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signal: Guarantee that SIGNAL_GROUP_EXIT is set on process exit

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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.

Link: https://lkml.kernel.org/r/87bkukd4tc.fsf_-_@email.froward.int.ebiederm.org
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
This commit is contained in:
Eric W. Biederman 2022-06-21 14:38:52 -05:00
parent cbe9dac379
commit d80f7d7b2c
3 changed files with 21 additions and 0 deletions
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1
include/linux/sched/signal.h
View File

@ -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() */

18
kernel/exit.c
View File

@ -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);

2
kernel/fork.c
View File

@ -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);