Re: [PATCH v3 3/3] linux/bits.h: Add compile time sanity check of GENMASK inputs

[Geert Uytterhoeven]

Hi Rikard,

On Sun, Aug 11, 2019 at 8:52 PM Rikard Falkeborn
<rikard.falkeborn@xxxxxxxxx> 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.
>
> Signed-off-by: Rikard Falkeborn <rikard.falkeborn@xxxxxxxxx>
> ---
> 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() that
> needs to be merged before this to avoid build failures. Currently, 5 of
> the patches are not in Linus tree, and 2 are not in linux-next. There is
> also a patch pending by Nathan Chancellor that also needs to be merged
> before this patch is merged to avoid build failures.
>
>  include/linux/bits.h | 21 +++++++++++++++++++--
>  1 file changed, 19 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/bits.h b/include/linux/bits.h
> index 669d69441a62..4ba0fb609239 100644
> --- a/include/linux/bits.h
> +++ b/include/linux/bits.h
> @@ -18,12 +18,29 @@
>   * position @h. For example
>   * GENMASK_ULL(39, 21) gives us the 64bit vector 0x000000ffffe00000.
>   */
> -#define GENMASK(h, l) \
> +#ifndef __ASSEMBLY__
> +#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 */

This is now commit 0fd35cd30a2fece1 ("linux/bits.h: add compile time sanity
check of GENMASK inputs") in next-20191008.

<noreply@xxxxxxxxxxxxxx> reported the following failure in sun3_defconfig,
which I managed to reproduce with gcc-4.6.3:

    lib/842/842_compress.c: In function '__split_add_bits':
    lib/842/842_compress.c:164:25: error: first argument to
'__builtin_choose_expr' not a constant
    lib/842/842_compress.c:164:25: error: bit-field '<anonymous>'
width not an integer constant
    scripts/Makefile.build:265: recipe for target
'lib/842/842_compress.o' failed

__split_add_bits() calls GENMASK_ULL() with a non-constant.
However __split_add_bits() itself is called with constants only.
Apparently gcc fails to inline __split_add_bits().
Adding inline or always_inline doesn't help.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds