On Wednesday 18 May 2011 14:38:39 Antonio Quartulli wrote:
> Introduce two operations to handle comparison between packet sequence
> numbers taking into account overflow/wraparound. Batman-adv uses
> these functions already to check for successor packet even in case of
> overflow.
Thanks for your efforts to bring that to the kernel. But when you prepare a
patch then you have to add a signoff. And also David S. Miller is the
maintainer for this header - it would be interesting to ask him first when we
want to change that file.
> ---
> I added this two functions in net.h because I didn't really know where
> best placement is. I saw several modules that redefine their own functions
> for the same purpose.
>
> include/linux/net.h | 17 +++++++++++++++++
> 1 files changed, 17 insertions(+), 0 deletions(-)
>
> diff --git a/include/linux/net.h b/include/linux/net.h
> index 94de83c..c7bc9bf 100644
> --- a/include/linux/net.h
> +++ b/include/linux/net.h
> @@ -295,4 +295,21 @@ extern struct ratelimit_state net_ratelimit_state;
> #endif
>
> #endif /* __KERNEL__ */
> +
> +/* Returns the smallest signed integer in two's complement with the sizeof
> x */ +#define smallest_signed_int(x) (1u << (7u + 8u * (sizeof(x) - 1u)))
> +
> +/* Checks if a sequence number x is a predecessor/successor of y.
> + * they handle overflows/underflows and can correctly check for a
> + * predecessor/successor unless the variable sequence number has grown by
> + * more then 2**(bitwidth(x)-1)-1.
> + * This means that for a uint8_t with the maximum value 255, it would
> think: + * - when adding nothing - it is neither a predecessor nor a
> successor + * - before adding more than 127 to the starting value - it is
> a predecessor, + * - when adding 128 - it is neither a predecessor nor a
> successor, + * - after adding more than 127 to the starting value - it is
> a successor */ +#define seq_before(x, y) ({typeof(x) _dummy = (x - y); \
> + _dummy > smallest_signed_int(_dummy); })
> +#define seq_after(x, y) seq_before(y, x)
> +
> #endif /* _LINUX_NET_H */
I suggested yesterday (probably too late) that it would be good to check the
type of both parameters (similar to the min and max functions in
include/linux/kernel.h
#define seq_before(x, y) ({typeof(x) _d1 = (x); \
typeof(y) _d2 = (y); \
(void) (&_d1 == &_d2); \
typeof(x) _dummy = (_d1 - _d2); \
_dummy > smallest_signed_int(_dummy); })
And your seq_before/after conflicts with the one defined in ppp_generic.c
drivers/net/ppp_generic.c:232:0: warning: "seq_before" redefined [enabled by
default]
include/linux/net.h:312:0: note: this is the location of the previous
definition
drivers/net/ppp_generic.c:233:0: warning: "seq_after" redefined [enabled by
default]
include/linux/net.h:314:0: note: this is the location of the previous
definition
The definition there is only for u32 - thus you would have to remove it and
check that it always gives the same result:
#define seq_before(a, b) ((s32)((a) - (b)) < 0)
#define seq_after(a, b) ((s32)((a) - (b)) > 0)
But I would say that they have a different definition of seq_before. Changing
that behaviour for batman-adv would not be that problematic, but maybe for
ppp.
A defintion which should fulfil the requirements for ppp could be:
#define seq_after(x, y) ({typeof(x) _d1 = (x); \
typeof(y) _d2 = (y); \
(void) (&_d1 == &_d2); \
typeof(x) _dummy = (_d2 - _d1); \
_dummy > smallest_signed_int(_dummy); })
#define seq_before(x, y) ({typeof(x) _d1 = (x); \
typeof(y) _d2 = (y); \
(void) (&_d1 == &_d2); \
typeof(x) _dummy = (_d1 - _d2); \
_dummy >= smallest_signed_int(_dummy); })
Of course the comment above the seq_before/seq_after would be wrong.
/* Checks if a sequence number x is a predecessor/successor of y.
* they handle overflows/underflows and can correctly check for a
* predecessor/successor unless the variable sequence number has grown by
* more then 2**(bitwidth(x)-1).
* This means that for a uint8_t with the maximum value 255, it would think:
* - when adding nothing - it is neither a predecessor nor a successor
* - before adding more than 128 to the starting value - it is a predecessor,
* - after adding more than 127 to the starting value - it is a successor */
I think there could be more candidates which would like to use this abstract
functionality. Maybe some one else on linux-kernel or netdev has a suggestion.
Kind regards,
Sven
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