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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