Re: [PATCH 03/23] lib: uaccess: provide getline_from_user()

[Bartosz Golaszewski <bgolaszewski@xxxxxxxxxxxx>]

On Mon, Sep 7, 2020 at 12:19 PM Andy Shevchenko
<andy.shevchenko@xxxxxxxxx> wrote:
>
> On Mon, Sep 7, 2020 at 1:05 PM Bartosz Golaszewski
> <bgolaszewski@xxxxxxxxxxxx> wrote:
> > On Fri, Sep 4, 2020 at 6:35 PM Andy Shevchenko
> > <andriy.shevchenko@xxxxxxxxxxxxxxx> wrote:
> > > On Fri, Sep 04, 2020 at 05:45:27PM +0200, Bartosz Golaszewski wrote:
> > > > From: Bartosz Golaszewski <bgolaszewski@xxxxxxxxxxxx>
>
> > > Doesn't mm/util.c provides us something like this?
> > > strndup_user()?
> > >
> >
> > Yes, there's both strndup_user() as well as strncpy_from_user(). The
> > problem is that they rely on the strings being NULL-terminated. This
> > is not guaranteed for debugfs file_operations write callbacks. We need
> > some helper that takes the minimum of bytes provided by userspace and
> > the buffer size and figure out how many bytes to actually copy IMO.
>
> Wouldn't this [1] approach work?
>
> [1]: https://elixir.bootlin.com/linux/v5.9-rc3/source/arch/x86/kernel/cpu/mtrr/if.c#L93
>

Sure, but this is pretty much what I do in getline_from_user(). If
anything we should port mtrr_write() to using getline_from_user() once
it's available upstream, no?

Bart