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From: Alice Ryhl <aliceryhl@google.com>
Date: Mon, 22 Apr 2024 10:56:31 +0200
Message-ID: <CAH5fLgjCmzOXWa7VFg6yCjnTDfu2pcJr9=FngeGg28zCvV49eQ@mail.gmail.com>
Subject: Re: [PATCH 1/4] binder: introduce BINDER_SET_PROC_FLAGS ioctl
To: Carlos Llamas <cmllamas@google.com>
Cc: arve@android.com, brauner@kernel.org, gregkh@linuxfoundation.org, 
	joel@joelfernandes.org, kernel-team@android.com, linux-kernel@vger.kernel.org, 
	maco@android.com, surenb@google.com, tkjos@android.com
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On Sun, Apr 21, 2024 at 1:39=E2=80=AFAM Carlos Llamas <cmllamas@google.com>=
 wrote:
>
> On Thu, Apr 18, 2024 at 08:34:47AM +0000, Alice Ryhl wrote:
> > Carlos Llamas <cmllamas@google.com> writes:
> > > This new ioctl enables userspace to control the individual behavior o=
f
> > > the 'struct binder_proc' instance via flags. The driver validates and
> > > returns the supported subset. Some existing ioctls are migrated to us=
e
> > > these flags in subsequent commits.
> > >
> > > Suggested-by: Arve Hj=C3=B8nnev=C3=A5g <arve@android.com>
> > > Signed-off-by: Carlos Llamas <cmllamas@google.com>
> > > ---
> > >  drivers/android/binder.c            | 25 +++++++++++++++++++++++++
> > >  drivers/android/binder_internal.h   |  4 +++-
> > >  include/uapi/linux/android/binder.h |  6 ++++++
> > >  3 files changed, 34 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/android/binder.c b/drivers/android/binder.c
> > > index bad28cf42010..e0d193bfb237 100644
> > > --- a/drivers/android/binder.c
> > > +++ b/drivers/android/binder.c
> > > @@ -5334,6 +5334,26 @@ static int binder_ioctl_get_extended_error(str=
uct binder_thread *thread,
> > >     return 0;
> > >  }
> > >
> > > +static int binder_ioctl_set_proc_flags(struct binder_proc *proc,
> > > +                                  u32 __user *user)
> > > +{
> > > +   u32 flags;
> > > +
> > > +   if (get_user(flags, user))
> > > +           return -EFAULT;
> > > +
> > > +   binder_inner_proc_lock(proc);
> > > +   flags &=3D PF_SUPPORTED_FLAGS_MASK;
> > > +   proc->flags =3D flags;
> > > +   binder_inner_proc_unlock(proc);
> > > +
> > > +   /* confirm supported flags with user */
> > > +   if (put_user(flags, user))
> > > +           return -EFAULT;
> > > +
> > > +   return 0;
> > > +}
> >
> > I'm just thinking out loud here, but is this the best API for this
> > ioctl? Using this API, if I want to toggle the oneway-spam-detection
> > flag, then I can't do so without knowing the value of all other flags,
> > and I also need to synchronize all calls to this ioctl.
> >
> > That's fine for the current use-case where these flags are only set
> > during startup, but are we confident that no future flag will be toggle=
d
> > while a process is active?
>
> hmmm, this is a very good point. It would probably lead to userspace
> having to cache its flags for every binder instance. This is not a good
> solution at all.
>
> >
> > How about these alternatives?
> >
> > 1. Userspace passes two masks, one containing bits to set, and another
> >    containing bits to unset. Userspace returns new value of flags. (If
> >    the same bit is set in both masks, we can fail with EINVAL.)

To add to this one, one could also say that if a bit is set in both,
then the value is toggled.

> > 2. Compare and swap. Userspace passes the expected previous value and
> >    the desired new value. The kernel returns the actual previous value
> >    and updates it only if userspace gave the right previous value.
> >
> > 3. Set or unset only. Userspace passes a boolean and a mask. Boolean
> >    determines whether userspace wants to set or unset the bits set in
> >    the mask.
> >
> > I don't know what the usual kernel convention is for this kind of
> > ioctl, so I'm happy with whatever you all think is best.
>
> I've never come across these types of alternatives personally. What I've
> seen however, is the typical SET/GET ioctl pairs. This is a "simpler"
> interface I guess but it has the downside of an extra roundtrip. e.g.
>
>         ioctl(fd, BINDER_GET_PROC_FLAGS, &flags);
>         flags |=3D BF_LARGE_HANDLES;
>         ioctl(fd, BINDER_SET_PROC_FLAGS, &flags);
>
> What seems tempting about the SET/GET pair is that we could replace the
> BINDER_ENABLE_ONEWAY_SPAM_DETECTION with the SET. Instead of maintaining
> legacy code for the "deprecated" ioctl.
>
> wdyt?
>
> I'll have a second look at the alternatives you mentioned. Perhaps I can
> reference some other existing ioctl that does something similar.

Hmm. I don't think a get/set pair improves the situation much.
Userspace still needs a global mutex for making changes to the flag in
that case. Otherwise, two threads changing two different flags in
parallel could result in a race condition where one of the changes is
lost.

Alice