Re: [PATCH v2 6/9] debugfs: Implement debugfs_create_str()

[Peter Zijlstra]

On Fri, Mar 26, 2021 at 01:53:59PM +0100, Rasmus Villemoes wrote:
> On 26/03/2021 12.38, Peter Zijlstra wrote:

> > +
> > +again:
> > +   rcu_read_lock();
> > +   str = rcu_dereference(*(char **)file->private_data);
> > +   len = strlen(str) + 1;
> > +
> > +   if (!copy || copy_len < len) {
> > +           rcu_read_unlock();
> > +           kfree(copy);
> > +           copy = kmalloc(len + 1, GFP_KERNEL);
> > +           if (!copy) {
> > +                   debugfs_file_put(dentry);
> > +                   return -ENOMEM;
> > +           }
> > +           copy_len = len;
> > +           goto again;
> > +   }
> > +
> > +   strncpy(copy, str, len);
> > +   copy[len] = '\n';
> > +   copy[len+1] = '\0';
> > +   rcu_read_unlock();
> 
> As noted (accidentally off-list), this is broken. I think you want this
> on top
> 
> - len = strlen(str) + 1;
> + len = strlen(str);

  kmalloc(len + 2, ...);

> - strncpy(copy, str, len);
> + memcpy(copy, str, len);
>   copy[len] = '\n';
> - copy[len+1] = '\0';

I'll go with strscpy() I tihnk, something like:

        len = strscpy(copy, str, len);
        if (len < 0)
                return len;
        copy[len] = '\n';
        copy[len + 1] = '\0';

> > +EXPORT_SYMBOL_GPL(debugfs_read_file_str);
> 
> Why?

Copy-pasta from debugfs_*_bool(). This thing seems to export everything
and I figured I'd go along with that.