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From:   Bartosz Golaszewski <bgolaszewski@baylibre.com>
Date:   Mon, 7 Sep 2020 12:28:05 +0200
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Subject: Re: [PATCH 03/23] lib: uaccess: provide getline_from_user()
To:     Andy Shevchenko <andy.shevchenko@gmail.com>
Cc:     Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Bartosz Golaszewski <brgl@bgdev.pl>,
        Linus Walleij <linus.walleij@linaro.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Mika Westerberg <mika.westerberg@linux.intel.com>,
        Kent Gibson <warthog618@gmail.com>,
        linux-gpio <linux-gpio@vger.kernel.org>,
        linux-doc <linux-doc@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        ACPI Devel Maling List <linux-acpi@vger.kernel.org>
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On Mon, Sep 7, 2020 at 12:19 PM Andy Shevchenko
<andy.shevchenko@gmail.com> wrote:
>
> On Mon, Sep 7, 2020 at 1:05 PM Bartosz Golaszewski
> <bgolaszewski@baylibre.com> wrote:
> > On Fri, Sep 4, 2020 at 6:35 PM Andy Shevchenko
> > <andriy.shevchenko@linux.intel.com> wrote:
> > > On Fri, Sep 04, 2020 at 05:45:27PM +0200, Bartosz Golaszewski wrote:
> > > > From: Bartosz Golaszewski <bgolaszewski@baylibre.com>
>
> > > 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