正确处理 SSL_shutdown

问题描述

关于 SSL_shutdown 的 OpenSSL 文档指出:

The OpenSSL documentation on SSL_shutdown states that:

因此建议检查SSL_shutdown()的返回值并再次调用SSL_shutdown()，如果双向关闭尚未完成(返回值第一个调用是 0).

It is therefore recommended, to check the return value of SSL_shutdown() and call SSL_shutdown() again, if the bidirectional shutdown is not yet complete (return value of the first call is 0).

https://www.openssl.org/docs/ssl/SSL_shutdown.html

我在下面有一个代码片段，我在其中检查 SSL_shutdown 的返回值 0 并再次调用它，我一直在使用它.我的问题是，是否可以在第二次调用时忽略 SSL_shutdown 的返回值，或者我们应该继续重试 SSL_shutdown 直到返回 1(双向关闭完成).

I have a code snippet below where I check for return value 0 from SSL_shutdown and call it again, which I have been using. My question is, is it okay to disregard the return value of SSL_shutdown on the second call or we should keep retrying SSL_shutdown until a 1 (bidirectional shutdown complete) is returned.

int r = SSL_shutdown(ssl);
//error handling here if r < 0 
if(!r)
{
    shutdown(fd,1);
    SSL_shutdown(ssl); //how should I handle return value and error handling here is it required?? 
}
SSL_free(ssl);
SSLMap.erase(fd);
shutdown(fd,2);
close(fd);

推荐答案

openssl 有点像黑暗艺术.

openssl is a bit of a dark art.

首先，您引用的页面对返回值进行了 HTML 化处理.这是手册页实际上所说的:

Firstly the page you referenced has HTML-ified the return values badly. Here's what the man-page actually says:

  RETURN VALUES

   The following return values can occur:

   0   The shutdown is not yet finished. Call SSL_shutdown() for a second
       time, if a bidirectional shutdown shall be performed.  The output
       of SSL_get_error(3) may be misleading, as an erroneous
       SSL_ERROR_SYSCALL may be flagged even though no error occurred.

   1   The shutdown was successfully completed. The "close notify" alert
       was sent and the peer's "close notify" alert was received.

   -1  The shutdown was not successful because a fatal error occurred
       either at the protocol level or a connection failure occurred. It
       can also occur if action is need to continue the operation for non-
       blocking BIOs.  Call SSL_get_error(3) with the return value ret to
       find out the reason.

如果您有阻塞 BIO，事情就相对简单了.第一次调用时为 0 意味着如果您想要完全双向关闭，则需要再次调用 SSL_shutdown.基本上这意味着您发送了 close_notify 警报但还没有回复).1 表示您之前收到了来自其他对等方的 close_notify 警报，并且您已经完全完成了.-1 表示不可恢复的错误.在第二次调用时(只有在返回 0 时才这样做)，然后启动双向关闭(即现在等待对方向您发送close_notify"警报).逻辑决定你不能再得到一个 0(因为它是一个阻塞的 BIO 并且已经完成了第一步).-1 表示错误，1 表示完成成功.

If you have blocking BIOs, things are relatively simple. A 0 on the first call means you need to call SSL_shutdown again if you want a full bidirectional shutdown. Basically it means that you sent a close_notify alert but haven't one back yet). A 1 would mean you previously received a close_notify alert from the other peer, and you're totally done. A -1 means an unrecoverable error. On the second call (which you only do if you got a 0 back), then a bidirectional shutdown is initiated (i.e. now wait from the other side for them to send you their "close_notify" alert). Logic dictates you can't get a 0 back again (because it's a blocking BIO and will have completed the first step). A -1 indicates an error, and a 1 indicates completion success.

如果你有非阻塞 BIO，同样的可能是 0 然后 1"的返回值适用，除了你需要遍历整个 SSL_ERROR_WANT_READ 和 SSL_ERROR_WANT_WRITE 也很啰嗦，即:

If you have non-blocking BIOs, the same "possibly 0 then 1" return values apply, save for the fact you need to go through the whole SSL_ERROR_WANT_READ and SSL_ERROR_WANT_WRITE rigmarole as well, i.e.:

   If the underlying BIO is non-blocking, SSL_shutdown() will also return
   when the underlying BIO could not satisfy the needs of SSL_shutdown()
   to continue the handshake. In this case a call to SSL_get_error() with
   the return value of SSL_shutdown() will yield SSL_ERROR_WANT_READ or
   SSL_ERROR_WANT_WRITE. The calling process then must repeat the call
   after taking appropriate action to satisfy the needs of SSL_shutdown().
   The action depends on the underlying BIO. When using a non-blocking
   socket, nothing is to be done, but select() can be used to check for
   the required condition. When using a buffering BIO, like a BIO pair,
   data must be written into or retrieved out of the BIO before being able
   to continue.

所以你有两个级别的重复.您在第一次"调用 SSL_shutdown，但如果在绕过 select() 或 SSL_ERROR_WANT_WRITE 后得到 SSL_ERROR_WANT_READ 或 SSL_ERROR_WANT_WRITE，请重复> 以正常方式循环，如果您得到非 SSL_ERROR_WANT_ 错误代码(在这种情况下它失败)，则仅将第一个"SSL_shutdown 视为已完成，或者您得到0 或 1 返回.如果你得到 1 返回，你就完成了.如果您得到 0 返回，并且您想要双向关闭，那么您必须进行第二次调用，再次需要检查 SSL_ERROR_WANT_READ 或 SSL_ERROR_WANT_WRITE 并重试选择；不应返回 1，但可能返回 0 或错误.

So you have two levels of repetition. You call SSL_shutdown the 'first' time but repeat if you get SSL_ERROR_WANT_READ or SSL_ERROR_WANT_WRITE after going around the select() loop in the normal way, and only count the 'first' SSL_shutdown as done if you get a non SSL_ERROR_WANT_ error code (in which case it failed), or you get a 0 or 1 return. If you get a 1 return, you've done. If you get a 0 return, and you want a bidirectional shutdown, then you have to do the second call, on which again you will need to check for SSL_ERROR_WANT_READ or SSL_ERROR_WANT_WRITE and retry select; that should not return 1, but may return 0 or an error.

不简单.

来自 docs 的更多注释:调用 之后SSL_shutdown 并在第一次返回0"时，您可以选择调用 SSL_read 而不是 SSL_shutdown(以防对等方仍在向您发送任何数据在那个 SSL 套接字上)，并且我猜希望"他们最终从他们这边向您发送关闭消息，以刷新管道.

Couple more notes from the docs: after calling SSL_shutdown and getting a "0" back the first time, you could optionally then call SSL_read instead of SSL_shutdown (in case the peer is still sending you any data on that SSL socket), and, I guess, "hope" that they eventually send you a close message from their side, to flush the pipes.

此外，如果您计划在关闭完成后无论如何"关闭套接字，您可以完全跳过对 SSL_shutdown(0 然后 1"的1")的第二次调用，然后只需继续关闭套接字，内核应该注意丢弃现在被忽略"的 close_notify 警报，大概他们应该发送...

Also if you're planning on closing the socket after shutdown completion "anyway" you could entirely skip the second call to SSL_shutdown (the "1" of the "0 then 1") and just go ahead and close the socket, the kernel should take care of discarding the "now ignored" close_notify alert that presumably they should be about to send...

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