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Subject: Re: [PATCH] proc: allow killing processes via file descriptors
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <CAKOZuetfqvn1uVqKJ=16iEzG4g449YOjC_tLM60eKBSkv9u+bQ@mail.gmail.com>
Date:   Sun, 18 Nov 2018 13:28:41 -0700
Cc:     Aleksa Sarai <cyphar@cyphar.com>,
        Andy Lutomirski <luto@kernel.org>,
        Randy Dunlap <rdunlap@infradead.org>,
        Christian Brauner <christian@brauner.io>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        LKML <linux-kernel@vger.kernel.org>,
        "Serge E. Hallyn" <serge@hallyn.com>, Jann Horn <jannh@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Oleg Nesterov <oleg@redhat.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Linux FS Devel <linux-fsdevel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        Tim Murray <timmurray@google.com>,
        Kees Cook <keescook@chromium.org>,
        Jan Engelhardt <jengelh@inai.de>
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References: <CAKOZuev1JUGFWuwsKqS6rXcFMqpCHT1VAG2kwB4O=FHo6DAFiQ@mail.gmail.com> <CALCETrVLP_mudJTW6EQpRr5GZ7kfuGci+QCT1uPrOVDTWcod-A@mail.gmail.com> <a7f50692-667c-4efe-a2d0-fa324eebb90b@infradead.org> <CAKOZueutLc8d0Fe+7dNWiZKnALhTSST8+kCnOrL+OmB6coUmtA@mail.gmail.com> <CALCETrVg71XBv-gMOtL-m0Dd0HNz8_oXOUDSWin5LeViAL0UYA@mail.gmail.com> <CAKOZuesCKo4GH9fdum2EUFLrtTWam3aizcDQUn3-vCYg4T1P8w@mail.gmail.com> <CALCETrUeNZPfrSYa9vH5Ukrk1Y+Kb9GkZOh6LkqG6Z9NpK5P0w@mail.gmail.com> <CAKOZuevVk_aH_2TuiNcmxgMa+gHXMBXz6Uu5a6TDjoxjhaE36g@mail.gmail.com> <CALCETrVscRwQG55-j1SKc2TmSb1-=5861804ojUuviNzdyDOrA@mail.gmail.com> <CAKOZuevRq-igh06zS_nsGG400zXrKFCtajpEG9-xgU2+Rtb2Pw@mail.gmail.com> <20181118190504.ixglsqbn6mxkcdzu@yavin> <CAKOZuetfqvn1uVqKJ=16iEzG4g449YOjC_tLM60eKBSkv9u+bQ@mail.gmail.com>
To:     Daniel Colascione <dancol@google.com>
Sender: linux-kernel-owner@vger.kernel.org
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> On Nov 18, 2018, at 12:44 PM, Daniel Colascione <dancol@google.com> wrote:=

>=20

>=20
> That is, I'm proposing an API that looks like this:
>=20
> int process_kill(int procfs_dfd, int signo, const union sigval value)
>=20
> If, later, process_kill were to *also* accept process-capability FDs,
> nothing would break.

Except that this makes it ambiguous to the caller as to whether their curren=
t creds are considered.  So it would need to be a different syscall or at le=
ast a flag.  Otherwise a lot of those nice theoretical properties go away.

> Yes, that's what I have in mind. A siginfo_t is small enough that we
> could just store it as a blob allocated off the procfs inode or
> something like that without bothering with a shmfs file. You'd be able
> to read(2) the exit status as many times as you wanted.

I think that, if the syscall in question is read(2), then it should work *on=
ce* per struct file.  Otherwise running cat on the file would behave very od=
dly.

Read and poll have the same problem as write: we can=E2=80=99t check caps in=
 read or poll either.=