Re: [PATCH for 5.5 2/2] rseq/selftests: Clarify rseq_prepare_unload() helper requirements

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- On Dec 20, 2019, at 3:27 PM, Shuah Khan skhan@linuxfoundation.org wrote:

> 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@deneb.enyo.de>
>> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
>> Cc: Shuah Khan <skhan@linuxfoundation.org>
>> Cc: Florian Weimer <fw@deneb.enyo.de>
>> Cc: Thomas Gleixner <tglx@linutronix.de>
>> Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
>> Cc: "Paul E. McKenney" <paulmck@linux.ibm.com>
>> Cc: Boqun Feng <boqun.feng@gmail.com>
>> Cc: "H . Peter Anvin" <hpa@zytor.com>
>> Cc: Paul Turner <pjt@google.com>
>> Cc: Dmitry Vyukov <dvyukov@google.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

Good catch. I've done the same error in patch 1/2. I'll update both and
resend.

Thanks!

Mathieu

> 
>> + * 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

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com