On Tue, 27 Nov 2018 10:15:55 +0000 Slavomir Kaslev <kaslevs@xxxxxxxxxx> wrote: > > I don't see anything too wrong with it, accept the following test broke: > > > > $ git checkout trace-cmd-v2.6 > > $ make > > $ sudo cp trace-cmd /usr/local/bin/trace-cmd-v2.6 > > $ git checkout origin/master > > $ patch -p1 < this.patch > > $ make > > $ trace-cmd-v2.6 listen -p 12345 > > > > In another terminal: > > > > $ sudo trace-cmd record -N 127.0.0.1:12345 -e sched sleep 1 > > trace-cmd: No such file or directory > > Cannot handle the protocol > > > > > > Remember, we need to remain backward compatible. We also need to test > > this code running as a listener, and the trace-cmd-v2.6 (and earlier) > > as the recorder. > > This is a design bug (the best kind), metadata should really be written from the > recording side and not from the listener. A backward compatible fix should have > the newer recorder and listener detect they're talking to an older version and > fallback to broken behavior. This implies a new protocol version or extending > MSG_TINIT/MSG_RINIT so that we can infer the behavior on the other side and > fallback to being broken when necessary. Agreed. > > What would you suggest? I just want to stress that I feel as strong for backward compatibility as Linus feels for not breaking user space. That is, I'll go without a fix if it breaks backward compatibility. But the really good news is, your code didn't break backward compatibility. It uncovered a bug :-) :-) :-) The failure of the old code is that it looked at buf[0] without initializing it. The fix is this: diff --git a/tracecmd/trace-record.c b/tracecmd/trace-record.c index e1e2f433..143793da 100644 --- a/tracecmd/trace-record.c +++ b/tracecmd/trace-record.c @@ -2775,6 +2775,8 @@ static void check_protocol_version(struct tracecmd_msg_handle *msg_handle) */ write(fd, V2_CPU, sizeof(V2_CPU)); + buf[0] = 0; + /* read a reply message */ n = read(fd, buf, BUFSIZ); That said, I have one minor nit with the patch. I'll reply to the patch itself. Thanks! -- Steve
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