linux-kernel - Re: [PATCH] bpf, doc: Correct one wrong value in "Register value tracking"

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Message-ID: <c17f532f-1259-f1dd-8c0d-7eb6a52da967@solarflare.com>
Date:   Wed, 24 Jan 2018 15:09:36 +0000
From:   Edward Cree <ecree@...arflare.com>
To:     Wang YanQing <udknight@...il.com>, <davem@...emloft.net>,
        <ast@...nel.org>, <daniel@...earbox.net>, <netdev@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] bpf, doc: Correct one wrong value in "Register value
 tracking"

On 24/01/18 07:48, Wang YanQing wrote:
> If we then OR this with 0x40, then the value of 6th bit (0th is first bit)
> become known, so the right mask is 0xbf instead of 0xcf.
>
> Signed-off-by: Wang YanQing <udknight@...il.com>
> ---
>  Documentation/networking/filter.txt | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/Documentation/networking/filter.txt b/Documentation/networking/filter.txt
> index 8781485..a4508ec 100644
> --- a/Documentation/networking/filter.txt
> +++ b/Documentation/networking/filter.txt
> @@ -1134,7 +1134,7 @@ The verifier's knowledge about the variable offset consists of:
>  mask and value; no bit should ever be 1 in both.  For example, if a byte is read
>  into a register from memory, the register's top 56 bits are known zero, while
>  the low 8 are unknown - which is represented as the tnum (0x0; 0xff).  If we
> -then OR this with 0x40, we get (0x40; 0xcf), then if we add 1 we get (0x0;
> +then OR this with 0x40, we get (0x40; 0xbf), then if we add 1 we get (0x0;
>  0x1ff), because of potential carries.
>  Besides arithmetic, the register state can also be updated by conditional
>  branches.  For instance, if a SCALAR_VALUE is compared > 8, in the 'true' branch

Acked-by: Edward Cree <ecree@...arflare.com>