tcp_bpf: fix return value of tcp_bpf_sendmsg()

When we cork messages in psock->cork, the last message triggers the
flushing will result in sending a sk_msg larger than the current
message size. In this case, in tcp_bpf_send_verdict(), 'copied' becomes
negative at least in the following case:

468         case __SK_DROP:
469         default:
470                 sk_msg_free_partial(sk, msg, tosend);
471                 sk_msg_apply_bytes(psock, tosend);
472                 *copied -= (tosend + delta); // <==== HERE
473                 return -EACCES;

Therefore, it could lead to the following BUG with a proper value of
'copied' (thanks to syzbot). We should not use negative 'copied' as a
return value here.

  ------------[ cut here ]------------
  kernel BUG at net/socket.c:733!
  Internal error: Oops - BUG: 00000000f2000800 [#1] PREEMPT SMP
  Modules linked in:
  CPU: 0 UID: 0 PID: 3265 Comm: syz-executor510 Not tainted 6.11.0-rc3-syzkaller-00060-gd07b43284ab3 #0
  Hardware name: linux,dummy-virt (DT)
  pstate: 61400009 (nZCv daif +PAN -UAO -TCO +DIT -SSBS BTYPE=--)
  pc : sock_sendmsg_nosec net/socket.c:733 [inline]
  pc : sock_sendmsg_nosec net/socket.c:728 [inline]
  pc : __sock_sendmsg+0x5c/0x60 net/socket.c:745
  lr : sock_sendmsg_nosec net/socket.c:730 [inline]
  lr : __sock_sendmsg+0x54/0x60 net/socket.c:745
  sp : ffff800088ea3b30
  x29: ffff800088ea3b30 x28: fbf00000062bc900 x27: 0000000000000000
  x26: ffff800088ea3bc0 x25: ffff800088ea3bc0 x24: 0000000000000000
  x23: f9f00000048dc000 x22: 0000000000000000 x21: ffff800088ea3d90
  x20: f9f00000048dc000 x19: ffff800088ea3d90 x18: 0000000000000001
  x17: 0000000000000000 x16: 0000000000000000 x15: 000000002002ffaf
  x14: 0000000000000000 x13: 0000000000000000 x12: 0000000000000000
  x11: 0000000000000000 x10: ffff8000815849c0 x9 : ffff8000815b49c0
  x8 : 0000000000000000 x7 : 000000000000003f x6 : 0000000000000000
  x5 : 00000000000007e0 x4 : fff07ffffd239000 x3 : fbf00000062bc900
  x2 : 0000000000000000 x1 : 0000000000000000 x0 : 00000000fffffdef
  Call trace:
   sock_sendmsg_nosec net/socket.c:733 [inline]
   __sock_sendmsg+0x5c/0x60 net/socket.c:745
   ____sys_sendmsg+0x274/0x2ac net/socket.c:2597
   ___sys_sendmsg+0xac/0x100 net/socket.c:2651
   __sys_sendmsg+0x84/0xe0 net/socket.c:2680
   __do_sys_sendmsg net/socket.c:2689 [inline]
   __se_sys_sendmsg net/socket.c:2687 [inline]
   __arm64_sys_sendmsg+0x24/0x30 net/socket.c:2687
   __invoke_syscall arch/arm64/kernel/syscall.c:35 [inline]
   invoke_syscall+0x48/0x110 arch/arm64/kernel/syscall.c:49
   el0_svc_common.constprop.0+0x40/0xe0 arch/arm64/kernel/syscall.c:132
   do_el0_svc+0x1c/0x28 arch/arm64/kernel/syscall.c:151
   el0_svc+0x34/0xec arch/arm64/kernel/entry-common.c:712
   el0t_64_sync_handler+0x100/0x12c arch/arm64/kernel/entry-common.c:730
   el0t_64_sync+0x19c/0x1a0 arch/arm64/kernel/entry.S:598
  Code: f9404463 d63f0060 3108441f 54fffe81 (d4210000)
  ---[ end trace 0000000000000000 ]---

Fixes: 4f738ad ("bpf: create tcp_bpf_ulp allowing BPF to monitor socket TX/RX data")
Reported-by: syzbot+58c03971700330ce14d8@syzkaller.appspotmail.com
Cc: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: Cong Wang <cong.wang@bytedance.com>
Reviewed-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20240821030744.320934-1-xiyou.wangcong@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv4/tcp_bpf.c

@@ -577,7 +577,7 @@ static int tcp_bpf_sendmsg(struct sock *sk, struct msghdr *msg, size_t size)
 		err = sk_stream_error(sk, msg->msg_flags, err);
 	release_sock(sk);
 	sk_psock_put(sk, psock);
-	return copied ? copied : err;
+	return copied > 0 ? copied : err;
 }
 
 enum {