linux-kernel - Re: [PATCH v3 1/7] binder: create userspace-to-binder-buffer copy function

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Message-ID: <20190214212549.GB185075@google.com>
Date:   Thu, 14 Feb 2019 16:25:49 -0500
From:   Joel Fernandes <joelaf@...gle.com>
To:     Todd Kjos <tkjos@...gle.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 Fernandes (Google)" <joel@...lfernandes.org>,
        Android Kernel Team <kernel-team@...roid.com>
Subject: Re: [PATCH v3 1/7] binder: create userspace-to-binder-buffer copy
 function

On Thu, Feb 14, 2019 at 03:53:54PM -0500, Joel Fernandes wrote:
> On Thu, Feb 14, 2019 at 3:42 PM Todd Kjos <tkjos@...gle.com> wrote:
> >
> > On Thu, Feb 14, 2019 at 11:45 AM Joel Fernandes <joelaf@...gle.com> wrote:
> [snip]
> > > > + * check_buffer() - verify that buffer/offset is safe to access
> > > > + * @alloc: binder_alloc for this proc
> > > > + * @buffer: binder buffer to be accessed
> > > > + * @offset: offset into @buffer data
> > > > + * @bytes: bytes to access from offset
> > > > + *
> > > > + * Check that the @offset/@...es are within the size of the given
> > > > + * @buffer and that the buffer is currently active and not freeable.
> > > > + * Offsets must also be multiples of sizeof(u32). The kernel is
> > >
> > > In all callers of binder_alloc_copy_user_to_buffer, the alignment of offsets
> > > is set to sizeof(void *). Then shouldn't this function check for sizeof(void *)
> > > alignment instead of u32?
> >
> > But there are other callers of check_buffer() later in the series that
> > don't require pointer-size alignment. u32 alignment is consistent with
> > the alignment requirements of the binder driver before this change.
> > The copy functions don't actually need to insist on alignment, but
> > these binder buffer objects have always used u32 alignment which has
> > been checked in the driver. If user code misaligned it, then errors
> > are returned. The alignment checks are really to be consistent with
> > previous binder driver behavior.
> 
> Got it, thanks.

One more thing I wanted to ask is, kmap() will now cause global lock
contention because of using spin_lock due to kmap_high().

Previously the binder driver was made to not use global lock (as you had
done). Now these paths will start global locking on 32-bit architectures.
Would that degrade performance?

Are we not using kmap_atomic() in this patch because of any concern that the
kmap fixmap space is limited and may run out?

thanks,

 - Joel