linux-kernel - Re: Behaviour of compat_msgsnd/compat

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Date:	Wed, 08 Nov 2006 10:27:06 -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

Corrected patch for some coding style errs..

Comments ?

Thanks

Suzuki

Suzuki K P wrote:
> Suzuki K P wrote:
> 
>>
>> -- 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-msg-size.diff" of type "text/x-patch" (4829 bytes)