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SubjectRe: [PATCH 6/9] signal: Fold do_group_exit into get_signal fixing io_uring threads
Kees Cook <keescook@chromium.org> writes:

> On Thu, Jun 24, 2021 at 02:02:16PM -0500, Eric W. Biederman wrote:
>>
>> Forld do_group_exit into get_signal as it is the last caller.
>>
>> Move the group_exit logic above the PF_IO_WORKER exit, ensuring
>> that if an PF_IO_WORKER catches SIGKILL every thread in
>> the thread group will exit not just the the PF_IO_WORKER.
>>
>> Now that the information is easily available only set PF_SIGNALED
>> when it was a signal that caused the exit.
>>
>> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
>> ---
>> include/linux/sched/task.h | 1 -
>> kernel/exit.c | 31 -------------------------------
>> kernel/signal.c | 35 +++++++++++++++++++++++++----------
>> 3 files changed, 25 insertions(+), 42 deletions(-)
>>
>> diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
>> index ef02be869cf2..45525512e3d0 100644
>> --- a/include/linux/sched/task.h
>> +++ b/include/linux/sched/task.h
>> @@ -77,7 +77,6 @@ static inline void exit_thread(struct task_struct *tsk)
>> {
>> }
>> #endif
>> -extern void do_group_exit(int);
>>
>> extern void exit_files(struct task_struct *);
>> extern void exit_itimers(struct signal_struct *);
>> diff --git a/kernel/exit.c b/kernel/exit.c
>> index 921519d80b56..635f434122b7 100644
>> --- a/kernel/exit.c
>> +++ b/kernel/exit.c
>> @@ -892,37 +892,6 @@ SYSCALL_DEFINE1(exit, int, error_code)
>> do_exit((error_code&0xff)<<8);
>> }
>>
>> -/*
>> - * Take down every thread in the group. This is called by fatal signals
>> - * as well as by sys_exit_group (below).
>> - */
>> -void
>> -do_group_exit(int exit_code)
>> -{
>> - struct signal_struct *sig = current->signal;
>> -
>> - BUG_ON(exit_code & 0x80); /* core dumps don't get here */
>> -
>> - if (signal_group_exit(sig))
>> - exit_code = sig->group_exit_code;
>> - else if (!thread_group_empty(current)) {
>> - struct sighand_struct *const sighand = current->sighand;
>> -
>> - spin_lock_irq(&sighand->siglock);
>> - if (signal_group_exit(sig))
>> - /* Another thread got here before we took the lock. */
>> - exit_code = sig->group_exit_code;
>> - else {
>> - sig->group_exit_code = exit_code;
>> - sig->flags = SIGNAL_GROUP_EXIT;
>> - zap_other_threads(current);
>
> Oh, now I see it: the "new code" in start_group_exit() is an open-coded
> zap_other_threads()? That wasn't clear to me, but makes sense now.

Pretty much. I think zap_other_threads has actually muddied the waters
quite a bit by putting reuse in the wrong place.

>> - }
>> - spin_unlock_irq(&sighand->siglock);
>> - }
>> -
>> - do_exit(exit_code);
>> - /* NOTREACHED */
>> -}
>>
>> /*
>> * this kills every thread in the thread group. Note that any externally
>> diff --git a/kernel/signal.c b/kernel/signal.c
>> index c79c010ca5f3..95a076af600a 100644
>> --- a/kernel/signal.c
>> +++ b/kernel/signal.c
>> @@ -2646,6 +2646,7 @@ bool get_signal(struct ksignal *ksig)
>> {
>> struct sighand_struct *sighand = current->sighand;
>> struct signal_struct *signal = current->signal;
>> + int exit_code;
>> int signr;
>>
>> if (unlikely(current->task_works))
>> @@ -2848,8 +2849,6 @@ bool get_signal(struct ksignal *ksig)
>> /*
>> * Anything else is fatal, maybe with a core dump.
>> */
>> - current->flags |= PF_SIGNALED;
>> -
>> if (sig_kernel_coredump(signr)) {
>> if (print_fatal_signals)
>> print_fatal_signal(ksig->info.si_signo);
>> @@ -2857,14 +2856,33 @@ bool get_signal(struct ksignal *ksig)
>> /*
>> * If it was able to dump core, this kills all
>> * other threads in the group and synchronizes with
>> - * their demise. If we lost the race with another
>> - * thread getting here, it set group_exit_code
>> - * first and our do_group_exit call below will use
>> - * that value and ignore the one we pass it.
>> + * their demise. If another thread makes it
>> + * to do_coredump first, it will set group_exit_code
>> + * which will be passed to do_exit.
>> */
>> do_coredump(&ksig->info);
>> }
>>
>> + /*
>> + * Death signals, no core dump.
>> + */
>> + exit_code = signr;
>> + if (signal_group_exit(signal)) {
>> + exit_code = signal->group_exit_code;
>> + } else {
>> + spin_lock_irq(&sighand->siglock);
>> + if (signal_group_exit(signal)) {
>> + /* Another thread got here before we took the lock. */
>> + exit_code = signal->group_exit_code;
>> + } else {
>> + start_group_exit_locked(signal, exit_code);
>
> And here's the "if we didn't already do start_group_exit(), do it here".
> And that state is entirely captured via the SIGNAL_GROUP_EXIT flag.
> Cool.

Yes. At least when the dust clears.

>> + }
>> + spin_unlock_irq(&sighand->siglock);
>> + }
>> +
>> + if (exit_code & 0x7f)
>> + current->flags |= PF_SIGNALED;
>> +
>> /*
>> * PF_IO_WORKER threads will catch and exit on fatal signals
>> * themselves. They have cleanup that must be performed, so
>> @@ -2873,10 +2891,7 @@ bool get_signal(struct ksignal *ksig)
>> if (current->flags & PF_IO_WORKER)
>> goto out;
>>
>> - /*
>> - * Death signals, no core dump.
>> - */
>> - do_group_exit(ksig->info.si_signo);
>> + do_exit(exit_code);
>> /* NOTREACHED */
>> }
>> spin_unlock_irq(&sighand->siglock);
>> --
>> 2.20.1
>>

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