[PATCH 2/2] net: socket: implement SO_DESCRIPTION

[Pascal Bouchareine]

This command attaches the zero terminated string in optval to the
socket for troubleshooting purposes. The free string is displayed in the
process fdinfo file for that fd (/proc/<pid>/fdinfo/<fd>).

One intended usage is to allow processes to self-document sockets
for netstat and friends to report

We ignore optlen and constrain the string to a static max size

Signed-off-by: Pascal Bouchareine <ka...@tfz.net>
---
 include/net/sock.h                |  4 ++++
 include/uapi/asm-generic/socket.h |  2 ++
 net/core/sock.c                   | 23 +++++++++++++++++++++++
 net/socket.c                      |  5 +++++
 4 files changed, 34 insertions(+)

diff --git a/include/net/sock.h b/include/net/sock.h
index 1183507df95b..6b4fd1383282 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -342,6 +342,7 @@ struct bpf_sk_storage;
   *    @sk_txtime_deadline_mode: set deadline mode for SO_TXTIME
   *    @sk_txtime_report_errors: set report errors mode for SO_TXTIME
   *    @sk_txtime_unused: unused txtime flags
+  *    @sk_description: user supplied with SO_DESCRIPTION
   */
 struct sock {
        /*
@@ -519,6 +520,9 @@ struct sock {
        struct bpf_sk_storage __rcu     *sk_bpf_storage;
 #endif
        struct rcu_head         sk_rcu;
+
+#define        SK_MAX_DESC_SIZE        256
+       char                    *sk_description;
 };
 
 enum sk_pacing {
diff --git a/include/uapi/asm-generic/socket.h 
b/include/uapi/asm-generic/socket.h
index 77f7c1638eb1..fb51c4bb7a12 100644
--- a/include/uapi/asm-generic/socket.h
+++ b/include/uapi/asm-generic/socket.h
@@ -119,6 +119,8 @@
 
 #define SO_DETACH_REUSEPORT_BPF 68
 
+#define SO_DESCRIPTION         69
+
 #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 2e5b7870e5d3..2cb44a0e38b7 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -828,6 +828,24 @@ void sock_set_rcvbuf(struct sock *sk, int val)
 }
 EXPORT_SYMBOL(sock_set_rcvbuf);
 
+int sock_set_description(struct sock *sk, char __user *user_desc)
+{
+       char *old, *desc;
+
+       desc = strndup_user(user_desc, SK_MAX_DESC_SIZE, GFP_KERNEL_ACCOUNT);
+       if (IS_ERR(desc))
+               return PTR_ERR(desc);
+
+       lock_sock(sk);
+       old = sk->sk_description;
+       sk->sk_description = desc;
+       release_sock(sk);
+
+       kfree(old);
+
+       return 0;
+}
+
 /*
  *     This is meant for all protocols to use and covers goings on
  *     at the socket level. Everything here is generic.
@@ -850,6 +868,9 @@ int sock_setsockopt(struct socket *sock, int level, int 
optname,
        if (optname == SO_BINDTODEVICE)
                return sock_setbindtodevice(sk, optval, optlen);
 
+       if (optname == SO_DESCRIPTION)
+               return sock_set_description(sk, optval);
+
        if (optlen < sizeof(int))
                return -EINVAL;
 
@@ -1792,6 +1813,8 @@ static void __sk_destruct(struct rcu_head *head)
                RCU_INIT_POINTER(sk->sk_filter, NULL);
        }
 
+       kfree(sk->sk_description);
+
        sock_disable_timestamp(sk, SK_FLAGS_TIMESTAMP);
 
 #ifdef CONFIG_BPF_SYSCALL
diff --git a/net/socket.c b/net/socket.c
index 976426d03f09..4f2c1a7744b0 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -134,6 +134,11 @@ static void sock_show_fdinfo(struct seq_file *m, struct 
file *f)
 {
        struct socket *sock = f->private_data;
 
+       lock_sock(sock->sk);
+       if (sock->sk->sk_description)
+               seq_printf(m, "desc:\t%s\n", sock->sk->sk_description);
+       release_sock(sock->sk);
+
        if (sock->ops->show_fdinfo)
                sock->ops->show_fdinfo(m, sock);
 }
-- 
2.25.1