linux-kernel - Re: [PATCH v3 01/12] crypto: LLVMLinux: Add macro to remove use of VLAIS in crypto code

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Date:	Wed, 17 Sep 2014 08:26:19 -0700
From:	Behan Webster <behanw@...verseincode.com>
To:	Herbert Xu <herbert@...dor.apana.org.au>,
	Dmitry Kasatkin <d.kasatkin@...sung.com>
CC:	agk@...hat.com, clm@...com, davem@...emloft.net,
	dm-devel@...hat.com, fabf@...net.be, jbacik@...com,
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	linux-btrfs@...r.kernel.org, linux-crypto@...r.kernel.org,
	linux-ima-devel@...ts.sourceforge.net,
	linux-ima-user@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
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	thomas.lendacky@....com, zohar@...ux.vnet.ibm.com,
	torvalds@...ux-foundation.org
Subject: Re: [PATCH v3 01/12] crypto: LLVMLinux: Add macro to remove use of
 VLAIS in crypto code

On 09/17/14 04:30, Herbert Xu wrote:
> On Wed, Sep 17, 2014 at 02:15:40PM +0300, Dmitry Kasatkin wrote:
>> On 17/09/14 12:22, Herbert Xu wrote:
>>> On Mon, Sep 15, 2014 at 12:30:23AM -0700, behanw@...verseincode.com wrote:
>>>> From: Behan Webster <behanw@...verseincode.com>
>>>>
>>>> Add a macro which replaces the use of a Variable Length Array In Struct (VLAIS)
>>>> with a C99 compliant equivalent. This macro instead allocates the appropriate
>>>> amount of memory using an char array.
>>>>
>>>> The new code can be compiled with both gcc and clang.
>>>>
>>>> struct shash_desc contains a flexible array member member ctx declared with
>>>> CRYPTO_MINALIGN_ATTR, so sizeof(struct shash_desc) aligns the beginning
>>>> of the array declared after struct shash_desc with long long.
>>>>
>>>> No trailing padding is required because it is not a struct type that can
>>>> be used in an array.
>>>>
>>>> The CRYPTO_MINALIGN_ATTR is required so that desc is aligned with long long
>>>> as would be the case for a struct containing a member with
>>>> CRYPTO_MINALIGN_ATTR.
>>>>
>>>> Signed-off-by: Behan Webster <behanw@...verseincode.com>
>>> Acked-by: Herbert Xu <herbert@...dor.apana.org.au>
>>>
>>> Thanks,
>> Just in case.
>> I would still follow advice from "Michał Mirosław" to use shash##__desc[]
Absolutely. I will be posting a v4 patchset . Just waiting a bit more 
for more comments on v3.

The macro from v4 will look like this which I believe will satisfy the 
concern and indeed be safer than my previous version.

+#define SHASH_DESC_ON_STACK(shash, tfm)                                  \
+       char __##shash##_desc[sizeof(struct shash_desc) +        \
+               crypto_shash_descsize(tfm)] CRYPTO_MINALIGN_ATTR; \
+       struct shash_desc *shash = (struct shash_desc *)__##shash##_desc

Hmm. Is it worth adding a comment with this macro explaining the reason this works? Essentially much of what is in the commit message?
  

> Oh yes of course.  My ack is more about the approach.

Wonderful!

Indeed. I would have asked for you to wait for v4 anyways. :)

Thank you,

Behan

-- 
Behan Webster
behanw@...verseincode.com

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