Re: [PATCH RFC v9 4/7] x86/entry: Erase kernel stack in syscall_trace_enter()

From: Linus Torvalds <torvalds@linux-foundation.org>
To: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Daniel Micay <danielmicay@gmail.com>,
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	X86 ML <x86@kernel.org>, LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH RFC v9 4/7] x86/entry: Erase kernel stack in syscall_trace_enter()
Date: Tue, 6 Mar 2018 11:16:51 -0800	[thread overview]
Message-ID: <CA+55aFxkzxhGaK=sp2CqM1YFEE=fL33wC1TR4KKqsRqPGto6yA@mail.gmail.com> (raw)
In-Reply-To: <CAKv+Gu_VaO39h8trgsNhU4JEVkdgoFWGoovBdmAL_YsHe7Nq1Q@mail.gmail.com>

On Tue, Mar 6, 2018 at 11:07 AM, Ard Biesheuvel
<ard.biesheuvel@linaro.org> wrote:
>
> The compiler usually does a pretty good job of detecting which scalar
> variables are never initialized by regular assignment.

Sure, but "usually" is not really the same as always. Sometimes scalar
types are initialized by passing a reference to them too.

> We could easily extend this to scalar and array types, but we'd first
> need to see what the performance impact is, because I don't think it
> will be negligible.

For scalar types, I suspect it will be entirely unnoticeable, because
they are not only small, but it's rare that this kind of "initialize
by passing a reference" happens in the first place.

For arrays, I agree. We very well may have arrays that we really want
to do magic things about. But even then I'd rather have a "don't
initialize this" flag for critical stuff that really *does* get
initialized some other way. Then we can grep for those things and be
more careful.

If somebody has big arrays on the stack, that's often a problem
anyway. It may be common in non-kernel code, but kernel code is very
special.

                    Linus