linux-kernel - Re: Dangerous code in cpumask_of

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Date:	Tue, 08 Jul 2008 11:28:32 +0200
From:	Johannes Weiner <hannes@...urebad.de>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Mike Travis <travis@....com>, linux-kernel@...r.kernel.org,
	"H. Anvin" <hpa@...or.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: Dangerous code in cpumask_of_cpu?


[ fixed christoph's address in cc]

Johannes Weiner <hannes@...urebad.de> writes:

>> I guess because we dereference it immediately again, the location is not
>> clobbered yet.  At least in my test case, gcc assembled it to code that
>> puts the address in eax and derefences it immediately, before eax is
>> reused:
>
> Gee, just ignore this bs.  The address is in eax, not the value.

My theory was half-right.  Since the code is a macro, there is no call
and hence no stack clean-up.  And although it is UB, it works correctly
as the value is not yet clobbered when we access it again.  Converting
foo to a macro yields this:

        movl    $42, -8(%ebp)
        leal    -8(%ebp), %eax
        movl    (%eax), %eax
	...
	ret

gcc only emits a warning if the scope we leak a local address from is
that of a function.

	Hannes
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