linux-kernel - Re: [PATCH for 5.5 2/2] rseq/selftests: Clarify rseq_prepare

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Date:   Fri, 20 Dec 2019 13:27:29 -0700
From:   Shuah Khan <skhan@...uxfoundation.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E . McKenney" <paulmck@...ux.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        "H . Peter Anvin" <hpa@...or.com>, Paul Turner <pjt@...gle.com>,
        linux-api@...r.kernel.org, stable@...r.kernel.org,
        Florian Weimer <fw@...eb.enyo.de>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        "skh >> Shuah Khan" <skhan@...uxfoundation.org>
Subject: Re: [PATCH for 5.5 2/2] rseq/selftests: Clarify rseq_prepare_unload()
 helper requirements

Hi Mathieu,

On 12/20/19 1:12 PM, Mathieu Desnoyers wrote:
> The rseq.h UAPI now documents that the rseq_cs field must be cleared
> before reclaiming memory that contains the targeted struct rseq_cs, but
> also that the rseq_cs field must be cleared before reclaiming memory of
> the code pointed to by the rseq_cs start_ip and post_commit_offset
> fields.
> 
> While we can expect that use of dlclose(3) will typically unmap
> both struct rseq_cs and its associated code at once, nothing would
> theoretically prevent a JIT from reclaiming the code without
> reclaiming the struct rseq_cs, which would erroneously allow the
> kernel to consider new code which is not a rseq critical section
> as a rseq critical section following a code reclaim.
> 
> Suggested-by: Florian Weimer <fw@...eb.enyo.de>
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> Cc: Shuah Khan <skhan@...uxfoundation.org>
> Cc: Florian Weimer <fw@...eb.enyo.de>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Peter Zijlstra (Intel) <peterz@...radead.org>
> Cc: "Paul E. McKenney" <paulmck@...ux.ibm.com>
> Cc: Boqun Feng <boqun.feng@...il.com>
> Cc: "H . Peter Anvin" <hpa@...or.com>
> Cc: Paul Turner <pjt@...gle.com>
> Cc: Dmitry Vyukov <dvyukov@...gle.com>
> ---
>   tools/testing/selftests/rseq/rseq.h | 12 +++++++-----
>   1 file changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/tools/testing/selftests/rseq/rseq.h b/tools/testing/selftests/rseq/rseq.h
> index d40d60e7499e..15cbd51d0818 100644
> --- a/tools/testing/selftests/rseq/rseq.h
> +++ b/tools/testing/selftests/rseq/rseq.h
> @@ -149,11 +149,13 @@ static inline void rseq_clear_rseq_cs(void)
>   /*
>    * rseq_prepare_unload() should be invoked by each thread executing a rseq
>    * critical section at least once between their last critical section and
> - * library unload of the library defining the rseq critical section
> - * (struct rseq_cs). This also applies to use of rseq in code generated by
> - * JIT: rseq_prepare_unload() should be invoked at least once by each
> - * thread executing a rseq critical section before reclaim of the memory
> - * holding the struct rseq_cs.
> + * library unload of the library defining the rseq critical section (struct
> + * rseq_cs) or the code refered to by the struct rseq_cs start_ip and

Nit: referred instead of refered

> + * post_commit_offset fields. This also applies to use of rseq in code
> + * generated by JIT: rseq_prepare_unload() should be invoked at least once by
> + * each thread executing a rseq critical section before reclaim of the memory
> + * holding the struct rseq_cs or reclaim of the code pointed to by struct
> + * rseq_cs start_ip and post_commit_offset fields.
>    */
>   static inline void rseq_prepare_unload(void)
>   {
> 

thanks,
-- Shuah