+linux-api, Andy Lutomirski, Eric Biederman
On Wed, Oct 31, 2018 at 3:12 AM Daniel Colascione <dancol@xxxxxxxxxx> 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@xxxxxxxxxx>
> ---
> 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;
> +
> +}
