linux-kernel - Re: [PATCH] kthread: move the cgroup info initialization early

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Message-ID: <20171107180558.GA4164960@devbig577.frc2.facebook.com>
Date:   Tue, 7 Nov 2017 10:05:58 -0800
From:   Tejun Heo <tj@...nel.org>
To:     Shaohua Li <shli@...com>
Cc:     LKML <linux-kernel@...r.kernel.org>, Jens Axboe <axboe@...nel.dk>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] kthread: move the cgroup info initialization early

On Tue, Nov 07, 2017 at 09:57:03AM -0800, Shaohua Li wrote:
> kthread() could bail out early before we initialize blkcg_css (if the
> kthread is killed very soon), which confuses free_kthread_struct. Move
> the blkcg_css initialization early.
> 
> Reported-by: syzbot <syzkaller@...glegroups.com>
> Fix: 05e3db9(kthread: add a mechanism to store cgroup info)
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Tejun Heo <tj@...nel.org>
> Signed-off-by: Shaohua Li <shli@...com>
> ---
>  kernel/kthread.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/kthread.c b/kernel/kthread.c
> index f87cd8b4..cf5c113 100644
> --- a/kernel/kthread.c
> +++ b/kernel/kthread.c
> @@ -205,6 +205,10 @@ static int kthread(void *_create)
>  	int ret;
>  
>  	self = kmalloc(sizeof(*self), GFP_KERNEL);
> +#ifdef CONFIG_BLK_CGROUP
> +	if (self)
> +		self->blkcg_css = NULL;
> +#endif

That's not a big struct and I can't imagine that avoiding zeroing it
matters in anyway.  Can't we just use kzalloc() instead and drop the
ugly ifdef?

Thanks.

-- 
tejun