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From:   Florian Weimer <fweimer@redhat.com>
To:     Ingo Molnar <mingo@kernel.org>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Peter Zijlstra <peterz@infradead.org>,
        linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        "Paul E . McKenney" <paulmck@kernel.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        "H . Peter Anvin" <hpa@zytor.com>, Paul Turner <pjt@google.com>,
        linux-api@vger.kernel.org, Peter Oskolkov <posk@posk.io>
Subject: Re: [PATCH 2/2] rseq: Kill process when unknown flags are
 encountered in ABI structures
References: <20220622194617.1155957-1-mathieu.desnoyers@efficios.com>
        <20220622194617.1155957-2-mathieu.desnoyers@efficios.com>
        <YufV3PmAOfo1Gt7g@gmail.com>
Date:   Mon, 01 Aug 2022 16:25:14 +0200
In-Reply-To: <YufV3PmAOfo1Gt7g@gmail.com> (Ingo Molnar's message of "Mon, 1
        Aug 2022 15:32:12 +0200")
Message-ID: <87tu6wm46t.fsf@oldenburg.str.redhat.com>
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* Ingo Molnar:

> * Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:
>
>> rseq_abi()->flags and rseq_abi()->rseq_cs->flags 29 upper bits are
>> currently unused.
>> 
>> The current behavior when those bits are set is to ignore them. This is
>> not an ideal behavior, because when future features will start using
>> those flags, if user-space fails to correctly validate that the kernel
>> indeed supports those flags (e.g. with a new sys_rseq flags bit) before
>> using them, it may incorrectly assume that the kernel will handle those
>> flags way when in fact those will be silently ignored on older kernels.
>> 
>> Validating that unused flags bits are cleared will allow a smoother
>> transition when those flags will start to be used by allowing
>> applications to fail early, and obviously, when they attempt to use the
>> new flags on an older kernel that does not support them.
>> 
>> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
>> ---
>>  kernel/rseq.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>> 
>> diff --git a/kernel/rseq.c b/kernel/rseq.c
>> index 81d7dc80787b..bda8175f8f99 100644
>> --- a/kernel/rseq.c
>> +++ b/kernel/rseq.c
>> @@ -176,7 +176,7 @@ static int rseq_need_restart(struct task_struct *t, u32 cs_flags)
>>  	u32 flags, event_mask;
>>  	int ret;
>>  
>> -	if (WARN_ON_ONCE(cs_flags & RSEQ_CS_NO_RESTART_FLAGS))
>> +	if (WARN_ON_ONCE(cs_flags & RSEQ_CS_NO_RESTART_FLAGS) || cs_flags)
>>  		return -EINVAL;
>>  
>>  	/* Get thread flags. */
>> @@ -184,7 +184,7 @@ static int rseq_need_restart(struct task_struct *t, u32 cs_flags)
>>  	if (ret)
>>  		return ret;
>>  
>> -	if (WARN_ON_ONCE(flags & RSEQ_CS_NO_RESTART_FLAGS))
>> +	if (WARN_ON_ONCE(flags & RSEQ_CS_NO_RESTART_FLAGS) || flags)
>>  		return -EINVAL;
>
> Just to make it clear: no existing libraries/tooling out there have learned 
> to rely on the old ABI that ignored unset flags, right? Only then is this 
> patch ABI-safe.

I believe glibc initializes the flag fields to zero before calling the
rseq system call.  (I don't know if the rseq system call does its own
initialization; maybe it should if it doesn't do so already.)

Thanks,
Florian