Re: [PATCH net-next] net: define and implement new SOL_SOCKET SO_RX_IFINDEX option

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

Maciej Żenczykowski wrote:
> On Thu, Oct 24, 2024 at 5:41 PM Maciej Żenczykowski <maze@google.com> wrote:
> >
> > This is currently only implemented for TCP and is not
> > guaranteed to return correct information for a multitude
> > of reasons (including multipath reception), but there are
> > scenarios where it is useful: in particular a strong host
> > model where connections are only viable via a single interface,
> > for example a VPN interface.  One could for example choose
> > to use this to SO_BINDTODEVICE.

Fair to say that this is the equivalent of ipi_ifindex in IP_PKTINFO,
but for non datagram sockets where skb_iff cannot be read directly?

> >
> > Test:
> >   // Python 2.7.18 (default, Jul 13 2022, 18:14:36)
> >   import socket
> >   SO_RX_IFINDEX=82
> >   s = socket.socket(socket.AF_INET6, socket.SOCK_STREAM, 0)
> >   c = socket.socket(socket.AF_INET6, socket.SOCK_STREAM, 0)
> >   s.bind(('::', 8888))
> >   s.listen(128)
> >   c.connect(('::', 8888))
> >   a = s.accept()
> >   print a  # (<socket._socketobject object>, ('::1', 58144, 0, 0))
> >   p=a[0]
> >   p.getsockname()  # ('::1', 8888, 0, 0)
> >   p.getpeername()  # ('::1', 58144, 0, 0)
> >   c.getsockname()  # ('::1', 58144, 0, 0)
> >   c.getpeername()  # ('::1', 8888, 0, 0)
> >   p.getsockopt(socket.SOL_SOCKET, SO_RX_IFINDEX)  # 1 (lo)
> >   c.getsockopt(socket.SOL_SOCKET, SO_RX_IFINDEX)  # 0 (unknown)
> >   c.send(b'X')  # 1
> >   p.recv(2)  # 'X'
> >   p.getsockopt(socket.SOL_SOCKET, SO_RX_IFINDEX)  # 1 (lo)
> >   c.getsockopt(socket.SOL_SOCKET, SO_RX_IFINDEX)  # 0 (unknown)
> >   p.send(b'Z')  # 1
> >   c.recv(2)  # 'Z'
> >   p.getsockopt(socket.SOL_SOCKET, SO_RX_IFINDEX)  # 1 (lo)
> >   c.getsockopt(socket.SOL_SOCKET, SO_RX_IFINDEX)  # 1 (lo)
> >
> > Which shows we should possibly fix the 3-way handshake SYN-ACK
> > to set sk->sk_rx_dst_ifindex.
> >
> > Cc: Lorenzo Colitti <lorenzo@google.com>
> > Cc: Eric Dumazet <edumazet@google.com>
> > Signed-off-by: Maciej Żenczykowski <maze@google.com>
> > ---
> >  arch/alpha/include/uapi/asm/socket.h  | 2 ++
> >  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/uapi/asm-generic/socket.h     | 2 ++
> >  net/core/sock.c                       | 4 ++++
> >  6 files changed, 14 insertions(+)
> >
> > diff --git a/arch/alpha/include/uapi/asm/socket.h b/arch/alpha/include/uapi/asm/socket.h
> > index 302507bf9b5d..5f139b095a49 100644
> > --- a/arch/alpha/include/uapi/asm/socket.h
> > +++ b/arch/alpha/include/uapi/asm/socket.h
> > @@ -148,6 +148,8 @@
> >
> >  #define SCM_TS_OPT_ID          81
> >
> > +#define SO_RX_IFINDEX          82
> > +
> >  #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 d118d4731580..ff25d24b4dea 100644
> > --- a/arch/mips/include/uapi/asm/socket.h
> > +++ b/arch/mips/include/uapi/asm/socket.h
> > @@ -159,6 +159,8 @@
> >
> >  #define SCM_TS_OPT_ID          81
> >
> > +#define SO_RX_IFINDEX          82
> > +
> >  #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 d268d69bfcd2..3f89c388e356 100644
> > --- a/arch/parisc/include/uapi/asm/socket.h
> > +++ b/arch/parisc/include/uapi/asm/socket.h
> > @@ -140,6 +140,8 @@
> >
> >  #define SCM_TS_OPT_ID          0x404C
> >
> > +#define SO_RX_IFINDEX          82
> > +
> >  #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 113cd9f353e3..f1af74f5f1ad 100644
> > --- a/arch/sparc/include/uapi/asm/socket.h
> > +++ b/arch/sparc/include/uapi/asm/socket.h
> > @@ -141,6 +141,8 @@
> >
> >  #define SCM_TS_OPT_ID            0x005a
> >
> > +#define SO_RX_IFINDEX            0x005b
> > +
> >  #if !defined(__KERNEL__)
> >
> >
> > diff --git a/include/uapi/asm-generic/socket.h b/include/uapi/asm-generic/socket.h
> > index deacfd6dd197..b16c69e22606 100644
> > --- a/include/uapi/asm-generic/socket.h
> > +++ b/include/uapi/asm-generic/socket.h
> > @@ -143,6 +143,8 @@
> >
> >  #define SCM_TS_OPT_ID          81
> >
> > +#define SO_RX_IFINDEX          82
> > +
> >  #if !defined(__KERNEL__)
> >
> >  #if __BITS_PER_LONG == 64 || (defined(__x86_64__) && defined(__ILP32__))
> > diff --git a/net/core/sock.c b/net/core/sock.c
> > index 7f398bd07fb7..6c985413c21f 100644
> > --- a/net/core/sock.c
> > +++ b/net/core/sock.c
> > @@ -1932,6 +1932,10 @@ int sk_getsockopt(struct sock *sk, int level, int optname,
> >                 v.val = READ_ONCE(sk->sk_mark);
> >                 break;
> >
> > +       case SO_RX_IFINDEX:
> > +               v.val = READ_ONCE(sk->sk_rx_dst_ifindex);
> > +               break;
> > +

If it is limited to TCP, return error in other cases.

So that we can extend it later with well defined behavior.

> >         case SO_RCVMARK:
> >                 v.val = sock_flag(sk, SOCK_RCVMARK);
> >                 break;
> > --
> > 2.47.0.105.g07ac214952-goog
> >
> 
> Note: I'm not sure if I did the right thing with parisc...
> It has:
> #define SO_DEVMEM_LINEAR 78
> #define SCM_DEVMEM_LINEAR SO_DEVMEM_LINEAR
> #define SO_DEVMEM_DMABUF 79
> #define SCM_DEVMEM_DMABUF SO_DEVMEM_DMABUF
> #define SO_DEVMEM_DONTNEED 80
> which is weird...

This is a common pattern.

To define separate SCM constants for cmsg fields, even though they
have the same constant as their [gs]etsockopt counterparts.