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Date:   Tue, 10 May 2022 20:05:58 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Mark Rutland <mark.rutland@arm.com>
Cc:     Alexander Popov <alex.popov@linux.com>,
        linux-arm-kernel@lists.infradead.org, akpm@linux-foundation.org,
        catalin.marinas@arm.com, linux-kernel@vger.kernel.org,
        luto@kernel.org, will@kernel.org
Subject: Re: [PATCH v2 05/13] stackleak: clarify variable names
Message-ID: <202205102001.7EB50CBE18@keescook>
References: <20220427173128.2603085-1-mark.rutland@arm.com>
 <20220427173128.2603085-6-mark.rutland@arm.com>
 <e1cf0177-40a0-ffca-6be4-57fd97860c4a@linux.com>
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On Tue, May 10, 2022 at 02:01:49PM +0100, Mark Rutland wrote:
> On Sun, May 08, 2022 at 11:49:46PM +0300, Alexander Popov wrote:
> > On 27.04.2022 20:31, Mark Rutland wrote:
> > > The logic within __stackleak_erase() can be a little hard to follow, as
> > > `boundary` switches from being the low bound to the high bound mid way
> > > through the function, and `kstack_ptr` is used to represent the start of
> > > the region to erase while `boundary` represents the end of the region to
> > > erase.
> > > 
> > > Make this a little clearer by consistently using clearer variable names.
> > > The `boundary` variable is removed, the bounds of the region to erase
> > > are described by `erase_low` and `erase_high`, and bounds of the task
> > > stack are described by `task_stack_low` and `task_stck_high`.
> > 
> > A typo here in `task_stck_high`.
> 
> Ah; whoops.

No worries; I fixed this when I took the patch.

> > That was also the main reason why I reused the 'boundary' variable: I wanted
> > the compiler to allocate it in the register and I avoided creating many
> > local variables.
> >
> > Mark, did your refactoring make the compiler allocate local variables on the
> > stack instead of the registers?
> 
> Considering the whole series, testing with GCC 11.1.0:
> 
> * On arm64:
>      before: stackleak_erase() uses 48 bytes of stack
>      after: stackleak_erase() uses 0 bytes of stack
> 
>      Note: this is entirely due to patch 1; arm64 has enough GPRs that it
>      doesn't need to use the stack.
> 
> * On x86_64:
>      before: stackleak_erase() uses 0 bytes of stack
>      after:  stackleak_erase() uses 0 bytes of stack
> 
> * On i386
>      before: stackleak_erase() uses 8 bytes of stach
>      after:  stackleak_erase() uses 16 bytes of stack
> 
> The i386 case isn't ideal, but given that those bytes will easily be used by
> the entry triage code before getting to any syscall handling, I don't believe
> that's an issue in practice.

I am biased and totally fine with choosing a solution where 64-bit
improvement comes at a 32-bit cost.

-- 
Kees Cook