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Date:   Mon, 7 Oct 2019 16:00:22 +0200
From:   Ingo Molnar <mingo@kernel.org>
To:     Hans de Goede <hdegoede@redhat.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        linux-crypto@vger.kernel.org, x86@kernel.org,
        linux-kernel@vger.kernel.org, Arvind Sankar <nivedita@alum.mit.edu>
Subject: Re: [PATCH v2 5.4 regression fix] x86/boot: Provide memzero_explicit
Message-ID: <20191007140022.GA29008@gmail.com>
References: <20191007134724.4019-1-hdegoede@redhat.com>
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* Hans de Goede <hdegoede@redhat.com> wrote:

> The purgatory code now uses the shared lib/crypto/sha256.c sha256
> implementation. This needs memzero_explicit, implement this.
> 
> Reported-by: Arvind Sankar <nivedita@alum.mit.edu>
> Fixes: 906a4bb97f5d ("crypto: sha256 - Use get/put_unaligned_be32 to get input, memzero_explicit")
> Signed-off-by: Hans de Goede <hdegoede@redhat.com>
> ---
> Changes in v2:
> - Add barrier_data() call after the memset, making the function really
>   explicit. Using barrier_data() works fine in the purgatory (build)
>   environment.
> ---
>  arch/x86/boot/compressed/string.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/arch/x86/boot/compressed/string.c b/arch/x86/boot/compressed/string.c
> index 81fc1eaa3229..654a7164a702 100644
> --- a/arch/x86/boot/compressed/string.c
> +++ b/arch/x86/boot/compressed/string.c
> @@ -50,6 +50,12 @@ void *memset(void *s, int c, size_t n)
>  	return s;
>  }
>  
> +void memzero_explicit(void *s, size_t count)
> +{
> +	memset(s, 0, count);
> +	barrier_data(s);
> +}

So the barrier_data() is only there to keep LTO from optimizing out the 
seemingly unused function?

Is there no canonical way to do that, instead of a seemingly obscure 
barrier_data() call - which would require a comment at minimum.

We do have $(DISABLE_LTO), not sure whether it's applicable here though.

Thanks,

	Ingo