linux-kernel - Re: Behaviour of compat_msgsnd/compat

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Message-ID: <45521580.5000606@in.ibm.com>
Date:	Wed, 08 Nov 2006 09:36:00 -0800
From:	Suzuki K P <suzuki@...ibm.com>
To:	David Miller <davem@...emloft.net>
CC:	linux-kernel@...r.kernel.org, Andrew Morton <akpm@...l.org>
Subject: Re: Behaviour of compat_msgsnd/compat_msgrcv calls


-- Original Post ---
> Hi,
> 
> I have a question regarding the behaviour of the comapt_msgsnd/ compat_msgrcv ()s. Don't know if this has been discussed already or if as I could not find any threads in the archives. Please bear with me if this is really a stupid question.
> 
>  The maximum length of the message that can be sent or received in any of those functions above is MAXBUF-(sizeof (struct msgbuf)), where MAXBUF is 64k.
> 
> ipc/compat.c : compat_msgrcv()
>         if (second < 0 || (second >= MAXBUF - sizeof(struct msgbuf)))
>                           ^^^^^^
>                 return -EINVAL;
> 
> Is this limit due to the buffer allocation in user space as below ?
> 
>  And the way we are doing this is by allocating a buffer of msgsize on the userspace stack using compat_alloc_user_space() instead of using the buffer provided by the user and later copying the result back to the user buffer.
>> 
>>Is there any specific reason behind this ? Can't we just use the user 
>>buffer directly instead of doing an additional copy_in_user ?
>>ie,
>>	err = sys_msgrcv(first, uptr, second, msgtyp, third);
> 
> 
-- Dave suggested --
> It's the cleanest way to deal with the difference in
> "struct msgbuf" layout between native and compat userspace.
> 
> I guess we could make some common do_sys_msgrcv() function
> that passed in a pointer to the "msgbuf" and the buffer
> seperately.

Dave,

I have attached a patch which does the same here. It introduces
do_msgrcv()/ do_msgsnd() routines to service the sys_msgxxx variants.
The do_msgxxx variants accepts an additional "mtext" parameter along
with the sys_msgxxx paramters.
i.e,

asmlinkage long sys_msgrcv(int msqid, struct msgbuf __user *msgp,
				size_t msgsz, long msgtyp, int msgflg)
{
           return do_msgrcv(msqid, msgp, msgp->mtext, msgsz, msgtyp,
   			   msgflg);
}

Comments ?

Thanks,

-Suzuki







View attachment "fix-compat-ipc-msg-size-limit.diff" of type "text/x-patch" (4819 bytes)