linux-kernel - Re: [PATCH v4 1/3] bitfield: fix *_encode

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Date:   Mon, 18 Jun 2018 23:41:58 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Johannes Berg <johannes@...solutions.net>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH v4 1/3] bitfield: fix *_encode_bits()

On Mon, Jun 18, 2018 at 11:37 PM, Johannes Berg
<johannes@...solutions.net> wrote:
> There's a bug in *_encode_bits() in using ~field_multiplier() for
> the check whether or not the constant value fits into the field,
> this is wrong and clearly ~field_mask() was intended. This was
> triggering for me for both constant and non-constant values.
>
> Additionally, make this case actually into an compile error.
> Declaring the extern function that will never exist with just a
> warning is pointless as then later we'll just get a link error.
>
> While at it, also fix the indentation in those lines I'm touching.
>
> Finally, as suggested by Andy Shevchenko, add some tests and for
> that introduce also u8 helpers. The tests don't compile without
> the fix, showing that it's necessary.
>

Reviewed-by: Andy Shevchenko <andy.shevchenko@...il.com>


> Fixes: 00b0c9b82663 ("Add primitives for manipulating bitfields both in host- and fixed-endian.")
> Signed-off-by: Johannes Berg <johannes@...solutions.net>
> ---
> v2: replace stray tab by space
> v3: u8 helpers, tests
> v4: minor cleanup (Andy)
>     split into two patches (Andy)
>
> If you don't mind, I'd like to take this through the networking
> tree(s) as a fix since I have some pending code that depends on
> it.
> ---
>  include/linux/bitfield.h | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
> index cf2588d81148..147a7bb341dd 100644
> --- a/include/linux/bitfield.h
> +++ b/include/linux/bitfield.h
> @@ -104,7 +104,7 @@
>                 (typeof(_mask))(((_reg) & (_mask)) >> __bf_shf(_mask)); \
>         })
>
> -extern void __compiletime_warning("value doesn't fit into mask")
> +extern void __compiletime_error("value doesn't fit into mask")
>  __field_overflow(void);
>  extern void __compiletime_error("bad bitfield mask")
>  __bad_mask(void);
> @@ -121,8 +121,8 @@ static __always_inline u64 field_mask(u64 field)
>  #define ____MAKE_OP(type,base,to,from)                                 \
>  static __always_inline __##type type##_encode_bits(base v, base field) \
>  {                                                                      \
> -        if (__builtin_constant_p(v) && (v & ~field_multiplier(field))) \
> -                           __field_overflow();                         \
> +       if (__builtin_constant_p(v) && (v & ~field_mask(field)))        \
> +               __field_overflow();                                     \
>         return to((v & field_mask(field)) * field_multiplier(field));   \
>  }                                                                      \
>  static __always_inline __##type type##_replace_bits(__##type old,      \
> --
> 2.14.4
>



-- 
With Best Regards,
Andy Shevchenko