Subject: Re: [PATCH net-next v2 1/2] net_tstamp: add SCM_TS_OPT_ID to provide OPT_ID in control message

From: Vadim Fedorenko <[email-masked]>
Date: Mon, 2 Sep 2024 20:40:51 +0100

On 02/09/2024 16:51, Willem de Bruijn wrote:
> Jason Xing wrote:
>> On Mon, Sep 2, 2024 at 9:09 PM Vadim Fedorenko <[email-masked]> wrote:
>>>
>>> SOF_TIMESTAMPING_OPT_ID socket option flag gives a way to correlate TX
>>> timestamps and packets sent via socket. Unfortunately, there is no way
>>> to reliably predict socket timestamp ID value in case of error returned
>>> by sendmsg. For UDP sockets it's impossible because of lockless
>>> nature of UDP transmit, several threads may send packets in parallel. In
>>> case of RAW sockets MSG_MORE option makes things complicated. More
>>> details are in the conversation [1].
>>> This patch adds new control message type to give user-space
>>> software an opportunity to control the mapping between packets and
>>> values by providing ID with each sendmsg. This works fine for UDP
>>> sockets only, and explicit check is added to control message parser.
>>>
>>> [1] https://lore.kernel.org/netdev/[email-masked]/
>>>
>>> Signed-off-by: Vadim Fedorenko <[email-masked]>
>>> ---
>>>   Documentation/networking/timestamping.rst | 14 ++++++++++++++
>>>   arch/alpha/include/uapi/asm/socket.h      |  4 +++-
>>>   arch/mips/include/uapi/asm/socket.h       |  2 ++
>>>   arch/parisc/include/uapi/asm/socket.h     |  2 ++
>>>   arch/sparc/include/uapi/asm/socket.h      |  2 ++
>>>   include/net/inet_sock.h                   |  4 +++-
>>>   include/net/sock.h                        |  1 +
>>>   include/uapi/asm-generic/socket.h         |  2 ++
>>>   include/uapi/linux/net_tstamp.h           |  3 ++-
>>>   net/core/sock.c                           | 12 ++++++++++++
>>>   net/ethtool/common.c                      |  1 +
>>>   net/ipv4/ip_output.c                      | 16 ++++++++++++----
>>>   net/ipv6/ip6_output.c                     | 16 ++++++++++++----
>>>   13 files changed, 68 insertions(+), 11 deletions(-)
>>>
>>> diff --git a/Documentation/networking/timestamping.rst b/Documentation/networking/timestamping.rst
>>> index 5e93cd71f99f..93b0901e4e8e 100644
>>> --- a/Documentation/networking/timestamping.rst
>>> +++ b/Documentation/networking/timestamping.rst
>>> @@ -193,6 +193,20 @@ SOF_TIMESTAMPING_OPT_ID:
>>>     among all possibly concurrently outstanding timestamp requests for
>>>     that socket.
>>>
>>> +  With this option enabled user-space application can provide custom
>>> +  ID for each message sent via UDP socket with control message with
>>> +  type set to SCM_TS_OPT_ID::
>>> +
>>> +    struct msghdr *msg;
>>> +    ...
>>> +    cmsg                        = CMSG_FIRSTHDR(msg);
>>> +    cmsg->cmsg_level            = SOL_SOCKET;
>>> +    cmsg->cmsg_type             = SO_TIMESTAMPING;
>>> +    cmsg->cmsg_len              = CMSG_LEN(sizeof(__u32));
>>> +    *((__u32 *) CMSG_DATA(cmsg)) = opt_id;
>>> +    err = sendmsg(fd, msg, 0);
>>> +
>>> +
>>>   SOF_TIMESTAMPING_OPT_ID_TCP:
>>>     Pass this modifier along with SOF_TIMESTAMPING_OPT_ID for new TCP
>>>     timestamping applications. SOF_TIMESTAMPING_OPT_ID defines how the
>>> diff --git a/arch/alpha/include/uapi/asm/socket.h b/arch/alpha/include/uapi/asm/socket.h
>>> index e94f621903fe..0698e6662cdf 100644
>>> --- a/arch/alpha/include/uapi/asm/socket.h
>>> +++ b/arch/alpha/include/uapi/asm/socket.h
>>> @@ -10,7 +10,7 @@
>>>    * Note: we only bother about making the SOL_SOCKET options
>>>    * same as OSF/1, as that's all that "normal" programs are
>>>    * likely to set.  We don't necessarily want to be binary
>>> - * compatible with _everything_.
>>> + * compatible with _everything_.
>>>    */
>>>   #define SOL_SOCKET     0xffff
>>>
>>> @@ -140,6 +140,8 @@
>>>   #define SO_PASSPIDFD           76
>>>   #define SO_PEERPIDFD           77
>>>
>>> +#define SCM_TS_OPT_ID          78
>>> +
>>>   #if !defined(__KERNEL__)
>>>
>>>   #if __BITS_PER_LONG == 64
>>> diff --git a/arch/mips/include/uapi/asm/socket.h b/arch/mips/include/uapi/asm/socket.h
>>> index 60ebaed28a4c..bb3dc8feb205 100644
>>> --- a/arch/mips/include/uapi/asm/socket.h
>>> +++ b/arch/mips/include/uapi/asm/socket.h
>>> @@ -151,6 +151,8 @@
>>>   #define SO_PASSPIDFD           76
>>>   #define SO_PEERPIDFD           77
>>>
>>> +#define SCM_TS_OPT_ID          78
>>> +
>>>   #if !defined(__KERNEL__)
>>>
>>>   #if __BITS_PER_LONG == 64
