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From:   Jann Horn <jannh@google.com>
Date:   Wed, 31 Oct 2018 17:22:49 +0100
Message-ID: <CAG48ez01OWYVqEo+Qf65scG7a-48tDM4Gh3BOUKY+-b9g+9itw@mail.gmail.com>
Subject: Re: [RFC PATCH] Implement /proc/pid/kill
To:     dancol@google.com
Cc:     kernel list <linux-kernel@vger.kernel.org>, timmurray@google.com,
        Joel Fernandes <joelaf@google.com>, surenb@google.com,
        Andy Lutomirski <luto@kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        "Eric W. Biederman" <ebiederm@xmission.com>
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+linux-api, Andy Lutomirski, Eric Biederman

On Wed, Oct 31, 2018 at 3:12 AM Daniel Colascione <dancol@google.com> wrote:
> Add a simple proc-based kill interface. To use /proc/pid/kill, just
> write the signal number in base-10 ASCII to the kill file of the
> process to be killed: for example, 'echo 9 > /proc/$$/kill'.

This is a kernel API change, you should CC the linux-api list.

I think that getting the semantics of this right might be easier if
you used an ioctl handler instead of a write handler.

> Semantically, /proc/pid/kill works like kill(2), except that the
> process ID comes from the proc filesystem context instead of from an
> explicit system call parameter. This way, it's possible to avoid races
> between inspecting some aspect of a process and that process's PID
> being reused for some other process.
>
> With /proc/pid/kill, it's possible to write a proper race-free and
> safe pkill(1). An approximation follows. A real program might use
> openat(2), having opened a process's /proc/pid directory explicitly,
> with the directory file descriptor serving as a sort of "process
> handle".
>
>     #!/bin/bash
>     set -euo pipefail
>     pat=$1
>     for proc_status in /proc/*/status; do (
>         cd $(dirname $proc_status)
>         readarray proc_argv -d'' < cmdline
>         if ((${#proc_argv[@]} > 0)) &&
>                [[ ${proc_argv[0]} = *$pat* ]];
>         then
>             echo 15 > kill
>         fi
>     ) || true; done
>
> Signed-off-by: Daniel Colascione <dancol@google.com>
> ---
>  fs/proc/base.c | 39 +++++++++++++++++++++++++++++++++++++++
>  1 file changed, 39 insertions(+)
>
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index 7e9f07bf260d..923d62b21e67 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -205,6 +205,44 @@ static int proc_root_link(struct dentry *dentry, struct path *path)
>         return result;
>  }
>
> +static ssize_t proc_pid_kill_write(struct file *file,
> +                                  const char __user *buf,
> +                                  size_t count, loff_t *ppos)
> +{
> +       ssize_t res;
> +       int sig;
> +       char buffer[4];
> +
> +       res = -EINVAL;
> +       if (*ppos != 0)
> +               goto out;
> +
> +       res = -EINVAL;
> +       if (count > sizeof(buffer) - 1)
> +               goto out;
> +
> +       res = -EFAULT;
> +       if (copy_from_user(buffer, buf, count))
> +               goto out;
> +
> +       buffer[count] = '\0';
> +       res = kstrtoint(strstrip(buffer), 10, &sig);
> +       if (res)
> +               goto out;
> +
> +       res = kill_pid(proc_pid(file_inode(file)), sig, 0);
> +       if (res)
> +               goto out;
> +       res = count;
> +out:
> +       return res;
> +
> +}