linux-kernel - Re: API for changing UIDs of other processes

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Date:	Fri, 2 May 2008 16:07:46 +0200 (CEST)
From:	Jan Engelhardt <jengelh@...ozas.de>
To:	Enrico Weigelt <weigelt@...ux.de>
cc:	linux kernel list <linux-kernel@...r.kernel.org>
Subject: Re: API for changing UIDs of other processes


On Friday 2008-05-02 14:16, Enrico Weigelt wrote:
>Hi folks,
>
>
>I'd like to build some authentication daemon which alters the
>privileges of another process (like factotum does on plan9).
>But I couldn't find any interface for that (setuid() and friends
>only operate on the current process). So I'm now going to create
>my own interface.
>
>As a little learning example I just added a few files to the
>per-pid dirs: uid, euid, suid, fsuid. This was trivial :)
>
>Now I'd like to add an write capability to these files: 
>simply writing another number changes the (|s|fs)uid.
>
>But this doesnt seem that trivial. Perhaps someone could give
>me some advice ?

Not really hard, is it? Just look at something like oom_adj

static ssize_t myprocpid_euid_write(struct file *file,
    const char __user *inbuf, size_t size)
{
	struct task_struct *task;
	char buf[sizeof("4294967296")];
	unsigned int amount = min(size, sizeof(buf) - 1);

	if (copy_from_user(buf, inbuf, amount) != 0)
		return -EFAULT;
	buf[amount] = '\0';

	task = get_proc_task(file->f_path.dentry->d_inode);
	if (task == NULL)
		return -ESRCH;

	/* do error checking */
	task->euid = simple_strtoul(buf, NULL, 0);
	put_task_struct(task);
	return size;
}

So far the theory..
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