>>> diff --git a/arch/parisc/include/uapi/asm/socket.h b/arch/parisc/include/uapi/asm/socket.h
>>> index be264c2b1a11..c3ab3b3289eb 100644
>>> --- a/arch/parisc/include/uapi/asm/socket.h
>>> +++ b/arch/parisc/include/uapi/asm/socket.h
>>> @@ -132,6 +132,8 @@
>>>   #define SO_PASSPIDFD           0x404A
>>>   #define SO_PEERPIDFD           0x404B
>>>
>>> +#define SCM_TS_OPT_ID          0x404C
>>> +
>>>   #if !defined(__KERNEL__)
>>>
>>>   #if __BITS_PER_LONG == 64
>>> diff --git a/arch/sparc/include/uapi/asm/socket.h b/arch/sparc/include/uapi/asm/socket.h
>>> index 682da3714686..9b40f0a57fbc 100644
>>> --- a/arch/sparc/include/uapi/asm/socket.h
>>> +++ b/arch/sparc/include/uapi/asm/socket.h
>>> @@ -133,6 +133,8 @@
>>>   #define SO_PASSPIDFD             0x0055
>>>   #define SO_PEERPIDFD             0x0056
>>>
>>> +#define SCM_TS_OPT_ID            0x0057
>>> +
>>>   #if !defined(__KERNEL__)
>>>
>>>
>>> diff --git a/include/net/inet_sock.h b/include/net/inet_sock.h
>>> index 394c3b66065e..2161d50cf0fd 100644
>>> --- a/include/net/inet_sock.h
>>> +++ b/include/net/inet_sock.h
>>> @@ -174,6 +174,7 @@ struct inet_cork {
>>>          __s16                   tos;
>>>          char                    priority;
>>>          __u16                   gso_size;
>>> +       u32                     ts_opt_id;
>>>          u64                     transmit_time;
>>>          u32                     mark;
>>>   };
>>> @@ -241,7 +242,8 @@ struct inet_sock {
>>>          struct inet_cork_full   cork;
>>>   };
>>>
>>> -#define IPCORK_OPT     1       /* ip-options has been held in ipcork.opt */
>>> +#define IPCORK_OPT             1       /* ip-options has been held in ipcork.opt */
>>> +#define IPCORK_TS_OPT_ID       2       /* timestmap opt id has been provided in cmsg */
>>>
>>>   enum {
>>>          INET_FLAGS_PKTINFO      = 0,
>>> diff --git a/include/net/sock.h b/include/net/sock.h
>>> index f51d61fab059..73e21dad5660 100644
>>> --- a/include/net/sock.h
>>> +++ b/include/net/sock.h
>>> @@ -1794,6 +1794,7 @@ struct sockcm_cookie {
>>>          u64 transmit_time;
>>>          u32 mark;
>>>          u32 tsflags;
>>> +       u32 ts_opt_id;
>>>   };
>>>
>>>   static inline void sockcm_init(struct sockcm_cookie *sockc,
>>> diff --git a/include/uapi/asm-generic/socket.h b/include/uapi/asm-generic/socket.h
>>> index 8ce8a39a1e5f..db3df3e74b01 100644
>>> --- a/include/uapi/asm-generic/socket.h
>>> +++ b/include/uapi/asm-generic/socket.h
>>> @@ -135,6 +135,8 @@
>>>   #define SO_PASSPIDFD           76
>>>   #define SO_PEERPIDFD           77
>>>
>>> +#define SCM_TS_OPT_ID          78
>>> +
>>>   #if !defined(__KERNEL__)
>>>
>>>   #if __BITS_PER_LONG == 64 || (defined(__x86_64__) && defined(__ILP32__))
>>> diff --git a/include/uapi/linux/net_tstamp.h b/include/uapi/linux/net_tstamp.h
>>> index a2c66b3d7f0f..e2f145e3f3a1 100644
>>> --- a/include/uapi/linux/net_tstamp.h
>>> +++ b/include/uapi/linux/net_tstamp.h
>>> @@ -32,8 +32,9 @@ enum {
>>>          SOF_TIMESTAMPING_OPT_TX_SWHW = (1<<14),
>>>          SOF_TIMESTAMPING_BIND_PHC = (1 << 15),
>>>          SOF_TIMESTAMPING_OPT_ID_TCP = (1 << 16),
>>> +       SOF_TIMESTAMPING_OPT_ID_CMSG = (1 << 17),
>>
>> I'm not sure if the new flag needs to be documented as well? After
>> this patch, people may search the key word in the documentation file
>> and then find nothing.
>>
>> If we have this flag here, normally it means we can pass it through
>> setsockopt, so is it expected? If it's an exception, I reckon that we
>> can forbid passing/setting this option in sock_set_timestamping() and
>> document this rule?
> 
> Good point, thanks.
> 
> It must definitely not be part of SOF_TIMESTAMPING_MASK. My bad for
> suggesting without giving it much thought.
> 
> The bit is kernel-internal. No need to even mention it in user-facing
> documentation. But anyone reading net_tstamp.h might wonder what it
> does.
> 
> It should not even be in a UAPI header, but in an internal one.
> Probably include/net/sock.h, near SK_FLAGS_TIMESTAMP.
> 
> Maybe we can reserve bit 31 in u32 sk_tsflags. And if we ever have
> to double that flag size, it can move up to 63, as it is not UAPI in
> any way. This is a workaround to having a separate flags field in
> sockcm_cookie.
> 
> And have a BUILD_BUG_ON if SOF_TIMESTAMPING_LAST reaches this reserved
> region.

Yeah, I was also thinking of it not being UAPI, that's why I tried to
avoid it in my RFC using 0 as a reserved value. Do you think
SK_FLAGS_CMSG_TS_OPT_ID is good naming for it?

Thanks,
Vadim