Behaviour of compat_msgsnd/compat_msgrcv calls

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.

if (!version) {
[...]
if (copy_from_user (&ipck, uptr, sizeof(ipck)))
goto out;
uptr = compat_ptr(ipck.msgp);
msgtyp = ipck.msgtyp;

}

p = compat_alloc_user_space(second + sizeof(struct msgbuf));

Do we really need this allocation ?

err = sys_msgrcv(first, p, second, msgtyp, third);

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);

Thanks,

-Suzuki
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