linux-hardening - Re: [PATCH 2/3] compiler.h: Introduce __must_be_char

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Message-ID: <20250207131305.0fe9622d@pumpkin>
Date: Fri, 7 Feb 2025 13:13:05 +0000
From: David Laight <david.laight.linux@...il.com>
To: Rasmus Villemoes <ravi@...vas.dk>
Cc: Kees Cook <kees@...nel.org>, Suren Baghdasaryan <surenb@...gle.com>,
 Kent Overstreet <kent.overstreet@...ux.dev>, Andy Shevchenko
 <andy@...nel.org>, Luc Van Oostenryck <luc.vanoostenryck@...il.com>, Nathan
 Chancellor <nathan@...nel.org>, Nick Desaulniers <ndesaulniers@...gle.com>,
 Bill Wendling <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>,
 Philipp Reisner <philipp.reisner@...bit.com>, Miguel Ojeda
 <ojeda@...nel.org>, linux-kernel@...r.kernel.org,
 linux-hardening@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH 2/3] compiler.h: Introduce __must_be_char_array()

On Fri, 07 Feb 2025 09:55:00 +0100
Rasmus Villemoes <ravi@...vas.dk> wrote:

> On Thu, Feb 06 2025, Kees Cook <kees@...nel.org> wrote:
> 
> > In preparation for adding stricter type checking to the str/mem*()
> > helpers, provide a way to check that a variable is a character array
> > via __must_be_char_array().
> >
> > Signed-off-by: Kees Cook <kees@...nel.org>
> > ---
> >  include/linux/compiler.h | 8 +++++++-
> >  1 file changed, 7 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> > index 7af999a131cb..a577fe0b1f8a 100644
> > --- a/include/linux/compiler.h
> > +++ b/include/linux/compiler.h
> > @@ -221,7 +221,13 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
> >  #endif /* __CHECKER__ */
> >  
> >  /* &a[0] degrades to a pointer: a different type from an array */
> > -#define __must_be_array(a)	__BUILD_BUG_ON_ZERO_MSG(__same_type((a), &(a)[0]), "must be array")
> > +#define __is_array(a)		(!__same_type((a), &(a)[0]))
> > +#define __must_be_array(a)	__BUILD_BUG_ON_ZERO_MSG(!__is_array(a), \
> > +							"must be array")
> > +
> > +#define __is_char_array(a)	(__is_array(a) && sizeof((a)[0]) == 1)
> > +#define __must_be_char_array(a)	__BUILD_BUG_ON_ZERO_MSG(!__is_char_array(a), \
> > +							"must be byte array")
> >  
> 
> It's probably unlikely to ever encounter an array of _Bool or array of
> structs-with-a-single-char-member in the wild, but it does seem a bit
> odd to base the test on sizeof(). Why not add a
> 
>   __is_character_type(t) (__same_type(t, char) ||
>                           __same_type(t, signed char) ||
>                           __same_type(t, unsigned char) )
> 
> helper and write the test using __is_character_type((a)[0])?
> 
> Or if you really mean that it must be an array of char, not any of the
> three "character types", simply replace the sizeof() by
> __same_type(a[0], char)

I'm not sure it matters enough to slow down the compilation by that much
cpp bloat.

	David