linux-kernel - Re: [GIT PULL] meminit fix for v5.3-rc2

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Date:   Sun, 28 Jul 2019 15:16:28 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Alexander Potapenko <glider@...gle.com>
Subject: Re: [GIT PULL] meminit fix for v5.3-rc2

On Sun, Jul 28, 2019 at 12:43:15PM -0700, Linus Torvalds wrote:
> On Sun, Jul 28, 2019 at 12:21 PM Kees Cook <keescook@...omium.org> wrote:
> >
> > Please pull this meminit fix for v5.3-rc2.
> 
> Side noe: I find "meminit" a confusing description for the structleak
> thing. When I hear it, it sounds like some generic memory
> initialization thing in the VM layer (which we obviously do also
> have), not the stack variable initialization.

I will find a better name. :) We dreamed up "meminit" as finding a name
for the umbrella of both stack and heap auto-initialization. But I
agree, it's confusing.

> Also, have you guys talked to gcc people about just making it a real
> feature, like I think it is for clang? In particular, I still suspect
> that we could/should  just make zero-filling the *default* in the long
> run, and say "our C standard is that local variables are initialized
> to zero, exactly the same way static variables are".

Yes, this is on the list for discussion at Plumber's. Having gcc do
auto-init is the first part. Convincing Clang that _zero_ init isn't
a language-breaking change is the second part. :P That's been a whole
other issue.

> I know you posted some numbers somewhere (well, I'm pretty sure you
> did) and the full stack initialization really was pretty cheap,
> wasn't it?

Yes, Clang's initialization (which is 0xAA not 0x00 in most cases) is
cheap. There are rumors(?) of some pathological workloads, though. I
haven't seen real numbers for that though.

I'll try to find the Clang numbers (maybe Alexander has them?) but I
remember it being the same as (or maybe better than) the gcc-plugin
version, which I measured here:

https://git.kernel.org/linus/81a56f6dcd20

-- 
Kees Cook