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Date: Sat, 2 Mar 2019 10:04:56 +0100
From: Ard Biesheuvel <ard.biesheuvel@...aro.org>
To: Arnd Bergmann <arnd@...db.de>
Cc: Kees Cook <keescook@...omium.org>, 
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Emese Revfy <re.emese@...il.com>, 
	Alexander Popov <alex.popov@...ux.com>, Laura Abbott <labbott@...hat.com>, Jann Horn <jannh@...gle.com>, 
	Alexander Potapenko <glider@...gle.com>, Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH 1/2] gcc-plugins: structleak: Generalize to all variable types

On Thu, 28 Feb 2019 at 21:27, Arnd Bergmann <arnd@...db.de> wrote:
>
> On Tue, Feb 12, 2019 at 7:08 PM Kees Cook <keescook@...omium.org> wrote:
> >
> > This adjusts structleak to also work with non-struct types when they
> > are passed by reference, since those variables may leak just like
> > anything else. This is exposed via an improved set of Kconfig options.
> > (This does mean structleak is slightly misnamed now.)
> >
> > Building with CONFIG_GCC_PLUGIN_STRUCTLEAK_BYREF_ALL should give the
> > kernel complete initialization coverage of all stack variables passed
> > by reference, including padding (see lib/test_stackinit.c).
> >
> > Using CONFIG_GCC_PLUGIN_STRUCTLEAK_VERBOSE to count added initializations
> > under defconfig:
> >
> >         ..._BYREF:      5945 added initializations
> >         ..._BYREF_ALL: 16606 added initializations
> >
> > There is virtually no change to text+data size (both have less than 0.05%
> > growth):
>
> I just resumed my randconfig build testing after a longer break, and found
> a regression for stack usage that I bisected to your change. It shows up in a
> variety of files depending on the configuration, so far the worst one is the
> configuration at https://pastebin.com/UK54qbKa that leads to
>
> ../drivers/media/dvb-frontends/stv090x.c: In function 'stv090x_start_search':
> ../drivers/media/dvb-frontends/stv090x.c:1595:1: error: the frame size
> of 5320 bytes is larger than 2048 bytes [-Werror=frame-larger-than=]
> ../drivers/media/dvb-frontends/stv090x.c: In function 'stv090x_optimize_track':
> ../drivers/media/dvb-frontends/stv090x.c:3090:1: error: the frame size
> of 5872 bytes is larger than 2048 bytes [-Werror=frame-larger-than=]
> ../drivers/media/dvb-frontends/stv090x.c: In function 'stv090x_algo':
> ../drivers/media/dvb-frontends/stv090x.c:3431:1: error: the frame size
> of 5144 bytes is larger than 2048 bytes [-Werror=frame-larger-than=]
>  }
>
> At least for this specific file, I also see a significant (though not alarming)
> increase in code size:
>
>    text       data        bss        dec        hex    filename
> 179196       4632        256     184084      2cf14
> obj-x86/drivers/media/dvb-frontends/stv090x-before.o
>  216740       4632        256     221628      361bc
> obj-x86/drivers/media/dvb-frontends/stv090x-after.o
>
> Part of the problem here is definitely interaction with the asan-stack
> sanitizer. Changing asan-stack=1 to asan-stack=0, it looks a lot
> better:
>
> ../drivers/media/dvb-frontends/stv090x.c: In function 'stv090x_start_search':
> ../drivers/media/dvb-frontends/stv090x.c:1595:1: warning: the frame
> size of 120 bytes is larger than 20 bytes [-Wframe-larger-than=]
> ../drivers/media/dvb-frontends/stv090x.c: In function 'stv090x_optimize_track':
> ../drivers/media/dvb-frontends/stv090x.c:3090:1: warning: the frame
> size of 168 bytes is larger than 20 bytes [-Wframe-larger-than=]
> ../drivers/media/dvb-frontends/stv090x.c: In function 'stv090x_algo':
> ../drivers/media/dvb-frontends/stv090x.c:3431:1: warning: the frame
> size of 192 bytes is larger than 20 bytes [-Wframe-larger-than=]
>
>    text       data        bss        dec        hex    filename
> 184061       4632        256     188949      2e215
> obj-x86/drivers/media/dvb-frontends/stv090x.o
>
> I get similar results with asan-stack=1 but without your plugin, only
> the combination of the two has the explosive stack size growth.
>
> I can help analyze this further, but maybe you can have a look first,
> there might be something obvious when you read the input to the
> plugin.
>

Is this before or after use-after-scope was disabled entirely?

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