Re: [PATCH] string: Check for "nonstring" attribute on strscpy() arguments

[Kees Cook <kees@kernel.org>]

On Tue, Aug 06, 2024 at 12:29:30PM +0200, Przemek Kitszel wrote:
> On 8/5/24 23:43, Kees Cook wrote:
> > GCC already checks for arguments that are marked with the "nonstring"[1]
> > attribute when used on standard C String API functions (e.g. strcpy). Gain
> > this compile-time checking also for the kernel's primary string copying
> > function, strscpy().
> > 
> > Note that Clang has neither "nonstring" nor __builtin_has_attribute().
> > 
> > Link: https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html#index-nonstring-variable-attribute [1]
> > Signed-off-by: Kees Cook <kees@kernel.org>
> 
> 
> 
> > diff --git a/include/linux/string.h b/include/linux/string.h
> > index 9edace076ddb..95b3fc308f4f 100644
> > --- a/include/linux/string.h
> > +++ b/include/linux/string.h
> > @@ -76,12 +76,16 @@ ssize_t sized_strscpy(char *, const char *, size_t);
> >    * known size.
> >    */
> >   #define __strscpy0(dst, src, ...)	\
> > -	sized_strscpy(dst, src, sizeof(dst) + __must_be_array(dst))
> > -#define __strscpy1(dst, src, size)	sized_strscpy(dst, src, size)
> > +	sized_strscpy(dst, src, sizeof(dst) + __must_be_array(dst) +	\
> > +				__must_be_cstr(dst) + __must_be_cstr(src))
> > +#define __strscpy1(dst, src, size)	\
> > +	sized_strscpy(dst, src, size + __must_be_cstr(dst) + __must_be_cstr(src))
> >   #define __strscpy_pad0(dst, src, ...)	\
> > -	sized_strscpy_pad(dst, src, sizeof(dst) + __must_be_array(dst))
> > -#define __strscpy_pad1(dst, src, size)	sized_strscpy_pad(dst, src, size)
> > +	sized_strscpy_pad(dst, src, sizeof(dst) + __must_be_array(dst) +	\
> > +				    __must_be_cstr(dst) + __must_be_cstr(src))
> > +#define __strscpy_pad1(dst, src, size)	\
> > +	sized_strscpy_pad(dst, src, size + __must_be_cstr(dst) + __must_be_cstr(src))
> 
> any way to avoid the usual caveat of repeating macro argument?
> 
> a variant of BUILD_BUG that is checking argument and otherwise pasting
> it would nail it, but I didn't pondered how to implement such

The use of __must_be_cstr() shouldn't cause side-effects, so from what I
can tell this is all okay. And since BUILD_BUG_ON_ZERO() resolves to a
constant expression, it shouldn't change the processing of the "size"
argument in the strscpy internals...

-- 
Kees Cook