[Bug 204812] pty read blocks after select, if ^S is written in between

bugzilla-noreply at freebsd.org bugzilla-noreply at freebsd.org
Wed Nov 25 17:34:27 UTC 2015


            Bug ID: 204812
           Summary: pty read blocks after select, if ^S is written in
           Product: Base System
           Version: 10.2-RELEASE
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: cgull at glup.org

The pts driver has an issue with breaking the promise made by select() that a
read will not block, if there is a write of ^S to the pty between the select()
and the read().

The reproduction for this is very simple:

  script /dev/null yes

Type ^S.

Type ^Q.  Nothing happens, because script is hung on a blocking read.  The read
will never complete, because script is hung and your ^Q will never be

I originally discovered this while working on mosh (net/mosh), but script
blocks in the same way.

The sequence of events that causes this:

* The pty slave has software flow control enabled.

* select() returns read status for the pty master fd.

* Something write()s ^S to the pty master.

* A read() is started on the pty master, with the expectation that it will not
block.  But it does block, and since the tty is stopped, it will never unblock
without outside intervention.

I hesitate to call this a bug, because the state of the pty *is* changed by
that write, and the fix for this issue is not clear.  But it violates POLA,
other kernels (Linux, OpenBSD) do not behave this way, and the consequences are

I see this issue on FreeBSD 10.2, 9.1, and 7.2.  An old 4.10 box doesn't have
this issue.

I originally discovered this bug while working on Mosh.  It's not a new bug
there.  I've now implemented pty packet mode, and the problem exists there too,
even though packet mode should always be able to avoid breaking this promise. 


I don't understand why nobody has run into this issue before though, either on
the FreeBSD or on the Mosh side.  (Mosh has this problem on FreeBSD and OS X.)

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