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Date: Tue, 20 Feb 2024 10:02:56 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Christian Brauner <brauner@kernel.org>
Cc: Andy Lutomirski <luto@amacapital.net>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	Tycho Andersen <tycho@tycho.pizza>, linux-api@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 2/2] pidfd: change pidfd_send_signal() to respect
 PIDFD_THREAD
Message-ID: <20240220090255.GA7783@redhat.com>
References: <20240210-dackel-getan-619c70fefa62@brauner>
 <20240210131518.GC27557@redhat.com>
 <20240210-chihuahua-hinzog-3945b6abd44a@brauner>
 <20240210165133.GD27557@redhat.com>
 <20240214123655.GB16265@redhat.com>
 <20240216-albern-aufwiegen-1de327c7dafd@brauner>
 <20240216130625.GA8723@redhat.com>
 <20240216-ohnedies-improvisieren-58edcc102b6a@brauner>
 <20240216181214.GA10393@redhat.com>
 <20240220-einwurf-depesche-d8682be0370c@brauner>
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On 02/20, Christian Brauner wrote:
>
> On Fri, Feb 16, 2024 at 07:12:14PM +0100, Oleg Nesterov wrote:
> > 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...
>
> My question was what the purpose of being able to to set si_code to
> e.g., SI_DETHREAD is and then to send a signal to yourself? Because it
> looks like that's what rt_{tg}sigqueueinfo() and pidfd_send_signal()
> allows the caller to do. I'm just trying to understand use-cases for
> this.

Ah. IIRC criu uses this hack to restore the pending (arbitrary) signals
collected at dump time.

I was a bit surprise sys_pidfd_send_signal() allows this hack too, I don't
think that criu uses pidfd at restore time, but I do not know.

Oleg.