Re: [PATCH] proc: augment /proc/pid/limits to allow setting of process limits.

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 28 Sep 2009 16:06:00 -0400
Neil Horman <nhorman@tuxdriver.com> wrote:

> Augment /proc/<pid>/limits file to support limit setting
> 
> It was suggested to me recently that we support a mechanism by which we can set
> various process limits from points external to the process.  The reasoning being
> that some processes are very long lived, and it would be beneficial to these
> long lived processes if we could modify their various limits without needing to
> kill them, adjust the limits for the user and restarting them.  While individual
> application can certainly export this control on their own, it would be nice if
> such functionality were available to a sysadmin, without needing to have each
> application re-invent the wheel.
> 
> As such, I've implemented the below patch, which makes /proc/pid/limits writable
> for each process.  By writing the following format:
> <limit> <current value> <max value>
> to the limits file, an administrator can now dynamically change the limits for
> the respective process.  Tested by myself with good results.
> 

Confused.  This appears to allow processes to cheerily exceed their
inherited limits, without bound.  See sys_setrliit()'s

	if (new_rlim.rlim_cur > new_rlim.rlim_max)
		return -EINVAL;

It might allow user A to diddle user B's limit too, I didn't check?

And it cheerily avoids security_task_setrlimit() too.

Apart from those somewhat fatal problems, it's all a bit unpleasing that
we now have two ways of setting rlimits, one of which is a superset of
the other.  Perhaps a better way would be a new sys_setrlimit2() which
takes a pid (in the current pid namespace, one assumes).  Then deprecate
sys_setrlimit().

>
> ...
>
> +static ssize_t proc_pid_limit_write(struct file *file, const char __user *buf,
> +		size_t count, loff_t *ppos)
> +{
> +	char *buffer;
> +	char *element, *vmc, *vmm;
> +	unsigned long long valuec, valuem;
> +	unsigned long flags;
> +	int i;
> +	int index = -1;
> +	size_t wcount = 0;
> +	struct task_struct *task = get_proc_task(file->f_path.dentry->d_inode);
> +
> +
> +	if (*ppos != 0)
> +		goto out;
> +
> +	if (count > 128)
> +		goto out;
> +	buffer = kzalloc(128, GFP_KERNEL);
> +
> +	if (!buffer)
> +		goto out;
> +
> +	element = kzalloc(sizeof(buffer), GFP_KERNEL);
> +	vmc = kzalloc(sizeof(buffer), GFP_KERNEL);
> +	vmm = kzalloc(sizeof(buffer), GFP_KERNEL);
> +
> +	if (!element || !vmm || !vmc)
> +		goto out_free;
> +
> +	wcount = count - copy_from_user(buffer, buf, count);
> +	if (wcount < count)
> +		goto out_free;
> +
> +	i = sscanf(buffer, "%s %s %s", element, vmc, vmm);
> +
> +	if (i < 3)
> +		goto out_free;
> +
> +	for (i = 0; i <= strlen(element); i++)
> +		element[i] = tolower(element[i]);
> +
> +	if (!strncmp(vmc, "unlimited", 9))
> +		valuec = RLIM_INFINITY;
> +	else
> +		valuec = simple_strtoull(vmc, NULL, 10);
> +
> +	if (!strncmp(vmm, "unlimited", 9))
> +		valuem = RLIM_INFINITY;
> +	else
> +		valuem = simple_strtoull(vmm, NULL, 10);
> +
> +	for (i = 0; i < RLIM_NLIMITS; i++) {
> +		if ((lnames[i].match) &&
> +		    !strncmp(element, lnames[i].match, 
> +		     strlen(lnames[i].match))) {
> +			index = i;
> +			break;
> +		}
> +	}
> +
> +	if (!lock_task_sighand(task, &flags))
> +		goto out_free;

The function silently does nothing if lock_task_sighand() fails.

> +	if (index >= 0) {
> +		task->signal->rlim[index].rlim_cur = valuec;
> +		task->signal->rlim[index].rlim_max = valuem;
> +	}
> +
> +	unlock_task_sighand(task, &flags);
> +
> +out_free:
> +	kfree(element);
> +	kfree(vmc);
> +	kfree(vmm);
> +	kfree(buffer);
> +out:
> +	*ppos += count;
> +	put_task_struct(task);
>  	return count;
>  }
>  
> +
> +static const struct file_operations proc_limit_operations = {
> +        .read           = proc_pid_limit_read,
> +	.write		= proc_pid_limit_write,

whitespace got munged.

> +};
> +