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Date: Wed, 9 Sep 2015 14:48:22 -0400
From: Zack Weinberg <>
To: Rich Felker <>
Cc:, GNU C Library <>,
Subject: Re: Compiler support for erasure of sensitive data

On 09/09/2015 01:13 PM, Rich Felker wrote:
> On Wed, Sep 09, 2015 at 12:47:10PM -0400, Zack Weinberg wrote:
>> On Wed, Sep 9, 2015 at 12:42 PM, Rich Felker <> wrote:
>>> You're making this harder than it needs to be. The "m" constraint is
>>> the wrong thing to use here. Simply use:
>>>         __asm__(""::"r"(ptr):"memory");
>> Please review my earlier conversation with Adhemerval on exactly this point.
> My understanding is that you consider this a "big hammer". Does that
> really matter if the intent is that it only be used in isolated,
> sensitive contexts? Are you just unhappy with the performance cost, or
> concerned that the clobber will cause more spilling of sensitive data?

Please review *all* of my earlier conversation with Adhemerval, in
particular the bit where I compiled libressl three different ways and
analyzed the assembly dumps.  I'm sure there's more to be said on the
topic, but *starting* from there.

> the hack with the "m" constraint is wrong and easily fixed

It's not wrong; it is in fact the documented way to express a fixed-size
read access to one block of memory.  Look for "ten bytes of a string"
(sorry, there don't appear to be anchors).

It merely doesn't work in C++, with Clang, or (maybe) with a block of
memory whose size cannot be determined at compile time.


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