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References: <20200302130430.201037-1-glider@google.com> <20200302130430.201037-3-glider@google.com>
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From:   Todd Kjos <tkjos@google.com>
Date:   Mon, 2 Mar 2020 08:56:50 -0800
Message-ID: <CAHRSSEwe=jZAEVhGw4ACBU0m-76TzZfJFv1Rzw=_UVm6HbTvAw@mail.gmail.com>
Subject: Re: [PATCH v2 3/3] sched/wait: avoid double initialization in ___wait_event()
To:     Alexander Potapenko <glider@google.com>
Cc:     keescook@chromium.org,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        =?UTF-8?B?QXJ2ZSBIasO4bm5ldsOlZw==?= <arve@android.com>,
        Ingo Molnar <mingo@redhat.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Jann Horn <jannh@google.com>,
        "open list:ANDROID DRIVERS" <devel@driverdev.osuosl.org>,
        Peter Zijlstra <peterz@infradead.org>,
        LKML <linux-kernel@vger.kernel.org>
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Sender: linux-kernel-owner@vger.kernel.org
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On Mon, Mar 2, 2020 at 5:04 AM <glider@google.com> wrote:
>
> With CONFIG_INIT_STACK_ALL enabled, the local __wq_entry is initialized
> twice. Because Clang is currently unable to optimize the automatic
> initialization away (init_wait_entry() is defined in another translation
> unit), remove it with the __no_initialize annotation.
>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Signed-off-by: Alexander Potapenko <glider@google.com>
>
> ---
>  v2:
>   - changed __do_not_initialize to __no_initialize as requested by Kees
>     Cook
> ---
>  drivers/android/binder.c | 4 ++--
>  include/linux/wait.h     | 3 ++-
>  2 files changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/android/binder.c b/drivers/android/binder.c
> index a59871532ff6b..66984e7c33094 100644
> --- a/drivers/android/binder.c
> +++ b/drivers/android/binder.c
> @@ -4827,7 +4827,7 @@ static int binder_ioctl_write_read(struct file *filp,
>         struct binder_proc *proc = filp->private_data;
>         unsigned int size = _IOC_SIZE(cmd);
>         void __user *ubuf = (void __user *)arg;
> -       struct binder_write_read bwr __no_initialize;
> +       struct binder_write_read bwr;

How did __no_initialize get set so that it can be removed here? Should
the addition of __no_initilize be removed earlier in the series (tip
doesn't have the __no_initialize).

>
>         if (size != sizeof(struct binder_write_read)) {
>                 ret = -EINVAL;
> @@ -5026,7 +5026,7 @@ static long binder_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
>                         goto err;
>                 break;
>         case BINDER_SET_MAX_THREADS: {
> -               int max_threads;
> +               int max_threads __no_initialize;

Is this really needed? A single integer in a rarely called ioctl()
being initialized twice doesn't warrant this optimization.

>
>                 if (copy_from_user(&max_threads, ubuf,
>                                    sizeof(max_threads))) {
> diff --git a/include/linux/wait.h b/include/linux/wait.h
> index 3283c8d021377..b52a9bb2c7727 100644
> --- a/include/linux/wait.h
> +++ b/include/linux/wait.h
> @@ -262,7 +262,8 @@ extern void init_wait_entry(struct wait_queue_entry *wq_entry, int flags);
>  #define ___wait_event(wq_head, condition, state, exclusive, ret, cmd)          \
>  ({                                                                             \
>         __label__ __out;                                                        \
> -       struct wait_queue_entry __wq_entry;                                     \
> +       /* Unconditionally initialized by init_wait_entry(). */                 \
> +       struct wait_queue_entry __wq_entry __no_initialize;                     \
>         long __ret = ret;       /* explicit shadow */                           \
>                                                                                 \
>         init_wait_entry(&__wq_entry, exclusive ? WQ_FLAG_EXCLUSIVE : 0);        \
> --
> 2.25.0.265.gbab2e86ba0-goog
>