Re: [patch -mm v2] mm: introduce oom_adj_child

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

On Tue, 28 Jul 2009 21:27:15 -0700 (PDT)
David Rientjes <rientjes@google.com> wrote:

> +static ssize_t oom_adj_child_write(struct file *file, const char __user *buf,
> +				size_t count, loff_t *ppos)
> +{
> +	struct task_struct *task;
> +	char buffer[PROC_NUMBUF], *end;
> +	int oom_adj_child;
> +
> +	memset(buffer, 0, sizeof(buffer));
> +	if (count > sizeof(buffer) - 1)
> +		count = sizeof(buffer) - 1;
> +	if (copy_from_user(buffer, buf, count))
> +		return -EFAULT;
> +	oom_adj_child = simple_strtol(buffer, &end, 0);
> +	if ((oom_adj_child < OOM_ADJUST_MIN ||
> +	     oom_adj_child > OOM_ADJUST_MAX) && oom_adj_child != OOM_DISABLE)
> +		return -EINVAL;
> +	if (*end == '\n')
> +		end++;
> +	task = get_proc_task(file->f_path.dentry->d_inode);
> +	if (!task)
> +		return -ESRCH;
> +	task_lock(task);
> +	if (task->mm && oom_adj_child < task->mm->oom_adj &&
> +	    !capable(CAP_SYS_RESOURCE)) {
> +		task_unlock(task);
> +		put_task_struct(task);
> +		return -EINVAL;
> +	}
> +	task_unlock(task);
> +	task->oom_adj_child = oom_adj_child;
> +	put_task_struct(task);
> +	if (end - buffer == 0)
> +		return -EIO;
> +	return end - buffer;
> +}

Do we really need to do all that string hacking?  All it does is reads
a plain old integer from userspace.

It's weird that the obfuscated check for zero-length input happens
right at the end of the function, particularly as we couldn't have got
that far anyway, because we'd already have returned -EINVAL.

And even after all that, I suspect the function will permit illogical
input such as "12foo" - which is what strict_strtoul() is for (as
checkpatch points out!).



grumble.  At how many codesites do we read an ascii integer from
userspace?  Thousands, surely.  You'd think we'd have a little function
to do it by now.