linux-kernel - Re: [PATCH 3/3] futex: make the encoded

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Date:   Sun, 5 Mar 2017 08:55:25 +0100
From:   Jiri Slaby <jslaby@...e.cz>
To:     akpm@...ux-foundation.org
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] futex: make the encoded_op decoding readable

On 03/03/2017, 01:27 PM, Jiri Slaby wrote:
> Decoding of encoded_op is a bit unreadable. It contains shifts to the
> left and to the right by some constants. Make it clearly visible what
> part of the bit mask is taken and shift the values only to the right
> appropriatelly.
> 
> Signed-off-by: Jiri Slaby <jslaby@...e.cz>
> ---
>  kernel/futex.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/futex.c b/kernel/futex.c
> index c09424406560..5834df248f09 100644
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -1459,10 +1459,10 @@ futex_wake(u32 __user *uaddr, unsigned int flags, int nr_wake, u32 bitset)
>  
>  static int futex_atomic_op_inuser(unsigned int encoded_op, u32 __user *uaddr)
>  {
> -	int op = (encoded_op >> 28) & 7;
> -	int cmp = (encoded_op >> 24) & 15;
> -	int oparg = (encoded_op << 8) >> 20;
> -	int cmparg = (encoded_op << 20) >> 20;
> +	int op = (encoded_op	& 0x70000000) >> 28;
> +	int cmp = (encoded_op	& 0x0f000000) >> 24;
> +	int oparg = (encoded_op & 0x00fff000) >> 12;
> +	int cmparg = encoded_op & 0x00000fff;

And it turned out that this one is actually invalid as it doesn't
preserve signedness on oparg and cmdarg. So please avoid applying this one.

thanks,
-- 
js
suse labs