net/9p: avoid -ERESTARTSYS leak to userspace

If it was interrupted by a signal, the 9p client may need to send some
more requests to the server for cleanup before returning to userspace.

To avoid such a last minute request to be interrupted right away, the
client memorizes if a signal is pending, clears TIF_SIGPENDING, handles
the request and calls recalc_sigpending() before returning.

Unfortunately, if the transmission of this cleanup request fails for any
reason, the transport returns an error and the client propagates it
right away, without calling recalc_sigpending().

This ends up with -ERESTARTSYS from the initially interrupted request
crawling up to syscall exit, with TIF_SIGPENDING cleared by the cleanup
request.  The specific signal handling code, which is responsible for
converting -ERESTARTSYS to -EINTR is not called, and userspace receives
the confusing errno value:

  open: Unknown error 512 (512)

This is really hard to hit in real life.  I discovered the issue while
working on hot-unplug of a virtio-9p-pci device with an instrumented
QEMU allowing to control request completion.

Both p9_client_zc_rpc() and p9_client_rpc() functions have this buggy
error path actually.  Their code flow is a bit obscure and the best
thing to do would probably be a full rewrite: to really ensure this
situation of clearing TIF_SIGPENDING and returning -ERESTARTSYS can
never happen.

But given the general lack of interest for the 9p code, I won't risk
breaking more things.  So this patch simply fixes the buggy paths in
both functions with a trivial label+goto.

Thanks to Laurent Dufour for his help and suggestions on how to find the
root cause and how to fix it.

Link: http://lkml.kernel.org/r/152062809886.10599.7361006774123053312.stgit@bahia.lan
Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Yiwen Jiang <jiangyiwen@huawei.com>
Cc: Eric Van Hensbergen <ericvh@gmail.com>
Cc: Ron Minnich <rminnich@sandia.gov>
Cc: Latchesar Ionkov <lucho@ionkov.net>
Cc: David Miller <davem@davemloft.net>
Cc: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

net/9p/client.c

@@ -769,7 +769,7 @@ p9_client_rpc(struct p9_client *c, int8_t type, const char *fmt, ...)
 	if (err < 0) {
 		if (err != -ERESTARTSYS && err != -EFAULT)
 			c->status = Disconnected;
-		goto reterr;
+		goto recalc_sigpending;
 	}
 again:
 	/* Wait for the response */
@@ -804,6 +804,7 @@ again:
 		if (req->status == REQ_STATUS_RCVD)
 			err = 0;
 	}
+recalc_sigpending:
 	if (sigpending) {
 		spin_lock_irqsave(&current->sighand->siglock, flags);
 		recalc_sigpending();
@@ -867,7 +868,7 @@ static struct p9_req_t *p9_client_zc_rpc(struct p9_client *c, int8_t type,
 		if (err == -EIO)
 			c->status = Disconnected;
 		if (err != -ERESTARTSYS)
-			goto reterr;
+			goto recalc_sigpending;
 	}
 	if (req->status == REQ_STATUS_ERROR) {
 		p9_debug(P9_DEBUG_ERROR, "req_status error %d\n", req->t_err);
@@ -885,6 +886,7 @@ static struct p9_req_t *p9_client_zc_rpc(struct p9_client *c, int8_t type,
 		if (req->status == REQ_STATUS_RCVD)
 			err = 0;
 	}
+recalc_sigpending:
 	if (sigpending) {
 		spin_lock_irqsave(&current->sighand->siglock, flags);
 		recalc_sigpending();