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. ---
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 */