linux-kernel - Re: [PATCH v2 2/2] pidfd: change pidfd_send_signal() to respect PIDFD

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Date: Fri, 16 Feb 2024 19:12:14 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: Christian Brauner <brauner@...nel.org>
Cc: Andy Lutomirski <luto@...capital.net>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Tycho Andersen <tycho@...ho.pizza>, linux-api@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] pidfd: change pidfd_send_signal() to respect
 PIDFD_THREAD

On 02/16, Christian Brauner wrote:
>
> > SI_USER means that the target can trust the values of si_pid/si_uid
> > in siginfo.
>
> Bah, what an annoying nonsense. I see that this can be used to emulate
> stuff like SI_TIMER and SI_ASYNCIO. But I very much doubt the value of
> e.g., emulating SI_DETHREAD. Maybe I'm missing something very obvious.

I don't understand...

SI_USER/SI_TKILL means that the signal comes from the userspace (kill/etc),
but siginfo was filled by the kernel so the receiver can trust it.

> So wouldn't be better of just writing this as?
>
> if ((task_pid(current) != pid || type > PIDTYPE_TGID) &&
>     (kinfo.si_code >= 0 || kinfo.si_code == SI_TKILL))
>         goto err;
>
> So that we don't have to repeat the same exercise if we extend this to
> anything above PIDTYPE_PGID?

Heh ;)

I swear, this is how I wrote it originally, but then for some reason I
thought it would raise the questions, so I changed it to check PIDTYPE_PGID.

IOW, sure, I agree.

Oleg.