linux-kernel - Re: [PATCH] lib/string: make memzero_explicit inline instead of external

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Date:   Thu, 10 Oct 2019 10:52:29 +0800
From:   Dave Young <dyoung@...hat.com>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Hans de Goede <hdegoede@...hat.com>,
        linux-crypto@...r.kernel.org, linux-s390@...r.kernel.org,
        x86@...nel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Stephan Mueller <smueller@...onox.de>,
        kexec@...ts.infradead.org
Subject: Re: [PATCH] lib/string: make memzero_explicit inline instead of
 external

On 10/07/19 at 06:00pm, Arvind Sankar wrote:
> With the use of the barrier implied by barrier_data(), there is no need
> for memzero_explicit to be extern. Making it inline saves the overhead
> of a function call, and allows the code to be reused in arch/*/purgatory
> without having to duplicate the implementation.
> 
> Fixes: 906a4bb97f5d ("crypto: sha256 - Use get/put_unaligned_be32 to get input, memzero_explicit")
> Reviewed-by: Hans de Goede <hdegoede@...hat.com>
> Tested-by: Hans de Goede <hdegoede@...hat.com>
> Signed-off-by: Arvind Sankar <nivedita@...m.mit.edu>
> ---
>  include/linux/string.h | 21 ++++++++++++++++++++-
>  lib/string.c           | 21 ---------------------
>  2 files changed, 20 insertions(+), 22 deletions(-)
> 
> diff --git a/include/linux/string.h b/include/linux/string.h
> index b2f9df7f0761..b6ccdc2c7f02 100644
> --- a/include/linux/string.h
> +++ b/include/linux/string.h
> @@ -227,7 +227,26 @@ static inline bool strstarts(const char *str, const char *prefix)
>  }
>  
>  size_t memweight(const void *ptr, size_t bytes);
> -void memzero_explicit(void *s, size_t count);
> +
> +/**
> + * memzero_explicit - Fill a region of memory (e.g. sensitive
> + *		      keying data) with 0s.
> + * @s: Pointer to the start of the area.
> + * @count: The size of the area.
> + *
> + * Note: usually using memset() is just fine (!), but in cases
> + * where clearing out _local_ data at the end of a scope is
> + * necessary, memzero_explicit() should be used instead in
> + * order to prevent the compiler from optimising away zeroing.
> + *
> + * memzero_explicit() doesn't need an arch-specific version as
> + * it just invokes the one of memset() implicitly.
> + */
> +static inline void memzero_explicit(void *s, size_t count)
> +{
> +	memset(s, 0, count);
> +	barrier_data(s);
> +}
>  
>  /**
>   * kbasename - return the last part of a pathname.
> diff --git a/lib/string.c b/lib/string.c
> index cd7a10c19210..08ec58cc673b 100644
> --- a/lib/string.c
> +++ b/lib/string.c
> @@ -748,27 +748,6 @@ void *memset(void *s, int c, size_t count)
>  EXPORT_SYMBOL(memset);
>  #endif
>  
> -/**
> - * memzero_explicit - Fill a region of memory (e.g. sensitive
> - *		      keying data) with 0s.
> - * @s: Pointer to the start of the area.
> - * @count: The size of the area.
> - *
> - * Note: usually using memset() is just fine (!), but in cases
> - * where clearing out _local_ data at the end of a scope is
> - * necessary, memzero_explicit() should be used instead in
> - * order to prevent the compiler from optimising away zeroing.
> - *
> - * memzero_explicit() doesn't need an arch-specific version as
> - * it just invokes the one of memset() implicitly.
> - */
> -void memzero_explicit(void *s, size_t count)
> -{
> -	memset(s, 0, count);
> -	barrier_data(s);
> -}
> -EXPORT_SYMBOL(memzero_explicit);
> -
>  #ifndef __HAVE_ARCH_MEMSET16
>  /**
>   * memset16() - Fill a memory area with a uint16_t
> -- 

Thanks for the fix!  Ccing kexec list since the problem is kexec/kdump
related.  People can try it when they see same issue.

Dave