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SubjectPtrace updates: has_stopped_jobs [1/5]
I posted this patch a couple of weeks ago, here it is again.  The basic
problem it's solving is this: suppose you have GDB debugging an application,
using the new PTRACE_EVENT_FORK. The application forks. There can be a
point where GDB resumes the parent, which gets scheduled, and exits, before
GDB gets a chance to resume the child. has_stopped_jobs causes the kernel
to decide the still-stopped-in-GDB child (which will be an orphaned pgrp) is
going to be lost, so it sends a SIGHUP and SIGCONT to it, confusing GDB
terribly. Normally those signals wouldn't be sent.


The longer explanation:

POSIX has this terribly useful thing to say:

# If the exit of the process causes a process group to become orphaned, and if
# any member of the newly-orphaned process group is stopped, then a SIGHUP
# signal followed by a SIGCONT signal shall be sent to each process in the
# newly-orphaned process group.

The Rationale is at least a little chattier. See

http://www.opengroup.org/onlinepubs/007904975/functions/exit.html#tag_03_131
if you want to read it.

Basically, this is so that a stopped process group won't unintentionally
stay stopped when its shell no longer has a connection to it. For whatever
that's worth.

I think this patch is well within the spirit of that definition. If a
process is stopped, but there is a debugger attached to it and the stopping
signal is not one that would normally stop the process, then don't count it
as a stopped job. Without this, when you continue past a fork() call and
the parent quickly exits, the child will get an unaccountable SIGHUP.

It's not perfect, of course - the application might be SIG_IGN'ing SIGTSTP,
but stopped in the debugger for it anyway. It's not worth being that
complicated here, though.


# --------------------------------------------
# 03/01/18 drow@nevyn.them.org 1.957
# Tweak has_stopped_jobs for use with debugging
# --------------------------------------------

diff -Nru a/kernel/exit.c b/kernel/exit.c
--- a/kernel/exit.c Thu Feb 6 16:57:39 2003
+++ b/kernel/exit.c Thu Feb 6 16:57:39 2003
@@ -203,6 +203,17 @@
for_each_task_pid(pgrp, PIDTYPE_PGID, p, l, pid) {
if (p->state != TASK_STOPPED)
continue;
+
+ /* If p is stopped by a debugger on a signal that won't
+ stop it, then don't count p as stopped. This isn't
+ perfect but it's a good approximation. */
+ if (unlikely (p->ptrace)
+ && p->exit_code != SIGSTOP
+ && p->exit_code != SIGTSTP
+ && p->exit_code != SIGTTOU
+ && p->exit_code != SIGTTIN)
+ continue;
+
retval = 1;
break;
}
--
Daniel Jacobowitz
MontaVista Software Debian GNU/Linux Developer
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