Subject: Re: [PATCH -next v3 1/3] syncookies: avoid magic values and document which-bit-is-what-option

From: Daniel Borkmann <[email-masked]>
Date: Mon, 03 Nov 2014 15:33:36 +0100

On 11/03/2014 03:24 PM, David Laight wrote:
> From: Florian Westphal
>> Was a bit more difficult to read than needed due to magic shifts;
>> add defines and document the used encoding scheme.
>>
>> Joint work with Daniel Borkmann.
>>
>> Signed-off-by: Daniel Borkmann <[email-masked]>
>> Signed-off-by: Florian Westphal <[email-masked]>
>> ---
>>   This patch was not part of earlier versions of the set.
>>
>>   net/ipv4/syncookies.c | 50 +++++++++++++++++++++++++++++++++++---------------
>>   1 file changed, 35 insertions(+), 15 deletions(-)
>>
>> diff --git a/net/ipv4/syncookies.c b/net/ipv4/syncookies.c
>> index 4ac7bca..c3792c0 100644
>> --- a/net/ipv4/syncookies.c
>> +++ b/net/ipv4/syncookies.c
>> @@ -19,10 +19,6 @@
>>   #include <net/tcp.h>
>>   #include <net/route.h>
>>
>> -/* Timestamps: lowest bits store TCP options */
>> -#define TSBITS 6
>> -#define TSMASK (((__u32)1 << TSBITS) - 1)
>> -
>>   extern int sysctl_tcp_syncookies;
>>
>>   static u32 syncookie_secret[2][16-4+SHA_DIGEST_WORDS] __read_mostly;
>> @@ -30,6 +26,30 @@ static u32 syncookie_secret[2][16-4+SHA_DIGEST_WORDS] __read_mostly;
>>   #define COOKIEBITS 24	/* Upper bits store count */
>>   #define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1)
>>
>> +/* TCP Timestamp: 6 lowest bits of timestamp sent in the cookie SYN-ACK
>> + * stores TCP options:
>> + *
>> + * MSB                               LSB
>> + * | 31 ...   6 |  5  |  4   | 3 2 1 0 |
>> + * |  Timestamp | ECN | SACK | WScale  |
>> + *
>> + * When we receive a valid cookie-ACK, we look at the echoed tsval (if
>> + * any) to figure out which TCP options we should use for the rebuilt
>> + * connection.
>> + *
>> + * A WScale setting of '0xf' (which is an invalid scaling value)
>> + * means that original syn did not include the TCP window scaling option.
>> + */
>> +#define TS_OPT_WSCALE_MASK	0xf
>> +#define TS_OPT_SACK		BIT(4)
>> +#define TS_OPT_ECN		BIT(5)
>> +/* There is no TS_OPT_TIMESTAMP:
>> + * if ACK contains timestamp option, we already know it was
>> + * requested/supported by the syn/synack exchange.
>> + */
>> +#define TSBITS	6
>> +#define TSMASK	(((__u32)1 << TSBITS) - 1)
>
> Personally I'd define all the values as hex constants instead of mixing
> and matching the defines.
>
> So probably just:
> #define TS_OPT_WSCALE_MASK	0x0f
> #define TS_OPT_SACK		0x10
> #define TS_OPT_ECN		0x20
> #define TSMASK                0x3f

If you look at the above comment and then take a peek at the actual TS_OPT_*,
it is much easier to follow. Moreover, how is having TSMASK as 0x3f better?!
Currently, it is a constant calculated based upon TSBITS.