linux-kernel - Re: C tricks for efficient stack zeroing

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Date:   Tue, 6 Mar 2018 20:17:08 -0600
From:   Julia Cartwright <juliac@....teric.us>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, pageexec@...email.hu
Subject: Re: C tricks for efficient stack zeroing

On Fri, Mar 02, 2018 at 08:50:17PM +0100, Jason A. Donenfeld wrote:
[..]
> What would be really nice would be to somehow keep track of the
> maximum stack depth, and just before the function returns, clear from
> the maximum depth to its stack base, all in one single call. This
> would not only make the code faster and less brittle, but it would
> also clean up some algorithms quite a bit.
> 
> Ideally this would take the form of a gcc attribute on the function,
> but I was unable to find anything of that nature. I started looking
> for little C tricks for this, and came up dry too. I realize I could
> probably just take the current stack address and zero out until _the
> very end_ but that seems to overshoot and would probably be bad for
> performance. The best I've been able to do come up with are some
> x86-specific macros, but that approach seems a bit underwhelming.
> Other approaches include adding a new attribute via the gcc plugin
> system, which could make this kind of thing more complete [cc'ing
> pipacs in case he's thought about that before].

Can objtool support a static stack usage analysis?

I'm wondering if it's possible to place these sensitive functions in a
special linker section, like .text.stackzero.<tag>; objtool could
collect static call data (as it already does) and stack usage, spitting
out a symbol definition stackzero_<tag>_max_depth, which you could then
use to bound your zeroing.

Obviously this is a static analysis, with the limitations therein.

   Julia