On Sat, Jul 22, 2023 at 8:40 AM Askar Safin <safinaskar@xxxxxxxxx> wrote: > shutdown(2) is underdocumented. Here is a lot of more details on > shutdown(2): https://github.com/WebAssembly/WASI/issues/547 . I > discovered them by experiment. So, please, document them > > -- > Askar Safin Documenting the asymmetry is probably a good idea: the TCP protocol only defines the equivalent of shutdown(SHUT_WR) and shutdown(SHUT_RDWR), and there's no natural equivalent of a shutdown(SHUT_RD), so I don't think the semantics themselves can easily be made more symmetric. To expand, the current behavior, where shutdown(SHUT_RD) by itself silently drops incoming data received before a shutdown(SHUT_WR), but replies with a RST to data received after a shutdown(SHUT_WR), is definitely pretty weird, even looking at the relevant RFCs. tcp_rcv_state_process() in net/ipv4/tcp_input.c implements this behavior: a RST is sent back if and only if the connection is in the FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, or LAST-ACK state (i.e., not in the ESTABLISHED state), data is received on the socket, and shutdown(SHUT_RD) has previously been called. The logic is accompanied by the comment: /* * RFC 793 says to queue data in these states, * RFC 1122 says we MUST send a reset. * BSD 4.4 also does reset. */ Looking at RFC 793 Section 3.5, it defines the CLOSE operation in a "simplex fashion": a FIN is sent and further SENDs are no longer allowed, but RECEIVEs are allowed until a FIN is sent from the remote host. This clearly corresponds to the shutdown(SHUT_WR) operation, so it doesn't appear to define any particular behavior for shutdown(SHUT_RD). Instead, the entire justification for this behavior lies in RFC 1122 Section 4.2.2.13: > A host MAY implement a "half-duplex" TCP close sequence, so > that an application that has called CLOSE cannot continue to > read data from the connection. If such a host issues a > CLOSE call while received data is still pending in TCP, or > if new data is received after CLOSE is called, its TCP > SHOULD send a RST to show that data was lost. And in its Discussion: > Some systems have not implemented half-closed > connections, presumably because they do not fit into > the I/O model of their particular operating system. On > these systems, once an application has called CLOSE, it > can no longer read input data from the connection; this > is referred to as a "half-duplex" TCP close sequence. First off, this isn't a MUST but a SHOULD; I don't know where that idea came from. Second off, we reach a bit of a conflict (IMO) between the wording and intent of this clause. It defines the RST behavior only following a CLOSE operation by the application, and a CLOSE still always implies a shutdown(SHUT_WR). So at best, by a strict interpretation, the application can be given a choice between shutdown(SHUT_WR) and shutdown(SHUT_RDWR). Thus, Linux doesn't send any RSTs until after a shutdown(SHUT_WR). However, the whole point here is "to show that data was lost", and silently dropping incoming data prior to a shutdown(SHUT_WR) is clearly contrary to this goal. Clearly, a RST isn't very nice to either host, but neither is lost data. So it seems at least defensible for a TCP implementation to unconditionally reply with a RST to data received after a shutdown(SHUT_RD). (As far as I know, this wouldn't break TCP itself from the remote host's end, since it allows hosts to send a RST whenever they feel like it. Higher-level protocols might be unhappy with it, though.) But of course, the current behavior is ancient, dating back to Linux 2.3.41pre2 from 2000. (Before then, a RST would only be sent after a full close(2).) So there's no changing it at this point in Linux, at least not without an explicit option. I do wonder if there are any other OSes that have a shutdown(SHUT_RD) with different behavior, though. Matthew House
