is:
	net/ipv4/tcp_minisocks.c:tcp_create_openreq_child().
this funtion in turn is only ever called from:
	net/ipv[46]/tcp_ipv[46].c:tcp_v[46]_syn_recv_sock()

the comment above the ipv4 version is (verbatim):

	/* 
	 * The three way handshake has completed - we got a valid synack - 
	 * now create the new socket. 
	 */

so if you need those experimental values of yours zeroed out on socket
creation i suggest replacing this snippet from
net/core/sock.c:sk_alloc()

        if(sk && zero_it) {
                memset(sk, 0, sizeof(struct sock));
                sk->family = family;
                sock_lock_init(sk);
        }

with

        if(  sk  ) {
		/* set your NULL init values here */
		if(  zero_it  ) {
	                memset(sk, 0, sizeof(struct sock));
	                sk->family = family;
	                sock_lock_init(sk);
		}
        }

doh!  i just re-read your mail and realized youre using the 2.2.15
kernel.
my examples are from the 2.4.2 sources...

looking at the 2.2.16 source (i have only 2.2.1[46] and not 2.2.15 for
the
2.2 series) the (roughly) congruent if block of code is:

        if(sk) {
                if (zero_it) 
                        memset(sk, 0, sizeof(struct sock));
                sk->family = family;
        }

and so if you're setting your init values to NULL under the "zero_it"
condition you would get the behavior that you reported.

hope that helps.

Re: creation of sock

[Jacob L E Blain Christen]

looking further at 
net/ipv4/tcp_ipv4.c:tcp_create_openreq_child() (for 2.2.16)
and
net/ipv4/tcp_minisocks.c:tcp_create_openreq_child() (for 2.4.x)

immediately after the sk_alloc() call (if it successful) it calls
	memcpy(newsk, sk, sizeof(*newsk))
i suggest setting your NULL initial values immediately after this line.

sorry for the premature email

Re: creation of sock

[Sourav Ghosh]

Jacob L E Blain Christen wrote:

> looking further at
> net/ipv4/tcp_ipv4.c:tcp_create_openreq_child() (for 2.2.16)
> and
> net/ipv4/tcp_minisocks.c:tcp_create_openreq_child() (for 2.4.x)
>
> immediately after the sk_alloc() call (if it successful) it calls
>         memcpy(newsk, sk, sizeof(*newsk))
> i suggest setting your NULL initial values immediately after this line.
>
> sorry for the premature email

Didn't help.
The problem persists.

Re: creation of sock

[Sourav Ghosh]

I found this condition only happens when the sock state is TCP_TIME_WAIT.
I don't know if this helps.


Sourav Ghosh wrote:

> Hello,
>
> I'm using linux 2.2.15 kernel on redhat.
> I have added some variables (pointers) on "sock" data structure.
> I was initializing them to NULL in sk_alloc() function.
>
> But it seems some sock structures are allocated for TCP bypassing this
> sk_alloc() and due to this my added pointers are not initialized to NULL
> all the time.
>
> Can anyone tell me which function is being called for generating sock
> for TCP connections ( I guess for a aprticular TCP packet type, not for
> all, as I'm getting into this problem intermittently, esp., when I try
> access some specified website from my PC) ?
>
> Thanks
> --
> Sourav

Re: creation of sock

[Sourav Ghosh]

Hello,

It seems like linux replaces sock with tcp_tw_bucket structure in the
sock list in order to "work around the memory consumption problems for
the heavilly loaded server". 

The header file tcp.h  has some comments before declaring
"tcp_tw_bucket" structure. It is done in tcp_input.c.

But anyway, tcp_tw_bucket is never identical to sock structure. This
is a serious hack and I'm suprised no linux kernel documentation talks
about it.

I just wanted to let others know so that no one else faces this
problem.

-- 
Sourav






















> 
> im a kernel newbie here so pardon "the blind leading the blind" ...
> 
> doing a quick search for all calls to sk_alloc in the entire kernel
> sources
> yields only one call that sets the "zero out the allocated struct"
> boolean
> to false and that is:
> 	net/ipv4/tcp_minisocks.c:tcp_create_openreq_child().
> this funtion in turn is only ever called from:
> 	net/ipv[46]/tcp_ipv[46].c:tcp_v[46]_syn_recv_sock()
> 
> the comment above the ipv4 version is (verbatim):
> 
> 	/* 
> 	 * The three way handshake has completed - we got a valid synack - 
> 	 * now create the new socket. 
> 	 */
> 
> so if you need those experimental values of yours zeroed out on socket
> creation i suggest replacing this snippet from
> net/core/sock.c:sk_alloc()
> 
>         if(sk && zero_it) {
>                 memset(sk, 0, sizeof(struct sock));
>                 sk->family = family;
>                 sock_lock_init(sk);
>         }
> 
> with
> 
>         if(  sk  ) {
> 		/* set your NULL init values here */
> 		if(  zero_it  ) {
> 	                memset(sk, 0, sizeof(struct sock));
> 	                sk->family = family;
> 	                sock_lock_init(sk);
> 		}
>         }
> 
> doh!  i just re-read your mail and realized youre using the 2.2.15
> kernel.
> my examples are from the 2.4.2 sources...
> 
> looking at the 2.2.16 source (i have only 2.2.1[46] and not 2.2.15 for
> the
> 2.2 series) the (roughly) congruent if block of code is:
> 
>         if(sk) {
>                 if (zero_it) 
>                         memset(sk, 0, sizeof(struct sock));
>                 sk->family = family;
>         }
> 
> and so if you're setting your init values to NULL under the "zero_it"
> condition you would get the behavior that you reported.
> 
> hope that helps.