job control: introduce JOBCTL_TRAP_STOP and use it for group stop trap

do_signal_stop() implemented both normal group stop and trap for group
stop while ptraced.  This approach has been enough but scheduled
changes require trap mechanism which can be used in more generic
manner and using group stop trap for generic trap site simplifies both
userland visible interface and implementation.

This patch adds a new jobctl flag - JOBCTL_TRAP_STOP.  When set, it
triggers a trap site, which behaves like group stop trap, in
get_signal_to_deliver() after checking for pending signals.  While
ptraced, do_signal_stop() doesn't stop itself.  It initiates group
stop if requested and schedules JOBCTL_TRAP_STOP and returns.  The
caller - get_signal_to_deliver() - is responsible for checking whether
TRAP_STOP is pending afterwards and handling it.

ptrace_attach() is updated to use JOBCTL_TRAP_STOP instead of
JOBCTL_STOP_PENDING and __ptrace_unlink() to clear all pending trap
bits and TRAPPING so that TRAP_STOP and future trap bits don't linger
after detach.

While at it, add proper function comment to do_signal_stop() and make
it return bool.

-v2: __ptrace_unlink() updated to clear JOBCTL_TRAP_MASK and TRAPPING
     instead of JOBCTL_PENDING_MASK.  This avoids accidentally
     clearing JOBCTL_STOP_CONSUME.  Spotted by Oleg.

-v3: do_signal_stop() updated to return %false without dropping
     siglock while ptraced and TRAP_STOP check moved inside for(;;)
     loop after group stop participation.  This avoids unnecessary
     relocking and also will help avoiding unnecessary traps by
     consuming group stop before handling pending traps.

-v4: Jobctl trap handling moved into a separate function -
     do_jobctl_trap().

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>

1 files changed, 9 insertions, 3 deletions

diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 7f05f3a1267b..45a8a4c5d8b2 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -83,6 +83,13 @@ void __ptrace_unlink(struct task_struct *child)
         spin_lock(&child->sighand->siglock);
 
         /*
+         * Clear all pending traps and TRAPPING.  TRAPPING should be
+         * cleared regardless of JOBCTL_STOP_PENDING.  Do it explicitly.
+         */
+        task_clear_jobctl_pending(child, JOBCTL_TRAP_MASK);
+        task_clear_jobctl_trapping(child);
+
+        /*
          * Reinstate JOBCTL_STOP_PENDING if group stop is in effect and
          * @child isn't dead.
          */
@@ -246,7 +253,7 @@ static int ptrace_attach(struct task_struct *task)
         spin_lock(&task->sighand->siglock);
 
         /*
-         * If the task is already STOPPED, set JOBCTL_STOP_PENDING and
+         * If the task is already STOPPED, set JOBCTL_TRAP_STOP and
          * TRAPPING, and kick it so that it transits to TRACED.  TRAPPING
          * will be cleared if the child completes the transition or any
          * event which clears the group stop states happens.  We'll wait
@@ -263,8 +270,7 @@ static int ptrace_attach(struct task_struct *task)
          * in and out of STOPPED are protected by siglock.
          */
         if (task_is_stopped(task) &&
-            task_set_jobctl_pending(task,
-                                    JOBCTL_STOP_PENDING | JOBCTL_TRAPPING))
+            task_set_jobctl_pending(task, JOBCTL_TRAP_STOP | JOBCTL_TRAPPING))
                 signal_wake_up(task, 1);
 
         spin_unlock(&task->sighand->siglock);