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Subject: Re: ftrace introduces instability into kernel 2.6.27(-rc2,-rc3)
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
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> Hmm, this was originally copied from x86, where we did a cmpxchg, but that 
> is probably not needed since all of this is done in kstop_machine. Also, 
> only the "get" is needed. If we don't fault there, we wont fault on the 
> put (unless we have permissions wrong, and that would be a bug).

Would it ? How do we make sure the kernel text is mapped writeable ?

> So are you recommending something like
> 
>         int cmd;
> 
> 	if (__get_user(cmd, ip))
> 		goto fault;
> 
> 	if (cmd != old)
> 		goto not_same;
> 
> 	WARN_ON_ONCE(__put_user(cmd, ip));
> 
> If we did this, we could probably put this into the generic code:

That would work I suppose, I'll give it a try.

> 	if (copy_from_user(cmd, ip, ARCH_CALL_SIZE))
> 		goto fault;
> 
> 	if (memcmp(cmd, old, ARCH_CALL_SIZE) != 0)
> 		goto not_same;
> 
> 	WARN_ON_ONCE(copy_to_user(cmd, ip, ARCH_CALL_SIZE));

You need the __ variants or the access_ok() checks will bite
you bad.

Ben.