linux-kernel - Re: [PATCH 1/5] linux/kernel.h: Add sizeof

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Date:   Wed, 25 Sep 2019 09:59:18 +0530
From:   "Bharadiya,Pankaj" <pankaj.laxminarayan.bharadiya@...el.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     pankaj.bharadiya@...il.com, andriy.shevchenko@...ux.intel.com,
        kernel-hardening@...ts.openwall.com, akpm@...ux-foundation.org,
        mayhs11saini@...il.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/5] linux/kernel.h: Add sizeof_member macro

On Tue, Sep 24, 2019 at 09:22:10AM -0700, Kees Cook wrote:
> On Tue, Sep 24, 2019 at 04:28:35PM +0530, Pankaj Bharadiya wrote:
> > At present we have 3 different macros to calculate the size of a
> > member of a struct:
> >   - SIZEOF_FIELD
> >   - FIELD_SIZEOF
> >   - sizeof_field
> > 
> > To bring uniformity in entire kernel source tree let's add
> > sizeof_member macro.
> > 
> > Replace all occurrences of above 3 macro's with sizeof_member in
> > future patches.
> > 
> > Signed-off-by: Pankaj Bharadiya <pankaj.laxminarayan.bharadiya@...el.com>
> > ---
> >  include/linux/kernel.h | 9 +++++++++
> >  1 file changed, 9 insertions(+)
> 
> Since stddef.h ends up needing this macro, and kernel.h includes
> stddef.h, why not put this macro in stddef.h instead? Then the
> open-coded version of it in stddef (your last patch) can use
> sizeof_member()?
> 

If I understood correctly, Andrew suggested to add such macros in kernel.h
https://www.openwall.com/lists/kernel-hardening/2019/06/11/5

Moreover similar type of other macros (like typeof_member & ARRAY_SIZE)
are defined in kernel.h
But as you pointed out, looks like stddef.h is the right place for this macro.

> Otherwise, yes, looks good. (Though I might re-order the patches so the
> last patch is the tree-wide swap -- then you don't need the exclusions,
> I think?)
>

I went through your tree. 
https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/log/?h=kspp/sizeof_member/full
Thank you for reordering the patches.

Thanks,
Pankaj

> -Kees
> 
> > 
> > diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> > index 4fa360a13c1e..0b80d8bb3978 100644
> > --- a/include/linux/kernel.h
> > +++ b/include/linux/kernel.h
> > @@ -79,6 +79,15 @@
> >   */
> >  #define round_down(x, y) ((x) & ~__round_mask(x, y))
> >  
> > +/**
> > + * sizeof_member - get the size of a struct's member
> > + * @T: the target struct
> > + * @m: the target struct's member
> > + * Return: the size of @m in the struct definition without having a
> > + * declared instance of @T.
> > + */
> > +#define sizeof_member(T, m) (sizeof(((T *)0)->m))
> > +
> >  /**
> >   * FIELD_SIZEOF - get the size of a struct's field
> >   * @t: the target struct
> > -- 
> > 2.17.1
> > 
> 
> -- 
> Kees Cook