linux-kernel - Re: [RFC v3 7/8] binder: use __kernel_pid_t and __kernel_uid

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Message-ID: <CAHRSSEx0ggFW5JqvQKJLzOLsSK4A4gERM62aXNU6XVkJn8QK7Q@mail.gmail.com>
Date:   Mon, 14 Mar 2022 17:11:25 -0700
From:   Todd Kjos <tkjos@...gle.com>
To:     "T.J. Mercier" <tjmercier@...gle.com>
Cc:     Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Jonathan Corbet <corbet@....net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arve Hjønnevåg <arve@...roid.com>,
        Todd Kjos <tkjos@...roid.com>,
        Martijn Coenen <maco@...roid.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Christian Brauner <brauner@...nel.org>,
        Hridya Valsaraju <hridya@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Christian König <christian.koenig@....com>,
        Benjamin Gaignard <benjamin.gaignard@...aro.org>,
        Liam Mark <lmark@...eaurora.org>,
        Laura Abbott <labbott@...hat.com>,
        Brian Starkey <Brian.Starkey@....com>,
        John Stultz <john.stultz@...aro.org>,
        Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Shuah Khan <shuah@...nel.org>,
        Kalesh Singh <kaleshsingh@...gle.com>, Kenny.Ho@....com,
        dri-devel@...ts.freedesktop.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-media@...r.kernel.org,
        linaro-mm-sig@...ts.linaro.org, cgroups@...r.kernel.org,
        linux-kselftest@...r.kernel.org
Subject: Re: [RFC v3 7/8] binder: use __kernel_pid_t and __kernel_uid_t for userspace

On Mon, Mar 14, 2022 at 4:45 PM T.J. Mercier <tjmercier@...gle.com> wrote:
>
> On Thu, Mar 10, 2022 at 11:33 AM Todd Kjos <tkjos@...gle.com> wrote:
> >
> > On Wed, Mar 9, 2022 at 8:52 AM T.J. Mercier <tjmercier@...gle.com> wrote:
> > >
> > > The kernel interface should use types that the kernel defines instead of
> > > pid_t and uid_t, whose definiton is owned by libc. This fixes the header
> > > so that it can be included without first including sys/types.h.
> > >
> > > Signed-off-by: T.J. Mercier <tjmercier@...gle.com>
> > > ---
> > >  include/uapi/linux/android/binder.h | 4 ++--
> > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/include/uapi/linux/android/binder.h b/include/uapi/linux/android/binder.h
> > > index 169fd5069a1a..aa28454dbca3 100644
> > > --- a/include/uapi/linux/android/binder.h
> > > +++ b/include/uapi/linux/android/binder.h
> > > @@ -289,8 +289,8 @@ struct binder_transaction_data {
> > >
> > >         /* General information about the transaction. */
> > >         __u32           flags;
> > > -       pid_t           sender_pid;
> > > -       uid_t           sender_euid;
> > > +       __kernel_pid_t  sender_pid;
> > > +       __kernel_uid_t  sender_euid;
> >
> > Are we guaranteed that this does not affect the UAPI at all? Userspace
> > code using this definition will have to run with kernels using the old
> > definition and visa-versa.
>
> A standards compliant userspace should be expecting a signed integer
> type here. So the only way I can think userspace would be affected is
> if:
> 1) pid_t is a long AND
> 2) sizeof(long) > sizeof(int) AND
> 3) Consumers of the pid_t definition actually attempt to mutate the
> result to make use of extra bits in the variable (which are not there)
>
> This seems extremely unlikely. For instance just on the topic of the
> first item, all of the C library implementations with pid_t
> definitions linked here use an int, except for Bionic which typdefs
> pid_t to __kernel_pid_t and Sortix which uses long.
> https://wiki.osdev.org/C_Library
>
> However I would argue this is already broken and should count as a bug
> fix since I can't do this:
>
> $ cat binder_include.c ; gcc binder_include.c
> #include <linux/android/binder.h>
> int main() {}
> In file included from binder_include.c:1:
> /usr/include/linux/android/binder.h:291:9: error: unknown type name ‘pid_t’
>   291 |         pid_t           sender_pid;
>       |         ^~~~~
> /usr/include/linux/android/binder.h:292:9: error: unknown type name ‘uid_t’
>   292 |         uid_t           sender_euid;
>       |         ^~~~~
>
> This is also the only occurrence of pid_t in all of
> include/uapi/linux. All 40+ other uses are __kernel_pid_t, and I don't
> see why the binder header should be different.

It looks like those other cases used to be pid_t, but were changed to
__kernel_pid_t.

Acked-by: Todd Kjos <tkjos@...gle.com>

>
>
> >
> > >         binder_size_t   data_size;      /* number of bytes of data */
> > >         binder_size_t   offsets_size;   /* number of bytes of offsets */
> > >
> > > --
> > > 2.35.1.616.g0bdcbb4464-goog
> > >