RE: [PATCH] seccomp: Improve performance by optimizing memory barrier

["Wanghongzhe (Hongzhe, EulerOS)" <wanghongzhe@huawei.com>]

> From: Leon Romanovsky [mailto:leon@kernel.org]
> Sent: Monday, February 8, 2021 2:44 PM
> To: Wanghongzhe (Hongzhe, EulerOS) <wanghongzhe@huawei.com>
> Cc: keescook@chromium.org; luto@amacapital.net; wad@chromium.org;
> ast@kernel.org; daniel@iogearbox.net; andrii@kernel.org; kafai@fb.com;
> songliubraving@fb.com; yhs@fb.com; john.fastabend@gmail.com;
> kpsingh@kernel.org; linux-kernel@vger.kernel.org; netdev@vger.kernel.org;
> bpf@vger.kernel.org
> Subject: Re: [PATCH] seccomp: Improve performance by optimizing memory
> barrier
> 
> On Mon, Feb 01, 2021 at 08:49:41PM +0800, wanghongzhe wrote:
> > If a thread(A)'s TSYNC flag is set from seccomp(), then it will
> > synchronize its seccomp filter to other threads(B) in same thread
> > group. To avoid race condition, seccomp puts rmb() between reading the
> > mode and filter in seccomp check patch(in B thread).
> > As a result, every syscall's seccomp check is slowed down by the
> > memory barrier.
> >
> > However, we can optimize it by calling rmb() only when filter is NULL
> > and reading it again after the barrier, which means the rmb() is
> > called only once in thread lifetime.
> >
> > The 'filter is NULL' conditon means that it is the first time
> > attaching filter and is by other thread(A) using TSYNC flag.
> > In this case, thread B may read the filter first and mode later in CPU
> > out-of-order exection. After this time, the thread B's mode is always
> > be set, and there will no race condition with the filter/bitmap.
> >
> > In addtion, we should puts a write memory barrier between writing the
> > filter and mode in smp_mb__before_atomic(), to avoid the race
> > condition in TSYNC case.
> >
> > Signed-off-by: wanghongzhe <wanghongzhe@huawei.com>
> > ---
> >  kernel/seccomp.c | 31 ++++++++++++++++++++++---------
> >  1 file changed, 22 insertions(+), 9 deletions(-)
> >
> > diff --git a/kernel/seccomp.c b/kernel/seccomp.c index
> > 952dc1c90229..b944cb2b6b94 100644
> > --- a/kernel/seccomp.c
> > +++ b/kernel/seccomp.c
> > @@ -397,8 +397,20 @@ static u32 seccomp_run_filters(const struct
> seccomp_data *sd,
> >  			READ_ONCE(current->seccomp.filter);
> >
> >  	/* Ensure unexpected behavior doesn't result in failing open. */
> > -	if (WARN_ON(f == NULL))
> > -		return SECCOMP_RET_KILL_PROCESS;
> > +	if (WARN_ON(f == NULL)) {
> > +		/*
> > +		 * Make sure the first filter addtion (from another
> > +		 * thread using TSYNC flag) are seen.
> > +		 */
> > +		rmb();
> > +
> > +		/* Read again */
> > +		f = READ_ONCE(current->seccomp.filter);
> > +
> > +		/* Ensure unexpected behavior doesn't result in failing open. */
> > +		if (WARN_ON(f == NULL))
> > +			return SECCOMP_RET_KILL_PROCESS;
> > +	}
> 
> IMHO, double WARN_ON() for the fallback flow is too much.
> Also according to the description, this "f == NULL" check is due to races and
> not programming error which WARN_ON() are intended to catch.
> 
> Thanks

Maybe you are right. I think 'if (f == NULL)' is enough for this optimizing.