Re: [PATCH v5] linux/bits.h: Add compile time sanity check of GENMASK inputs

[Kees Cook]

On Sun, Mar 08, 2020 at 08:39:54PM +0100, Rikard Falkeborn wrote:
> GENMASK() and GENMASK_ULL() are supposed to be called with the high bit
> as the first argument and the low bit as the second argument. Mixing
> them will return a mask with zero bits set.
> 
> Recent commits show getting this wrong is not uncommon, see e.g.
> commit aa4c0c9091b0 ("net: stmmac: Fix misuses of GENMASK macro") and
> commit 9bdd7bb3a844 ("clocksource/drivers/npcm: Fix misuse of GENMASK
> macro").
> 
> To prevent such mistakes from appearing again, add compile time sanity
> checking to the arguments of GENMASK() and GENMASK_ULL(). If both
> arguments are known at compile time, and the low bit is higher than the
> high bit, break the build to detect the mistake immediately.
> 
> Since GENMASK() is used in declarations, BUILD_BUG_ON_ZERO() must be
> used instead of BUILD_BUG_ON().
> 
> __builtin_constant_p does not evaluate is argument, it only checks if it
> is a constant or not at compile time, and __builtin_choose_expr does not
> evaluate the expression that is not chosen. Therefore, GENMASK(x++, 0)
> does only evaluate x++ once.
> 
> Commit 95b980d62d52 ("linux/bits.h: make BIT(), GENMASK(), and friends
> available in assembly") made the macros in linux/bits.h available in
> assembly. Since BUILD_BUG_OR_ZERO() is not asm compatible, disable the
> checks if the file is included in an asm file.
> 
> Due to bugs in GCC versions before 4.9 [0], disable the check if
> building with a too old GCC compiler.
> 
> [0]: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=19449
> 
> Signed-off-by: Rikard Falkeborn <rikard.falkeborn@xxxxxxxxx>

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

-Kees

> Reviewed-by: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
> ---
> Another attempt to get this merged. I've test built allmodconfig for
> i386, x86_64 and arm64 for linux-next 20200306 without issues. Also, the
> last known GENMASK issue was just merged into Linus tree [1].
> 
> [1]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=96b4ea324ae92386db2b0c73ace597c80cde1ecb 
> 
> Changes in v5:
>   - Added Masahiros Reviewed-by
>   - Waited for bugfixes to get merged
> 
> Changes in v4:
>   - Disable the argument check for GCC < 4.9 due to a compiler bug.
> 
> Changes in v3:
>   - Changed back to shorter macro argument names
>   - Remove casts and use 0 instead of UL(0) in GENMASK_INPUT_CHECK(),
>     since all results in GENMASK_INPUT_CHECK() are now ints. Update
>     commit message to reflect that.
> 
> Changes in v2:
>   - Add comment about why inputs are not checked when used in asm file
>   - Use UL(0) instead of 0
>   - Extract mask creation in a separate macro to improve readability
>   - Use high and low instead of h and l (part of this was extracted to a
>     separate patch)
>   - Updated commit message
> 
> Joe Perches sent a series to fix the existing misuses of GENMASK().
> Those patches have been merged into Linus tree except two places where
> the GENMASK misuse is in unused macros, which will not fail to build.
> There was also a patch by Nathan Chancellor that have now been merged.
> 
>  include/linux/bits.h | 22 ++++++++++++++++++++--
>  1 file changed, 20 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/bits.h b/include/linux/bits.h
> index 669d69441a62..f108302a3121 100644
> --- a/include/linux/bits.h
> +++ b/include/linux/bits.h
> @@ -18,12 +18,30 @@
>   * position @h. For example
>   * GENMASK_ULL(39, 21) gives us the 64bit vector 0x000000ffffe00000.
>   */
> -#define GENMASK(h, l) \
> +#if !defined(__ASSEMBLY__) && \
> +	(!defined(CONFIG_CC_IS_GCC) || CONFIG_GCC_VERSION >= 49000)
> +#include <linux/build_bug.h>
> +#define GENMASK_INPUT_CHECK(h, l) \
> +	(BUILD_BUG_ON_ZERO(__builtin_choose_expr( \
> +		__builtin_constant_p((l) > (h)), (l) > (h), 0)))
> +#else
> +/*
> + * BUILD_BUG_ON_ZERO is not available in h files included from asm files,
> + * disable the input check if that is the case.
> + */
> +#define GENMASK_INPUT_CHECK(h, l) 0
> +#endif
> +
> +#define __GENMASK(h, l) \
>  	(((~UL(0)) - (UL(1) << (l)) + 1) & \
>  	 (~UL(0) >> (BITS_PER_LONG - 1 - (h))))
> +#define GENMASK(h, l) \
> +	(GENMASK_INPUT_CHECK(h, l) + __GENMASK(h, l))
>  
> -#define GENMASK_ULL(h, l) \
> +#define __GENMASK_ULL(h, l) \
>  	(((~ULL(0)) - (ULL(1) << (l)) + 1) & \
>  	 (~ULL(0) >> (BITS_PER_LONG_LONG - 1 - (h))))
> +#define GENMASK_ULL(h, l) \
> +	(GENMASK_INPUT_CHECK(h, l) + __GENMASK_ULL(h, l))
>  
>  #endif	/* __LINUX_BITS_H */
> -- 
> 2.25.1
> 

-- 
Kees Cook