[PATCH 2/3] signal: Guarantee that SIGNAL_GROUP_EXIT is set on process exit

From: Eric W. Biederman
Date: Wed Jun 22 2022 - 12:47:52 EST



Track how many threads have not started exiting and when the last
thread starts exiting set SIGNAL_GROUP_EXIT.

This guarantees that SIGNAL_GROUP_EXIT will get set when a process
exits. In practice this achieves nothing as glibc's implementation of
_exit calls sys_group_exit then sys_exit. While glibc's implemenation
of pthread_exit calls exit (which cleansup and calls _exit) if it is
the last thread and sys_exit if it is the last thread.

This means the only way the kernel might observe a process that does
not set call exit_group is if the language runtime does not use glibc.

With more cleanups I hope to move the decrement of quick_threads
earlier.

Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
---
include/linux/sched/signal.h | 1 +
kernel/exit.c | 18 ++++++++++++++++++
kernel/fork.c | 2 ++
3 files changed, 21 insertions(+)

diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
index cafbe03eed01..20099268fa25 100644
--- a/include/linux/sched/signal.h
+++ b/include/linux/sched/signal.h
@@ -94,6 +94,7 @@ struct signal_struct {
refcount_t sigcnt;
atomic_t live;
int nr_threads;
+ int quick_threads;
struct list_head thread_head;

wait_queue_head_t wait_chldexit; /* for wait4() */
diff --git a/kernel/exit.c b/kernel/exit.c
index 96e4b12edea8..beaedb867bd3 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -733,11 +733,29 @@ static void check_stack_usage(void)
static inline void check_stack_usage(void) {}
#endif

+static void synchronize_group_exit(struct task_struct *tsk, long code)
+{
+ struct sighand_struct *sighand = tsk->sighand;
+ struct signal_struct *signal = tsk->signal;
+
+ spin_lock_irq(&sighand->siglock);
+ signal->quick_threads--;
+ if ((signal->quick_threads == 0) &&
+ !(signal->flags & SIGNAL_GROUP_EXIT)) {
+ signal->flags = SIGNAL_GROUP_EXIT;
+ signal->group_exit_code = code;
+ signal->group_stop_count = 0;
+ }
+ spin_unlock_irq(&sighand->siglock);
+}
+
void __noreturn do_exit(long code)
{
struct task_struct *tsk = current;
int group_dead;

+ synchronize_group_exit(tsk, code);
+
WARN_ON(tsk->plug);

kcov_task_exit(tsk);
diff --git a/kernel/fork.c b/kernel/fork.c
index 9d44f2d46c69..67813b25a567 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1692,6 +1692,7 @@ static int copy_signal(unsigned long clone_flags, struct task_struct *tsk)
return -ENOMEM;

sig->nr_threads = 1;
+ sig->quick_threads = 1;
atomic_set(&sig->live, 1);
refcount_set(&sig->sigcnt, 1);

@@ -2444,6 +2445,7 @@ static __latent_entropy struct task_struct *copy_process(
__this_cpu_inc(process_counts);
} else {
current->signal->nr_threads++;
+ current->signal->quick_threads++;
atomic_inc(&current->signal->live);
refcount_inc(&current->signal->sigcnt);
task_join_group_stop(p);
--
2.35.3