Re: [PATCH] Make kunmap_atomic() harder to misuse

[Rusty Russell <rusty@rustcorp.com.au>]

On Sun, 30 May 2010 01:12:56 pm Andrew Morton wrote:
> On Fri, 28 May 2010 07:53:13 -0300 Cesar Eduardo Barros <cesarb@cesarb.net> wrote:
> 
> > kunmap_atomic() is currently at level -4 on Rusty's "Hard To Misuse"
> > list[1] ("Follow common convention and you'll get it wrong"), except in
> > some architectures when CONFIG_DEBUG_HIGHMEM is set[2][3].
> > 
> > kunmap() takes a pointer to a struct page; kunmap_atomic(), however,
> > takes takes a pointer to within the page itself. This seems to once in a
> > while trip people up (the convention they are following is the one from
> > kunmap()).
> > 
> > Make it much harder to misuse, by moving it to level 9 on Rusty's
> > list[4] ("The compiler/linker won't let you get it wrong"). This is done
> > by refusing to build if the pointer passed to it is convertible to a
> > struct page * but it is not a void * (verified by trying to convert it
> > to a pointer to a dummy struct).
> > 
> > The real kunmap_atomic() is renamed to kunmap_atomic_notypecheck()
> > (which is what you would call in case for some strange reason calling it
> > with a pointer to a struct page is not incorrect in your code).
> > 
> 
> Fair enough, that's a 99% fix.  A long time ago I made kmap_atomic()
> return a char * (iirc) and kunmap_atomic() is passed a char*.  It
> worked, but I ended up throwing it away.  I don't precisely remember
> why - I think it was intrusiveness and general hassle rather than
> anything fundamental.
> 
> >
> > ...
> >
> > +/* Prevent people trying to call kunmap_atomic() as if it were kunmap() */
> > +struct __kunmap_atomic_dummy {};
> > +#define kunmap_atomic(addr, idx) do { \
> > +		BUILD_BUG_ON( \
> > +			__builtin_types_compatible_p(typeof(addr), struct page *) && \
> > +			!__builtin_types_compatible_p(typeof(addr), struct __kunmap_atomic_dummy *)); \
> > +		kunmap_atomic_notypecheck((addr), (idx)); \
> > +	} while (0)
> 
> <looks around>
> 
> OK, it seems that __builtin_types_compatible_p() is supported on all
> approved gcc versions.
> 
> We have a little __same_type() helper for this.  __must_be_array()
> should be using it, too.

Yep... but I think BUILD_BUG_ON(__same_type((addr), struct page *)); is
sufficient; void * is not compatible in my quick tests here.

Andrew, want to take this?

Subject: Use __same_type() in __must_be_array()

We should use the __same_type() helper in __must_be_array().

Reported-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
--- a/include/linux/compiler-gcc.h
+++ b/include/linux/compiler-gcc.h
@@ -35,8 +35,7 @@
     (typeof(ptr)) (__ptr + (off)); })
 
 /* &a[0] degrades to a pointer: a different type from an array */
-#define __must_be_array(a) \
-  BUILD_BUG_ON_ZERO(__builtin_types_compatible_p(typeof(a), typeof(&a[0])))
+#define __must_be_array(a) BUILD_BUG_ON_ZERO(__same_type((a), &(a)[0]))
 
 /*
  * Force always-inline if the user requests it so via the .config,