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

[Behan Webster]

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@xxxxxxxxxxxxxxxxxx wrote:
> > > > From: Behan Webster <behanw@xxxxxxxxxxxxxxxxxx>
> > > >
> > > > 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@xxxxxxxxxxxxxxxxxx>
> > > Acked-by: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
> > >
> > > Thanks,
> > Just in case.
> > I would still follow advice from "MichaÅ MirosÅaw" 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@xxxxxxxxxxxxxxxxxx

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