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

> +};
> +



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