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Date: Sat, 11 Nov 2017 20:21:05 -0800
From: Andy Lutomirski <>
To: "Hector Martin 'marcan'" <>
Cc: Andy Lutomirski <>, LKML <>, 
	"" <>, X86 ML <>
Subject: Re: Re: vDSO maximum stack usage, stack probes,
 and -fstack-check

On Fri, Nov 10, 2017 at 9:16 PM, Hector Martin 'marcan'
<> wrote:
> On 2017-11-11 07:04, Andy Lutomirski wrote:
>>> On Nov 10, 2017, at 8:36 AM, Hector Martin 'marcan' <> wrote:
>>>> On 2017-11-11 01:02, Hector Martin 'marcan' wrote:
>>>> Not entirely sure what's going on here.
>>> Actually, if you think about it, it doesn't matter that it skips the
>>> first page, since it's probing one page more. That just means the caller
>>> will have probed the previous page. So ultimately you're just probing
>>> ahead of where you need to, but that should be OK.
>> The whole point is to touch the stack pages in order.  Also, I see no
>> guarantee that the function would touch the intermediate page before
>> clobbering the probed page.  You're seeing exactly that behavior, in
>> fact.
> Only because Go is not C and is not compiled like this. If all the code
> is GCC-compiled C code and built with -fstack-check, it should always
> probe stack pages in order except for potentially the second page in the
> stack, which may be touched after the third page (but hopefully your
> stack is at least two pages long to begin with).

If you're generating code to improve stack overflow, assuming that you
have at least two pages left seems like an *awful* assumption to make.

> AIUI -fstack-check was not intended for stack clash protection (the
> latter isn't even in a GCC release yet), but in most circumstances it
> seems to me like it's an effective mitigation if all code is compiled
> with it. Qualys mentioned it as such in their advisory. This is probably
> why Gentoo Hardened enables it by default globally in their toolchain.

Gentoo Hardened should seriously consider turning it back off.  Do you
happen to know what exactly Gentoo does to cause the vdso to get build
with -fstack-check?  I'll write a patch to either fail the build or to
force it off.

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