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Message-ID: <201910281153.7B6F79DBD@keescook>
Date: Mon, 28 Oct 2019 11:58:34 -0700
From: Kees Cook <keescook@...omium.org>
To: Joe Perches <joe@...ches.com>
Cc: Julia Lawall <julia.lawall@...6.fr>,
Andrew Morton <akpm@...ux-foundation.org>,
LKML <linux-kernel@...r.kernel.org>,
Dan Carpenter <error27@...il.com>,
Thomas Gleixner <tglx@...utronix.de>,
zhanglin <zhang.lin16@....com.cn>
Subject: Re: [PATCH] kernel: sys.c: Avoid copying possible padding bytes in
copy_to_user
On Sun, Oct 27, 2019 at 03:47:21PM -0700, Joe Perches wrote:
> I think yes as at least it makes it consistent.
>
> From the link above, as I understand the __user
> gcc extension here
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=c61f13eaa1ee17728c41370100d2d45c254ce76f
>
> gcc does not clear padding from initialized structs
> marked with __user.
It seems to depend on how complete the initialization is and/or how
large the structure is. AFAICT, based on the tests I wrote[1], if
CONFIG_GCC_PLUGIN_STRUCTLEAK_BYREF (or ..._ALL) are used, even padding
will get initialized as long as things are in memory. (And the same is
true with Clang under CONFIG_INIT_STACK_ALL.)
> Though that doesn't force the compiler to not
> perform the possible register optimization shown
> in the first document above.
Right. This is the only case where things aren't clear. I haven't been
able to build a test where "store in registers" behavior is tripped.
-Kees
[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/lib/test_stackinit.c
--
Kees Cook
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