linux-kernel - Re: [PATCH 1/7] binder: create userspace-to-binder-buffer copy function

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Date:   Tue, 29 Jan 2019 16:32:09 -0800
From:   Todd Kjos <tkjos@...gle.com>
To:     Dan Carpenter <dan.carpenter@...cle.com>
Cc:     Todd Kjos <tkjos@...roid.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arve Hjønnevåg <arve@...roid.com>,
        "open list:ANDROID DRIVERS" <devel@...verdev.osuosl.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Martijn Coenen <maco@...gle.com>, joel@...lfernandes.org,
        Android Kernel Team <kernel-team@...roid.com>
Subject: Re: [PATCH 1/7] binder: create userspace-to-binder-buffer copy function

On Tue, Jan 29, 2019 at 12:12 AM Dan Carpenter <dan.carpenter@...cle.com> wrote:
>
> On Mon, Jan 28, 2019 at 04:49:28PM -0800, Todd Kjos wrote:
> > +/**
> > + * binder_alloc_copy_user_to_buffer() - copy src user to tgt user
> > + * @alloc: binder_alloc for this proc
> > + * @buffer: binder buffer to be accessed
> > + * @buffer_offset: offset into @buffer data
> > + * @from: userspace pointer to source buffer
> > + * @bytes: bytes to copy
> > + *
> > + * Copy bytes from source userspace to target buffer.
> > + *
> > + * Return: bytes remaining to be copied
> > + */
> > +unsigned long
> > +binder_alloc_copy_user_to_buffer(struct binder_alloc *alloc,
> > +                              struct binder_buffer *buffer,
> > +                              binder_size_t buffer_offset,
> > +                              const void __user *from,
> > +                              size_t bytes)
> > +{
> > +     if (!check_buffer(alloc, buffer, buffer_offset, bytes))
> > +             return bytes;
> > +
> > +     while (bytes) {
> > +             unsigned long size;
> > +             unsigned long ret;
> > +             struct page *page;
> > +             pgoff_t pgoff;
> > +             void *kptr;
> > +
> > +             page = binder_alloc_get_page(alloc, buffer,
> > +                                          buffer_offset, &pgoff);
> > +             size = min(bytes, (size_t)(PAGE_SIZE - pgoff));
>
> This code has so much more casting than necessary.  To me casting says
> that we haven't got the types correct or something.  I've just pulled
> this function out as an example really, but none of the casts here are
> required...  This could just be:
>
>                 size = min(bytes, PAGE_SIZE - pgoff);

Dan,

Thanks for the feedback.

The one above: "size = min(bytes, PAGE_SIZE - pgoff);", results in
32-bit compile warnings:

./include/linux/kernel.h:846:29: warning: comparison of distinct
pointer types lacks a cast
   (!!(sizeof((typeof(x) *)1 == (typeof(y) *)1)))
[...]
drivers/android/binder_alloc.c:1157:10: note: in expansion of macro ‘min’
   size = min(bytes, PAGE_SIZE - pgoff);

so, I took your suggestion:

  size = min_t(size_t, bytes, PAGE_SIZE - pgoff);

>
> Btw, if you really need to do a cast inside a min() (you don't in this
> cast) then use min_t().
>
> > +             kptr = (void *)((uintptr_t)kmap(page) + pgoff);
>
> This would be a lot cleaner as:
>
>                 kptr = kmap(page) + pgoff;

It definitely is cleaner, but the clean version is doing arithmetic on
a void pointer which is generally not considered good practice (but is
supported by gcc). Is this acceptable in the kernel? I don't see any
statement on it in Documentation/process/coding-style.rst.

If so, I agree a bunch of these casts can go as you suggest.

-Todd

[...]

>
> regards,
> dan carpenter
>