On Wed, Feb 21, 2018 at 2:47 PM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> And other attribute specifiers we encourage people to put in other
> parts of the type, like __user etc, so they don't have that same
> parsing issue.
Looking at other attributes we use on structs, we may have similar
risks for these:
__packed
____cacheline_aligned
____cacheline_aligned_in_smp
____cacheline_internodealigned_in_smp
But they just haven't been used in places that we could trip over it
as badly, AFAICT.
> I guess one _extreme_ fix for this would be to put
>
> extern struct nostruct __randomize_layout;
>
> in our include/linux/kconfig.h, which I think we end up always
> including first thanks to having it on the command line.
We could do that for all the above, but I wonder if the real problem
is our convention of using "regular" names for these kinds of
attributes instead of parameterized names. If we always used something
like:
#define __struct(x) __attribute__(x)
We'd avoid it, but we'd uglify our struct attributes:
struct thing { ... } __struct(randomize_layout);
though trying this now creates other problems. Hmmm.
(Regardless, let me send the nfs fix separately...)
-Kees
>
> Because if you do that, you actually get an error:
>
> CC [M] fs/nfsd/nfs4xdr.o
> In file included from ./include/linux/fs_struct.h:5:0,
> from fs/nfsd/nfs4xdr.c:36:
> ./include/linux/path.h:11:3: error: conflicting types for ‘__randomize_layout’
> } __randomize_layout;
> ^~~~~~~~~~~~~~~~~~
> In file included from <command-line>:0:0:
> ././include/linux/kconfig.h:8:28: note: previous declaration of
> ‘__randomize_layout’ was here
> extern struct nostruct __randomize_layout;
> ^~~~~~~~~~~~~~~~~~
> make[1]: *** [scripts/Makefile.build:317: fs/nfsd/nfs4xdr.o] Error 1
>
> and we would have figured this out immediately.
>
> Broken example patch appended, in case somebody wants to play with
> something like this or comes up with a better model entirely..
>
> Linus
>
> ---
>
> diff --git a/include/linux/kconfig.h b/include/linux/kconfig.h
> index fec5076eda91..537dacb83380 100644
> --- a/include/linux/kconfig.h
> +++ b/include/linux/kconfig.h
> @@ -4,6 +4,10 @@
>
> #include <generated/autoconf.h>
>
> +#ifndef __ASSEMBLY__
> + extern struct nostruct __randomize_layout;
> +#endif
> +
> #define __ARG_PLACEHOLDER_1 0,
> #define __take_second_arg(__ignored, val, ...) val
--
Kees Cook
Pixel Security
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