[okl4-developer] Shared memory howto

[Kalamkar, Dhiraj D]

Hi Remy,

One thing I couldn't understand is how client gets access rights for server's shared memsection? If just passing capability object is sufficient, client can forward it to anyone else say X and then X can access server's shared memsection, how to prevent that? Also, how server can specify what type of access is granted e.g. readonly or readwrite?

Thanks and regards,
Dhiraj Kalamkar

-----Original Message-----
From: developer-bounces at okl4.org [mailto:developer-bounces at okl4.org] On Behalf Of Remy Gottschalk
Sent: Friday, April 25, 2008 7:57 PM
To: developer
Subject: [okl4-developer] Shared memory howto

Here come the new howto. This one concern shared memory. By extending
the HelloWorld server previously presented it show a exemple of using a
piece of shared memory.
Great work was done by Nelson in explaining how to setup such mechanism
in this thread :
http://lists.okl4.org/pipermail/developer/2008-February/000676.html.
What was said there is pretty self-explanatory. But I add anyway this
howto to it since it may be useful to see a practical implementation.
As usual comments, corrections and upgrades are welcome.

-------------
Shared Memory
-------------

We will modify the HelloWorld server to make it use a shared memory.
This shared memory will be used by its clients to make it say specific
things.

In this example a single piece of shared memory is allocated by the
server and access is given to the client.

	1. The IDL4 interface

Firstly we will add two function to the IDL4 file :
      * get_shm to pass the shared memory's information to the client
        side
      * say_this_shm to order the server to "say" whatever is in the
        shared buffer

> cat libs/hello/include/interfaces/hello.idl4
import "iguana/types.h";

[uuid(45)]
interface hello_
{
void say_this_shm(void);
void get_shm(out cap_t cap);
};

	2. The server

We will add in the server part include to the necessary headers.
We declare the needed variables.
We modify the main function so it initialize the shared memory.
Lastly we implement the functions get_shm and say this_shm previously
declared in the IDL4 file.

> cat iguana/hello/src/hello_server.c 

#include <assert.h>
#include <stdio.h>
#include <string.h>
#include <interfaces/hello_serverdecls.h>
#include <iguana/memsection.h>
#include <iguana/cap.h>

static cap_t cap;
static uintptr_t base;

int main(int argc, char **argv)
{
memsection_ref_t ms;
ms = memsection_create(0x1000, &base);
cap.ref.obj = ms;
cap.passwd = 0;
[...]
}

void hello__get_shm_impl(CORBA_Object _caller, cap_t *out_cap,
idl4_server_environment *enc)
{
memcpy(out_cap, &cap, sizeof(cap));
} 

void hello__say_this_shm_impl(CORBA_Object _caller,
idl4_server_environment *env)
{
char *tmp_str = (char *)base;
printf("I can't resist to say (shared one): \"%s\"\n", tmp_str);
}

	3. The client side

Then we add in the client library the support of the shared memory to
mask it use from the end user. 
To achieve that we create the wrapper function say_this_shm.

> cat libs/hello/include/hello.h

#ifndef __HELLO_H__
#define __HELLO_H__
[...]
void say_this_shm(char *);

#endif

We implement the new wrapper function and we modify the hello_init
function to make it use get_shm and get the shared memory.

> cat libs/hello/src/hello.c 

#include <string.h>
#include <iguana/env.h>
#include <iguana/memsection.h>
#include <iguana/cap.h>
#include <iguana/thread.h>
#include <l4/thread.h>
#include <interfaces/hello_client.h>
#include <hello/hello.h>
#include <stdio.h>

static L4_ThreadId_t server_tid;
memsection_ref_t ms;
void *ms_base;
uintptr_t ms_size;

void hello_init(void)
{
thread_ref_t server_;
cap_t cap;


memsection_lookup((objref_t)
env_memsection_base(iguana_getenv("OKL4_HELLO_SERVER")),
&server_);

server_tid = thread_l4tid(server_);
hello__get_shm(server_tid, &cap, NULL);
clist_insert(default_clist, cap);
ms = (memsection_ref_t)cap.ref.obj;
ms_base = memsection_base(ms);
ms_size = memsection_size(ms);
}

void say_this_shm(char *in_str)
{
char *tmp;

tmp = (char *)ms_base;
strncpy(ms_base, in_str, ms_size);
hello__say_this_shm(server_tid, NULL);
}

	4. Using the server

Using the server doesn't change much. We now got the say_this_shm
function which make the server say the argument string. This string is
passed to the server using a piece of shared memory. Of course we still
need to call hello_init first to setup things.


-- 
Remy Gottschalk - rgottschalk at linagora.com
Ingénieur informatique embarquée
Groupe LINAGORA - http://www.linagora.com
Tél.: +33(0)1 58 18 68 28 - Fax : +33(0)1 58 18 68 29


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