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

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Date:   Wed, 31 Jul 2019 12:27:38 -0700
From:   Joe Perches <joe@...ches.com>
To:     Rikard Falkeborn <rikard.falkeborn@...il.com>
Cc:     akpm@...ux-foundation.org, johannes@...solutions.net,
        linux-kernel@...r.kernel.org, yamada.masahiro@...ionext.com
Subject: Re: [PATCH] linux/bits.h: Add compile time sanity check of GENMASK
 inputs

On Wed, 2019-07-31 at 21:03 +0200, 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 the
> 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_OR_ZERO() must be
> used instead of BUILD_BUG_ON(), and __is_constexpr() must be used instead
> of __builtin_constant_p().
> 
> Commit 95b980d62d52 ("linux/bits.h: make BIT(), GENMASK(), and friends
> available in assembly") made the macros in linux/bits.h available in
> assembly. Since neither BUILD_BUG_OR_ZERO() or __is_constexpr() are asm
> compatible, disable the checks if the file is included in an asm file.
> 
> Signed-off-by: Rikard Falkeborn <rikard.falkeborn@...il.com>
> ---
> Joe Perches sent a series to fix the existing misuses of GENMASK() that
> needs to be merged before this to avoid build failures. Currently, 7 of
> the patches were not in Linus tree, and 2 were not in linux-next.
> 
> Also, there's currently no asm users of bits.h, but since it was made
> asm-compatible just two weeks ago it would be a shame to break it right
> away...
[]
> diff --git a/include/linux/bits.h b/include/linux/bits.h
[]
> @@ -18,12 +18,22 @@
>   * position @h. For example
>   * GENMASK_ULL(39, 21) gives us the 64bit vector 0x000000ffffe00000.
>   */
> +#ifndef __ASSEMBLY__
> +#include <linux/build_bug.h>
> +#define GENMASK_INPUT_CHECK(h, l)  BUILD_BUG_ON_ZERO(__builtin_choose_expr( \
> +		__is_constexpr(h) && __is_constexpr(l), (l) > (h), 0))
> +#else
> +#define GENMASK_INPUT_CHECK(h, l) 0

A few things:

o Reading the final code is a bit confusing.
  Perhaps add a comment description saying it's not checked
  in asm .h uses.

o Maybe use:
  #define GENMASK_INPUT_CHECK(h, l) UL(0)

o The compiler error message when the arguments are in the
  wrong order isn't obvious.  Is there some way to improve
  the compiler error output, maybe by using BUILD_BUG_ON_MSG
  or some other mechanism?