[Bug 250701] [tty] [patch] Race condition between tty_wait_background() and doenterpgrp()

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=250701

            Bug ID: 250701
           Summary: [tty] [patch] Race condition between
                    tty_wait_background() and doenterpgrp()
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: j.piecuch96 at gmail.com

Created attachment 219174
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=219174&action=edit
Patch: check for process group change in tty_wait_background()

The calling process's process group can change between PROC_UNLOCK(p) and
PGRP_LOCK(pg) in tty_wait_background(), e.g. by a setpgid() call from another
process. If that happens, the signal is not sent to the calling process, even
if the prior checks determine that one should be sent.
The patch checks that the process group hasn't changed after acquiring the pgrp
lock, and if it has, it redoes the checks.

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