Re: [PATCH mptcp-next v2 2/2] mptcp: setsockopt support for TCP_MD5SIG

[Geliang Tang <geliang@kernel.org>]

Hi Christoph,

On Thu, 2025-07-31 at 12:18 -0700, Christoph Paasch wrote:
> On Thu, Jul 31, 2025 at 12:28 AM Geliang Tang <geliang@kernel.org>
> wrote:
> > 
> > From: Geliang Tang <tanggeliang@kylinos.cn>
> > 
> > This adds setsockopt support for TCP_MD5SIG and TCP_MD5SIG_EXT
> > options.
> > The implementation:
> > - Allows setting these options (getsockopt remains unsupported)
> > - Applies them only to the first subflow
> > - Forces fallback to TCP (since MD5 isn't compatible with MPTCP)
> > 
> > Setting these options triggers fallback to TCP to maintain MD5
> > compatibility.
> > 
> > Note that TCP_MD5SIG and TCP_MD5SIG_EXT are unsupported for TCP
> > too.
> 
> Just wondering : What does the above line mean ? "unsupported for TCP
> too" ?

It should be:

TCP_MD5SIG and TCP_MD5SIG_EXT getsockopts are unsupported for TCP too.

Thanks,
-Geliang

> 
> 
> Christoph
> 
> > 
> > Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/575
> > Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
> > ---
> >  net/mptcp/sockopt.c | 12 ++++++++++--
> >  1 file changed, 10 insertions(+), 2 deletions(-)
> > 
> > diff --git a/net/mptcp/sockopt.c b/net/mptcp/sockopt.c
> > index b264185b810d..3ffdeca694be 100644
> > --- a/net/mptcp/sockopt.c
> > +++ b/net/mptcp/sockopt.c
> > @@ -13,6 +13,7 @@
> >  #include <net/tcp.h>
> >  #include <net/mptcp.h>
> >  #include "protocol.h"
> > +#include "mib.h"
> > 
> >  #define MIN_INFO_OPTLEN_SIZE           16
> >  #define MIN_FULL_INFO_OPTLEN_SIZE      40
> > @@ -567,11 +568,12 @@ static bool mptcp_supported_sockopt(int
> > level, int optname)
> >                 case TCP_FASTOPEN_CONNECT:
> >                 case TCP_FASTOPEN_KEY:
> >                 case TCP_FASTOPEN_NO_COOKIE:
> > +               /* MD5 will force a fallback to TCP: OK to set
> > while not connected */
> > +               case TCP_MD5SIG:
> > +               case TCP_MD5SIG_EXT:
> >                         return true;
> >                 }
> > 
> > -               /* TCP_MD5SIG, TCP_MD5SIG_EXT are not supported,
> > MD5 is not compatible with MPTCP */
> > -
> >                 /* TCP_REPAIR, TCP_REPAIR_QUEUE, TCP_QUEUE_SEQ,
> > TCP_REPAIR_OPTIONS,
> >                  * TCP_REPAIR_WINDOW are not supported, better
> > avoid this mess
> >                  */
> > @@ -830,6 +832,12 @@ static int mptcp_setsockopt_sol_tcp(struct
> > mptcp_sock *msk, int optname,
> >                 /* See tcp.c: TCP_DEFER_ACCEPT does not fail */
> >                 mptcp_setsockopt_first_sf_only(msk, SOL_TCP,
> > optname, optval, optlen);
> >                 return 0;
> > +#ifdef CONFIG_TCP_MD5SIG
> > +       case TCP_MD5SIG:
> > +       case TCP_MD5SIG_EXT:
> > +               __mptcp_try_fallback(msk,
> > MPTCP_MIB_MD5SIGFALLBACK);
> > +               fallthrough;
> > +#endif
> >         case TCP_FASTOPEN:
> >         case TCP_FASTOPEN_CONNECT:
> >         case TCP_FASTOPEN_KEY:
> > --
> > 2.48.1
> > 
> >