[PATCH] bpf, doc: Correct one wrong value in "Register value tracking"

[PATCH] bpf, doc: Correct one wrong value in "Register value tracking"

[Wang YanQing]

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@gmail.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
-- 
1.8.5.6.2.g3d8a54e.dirty

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

[Edward Cree]

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@gmail.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@solarflare.com>

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

[Daniel Borkmann]

On 01/24/2018 08:48 AM, 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@gmail.com>

Applied to bpf-next, thanks Wang!