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From:   Daniel Colascione <dancol@google.com>
Date:   Sun, 18 Nov 2018 12:32:56 -0800
Message-ID: <CAKOZuet4uzYjvNznfUvid2RH8kAuxteWWc26vLhJHKSfS6MjAA@mail.gmail.com>
Subject: Re: [PATCH] proc: allow killing processes via file descriptors
To:     Andy Lutomirski <luto@amacapital.net>
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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On Sun, Nov 18, 2018 at 12:28 PM, Andy Lutomirski <luto@amacapital.net> wro=
te:
>> That is, I'm proposing an API that looks like this:
>>
>> int process_kill(int procfs_dfd, int signo, const union sigval value)
>>
>> 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 cur=
rent creds are considered.  So it would need to be a different syscall or a=
t least a flag.  Otherwise a lot of those nice theoretical properties go aw=
ay.

Sure. A flag might make for better ergonomics.

>> 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 =
*once* per struct file.  Otherwise running cat on the file would behave ver=
y oddly.

Why? The file pointer would work normally.

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

Why not? Reading /proc/pid/stat does an access check today and
conditionally replaces the exit status with zero.