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Date:   Thu, 30 Apr 2020 12:55:54 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To:     Florian Weimer <fweimer@redhat.com>
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Subject: Re: [RFC PATCH glibc 1/3] glibc: Perform rseq(2) registration at C
 startup and thread creation (v18)
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Thread-Topic: glibc: Perform rseq(2) registration at C startup and thread creation (v18)
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----- On Apr 30, 2020, at 12:36 PM, Florian Weimer fweimer@redhat.com wrote:

> * Mathieu Desnoyers:
> 
[...]
> 
>>>> +  if (__rseq_abi.cpu_id == RSEQ_CPU_ID_REGISTRATION_FAILED)
>>>> +    return;
>>>> +  ret = INTERNAL_SYSCALL_CALL (rseq, &__rseq_abi, sizeof (struct rseq),
>>>> +                              0, RSEQ_SIG);
>>>> +  if (INTERNAL_SYSCALL_ERROR_P (ret) &&
>>>> +      INTERNAL_SYSCALL_ERRNO (ret) != EBUSY)
>>>> +    __rseq_abi.cpu_id = RSEQ_CPU_ID_REGISTRATION_FAILED;
>>> 
>>> Sorry, I forgot: Please add a comment that the EBUSY error is ignored
>>> because registration may have already happened in a legacy library.
>>
>> Considering that we now disable signals across thread creation, and that
>> glibc's initialization happens before other libraries' constructors
>> (as far as I remember even before LD_PRELOADed library constructors),
>> in which scenario can we expect to have EBUSY here ?
> 
> That's a good point.
> 
>> Not setting __rseq_abi.cpu_id to RSEQ_CPU_ID_REGISTRATION_FAILED in case
>> of EBUSY is more a way to handle "unforeseen" scenarios where somehow the
>> registration would already be done. But I cannot find an "expected"
>> scenario which would lead to this now.
>>
>> So if EBUSY really is unexpected, how should we treat that ? I don't think
>> setting REGISTRATION_FAILED would be appropriate, because then it would
>> break assumption of the prior successful registration that have already
>> been done by this thread.
> 
> You could call __libc_fatal with an error message.  ENOSYS is definitely
> an expected error code here, and EPERM (and perhaps EACCES) can happen
> with seccomp filters.

If we go this way, I'd also recommend to treat any situation where
__rseq_abi.cpu_id is already initialized as a fatal error. Does the
code below seem OK to you ?

static inline void
rseq_register_current_thread (void)
{
  int ret;

  if (__rseq_abi.cpu_id != RSEQ_CPU_ID_UNINITIALIZED)
    __libc_fatal ("rseq already initialized for this thread\n");
  ret = INTERNAL_SYSCALL_CALL (rseq, &__rseq_abi, sizeof (struct rseq),
                              0, RSEQ_SIG);
  if (INTERNAL_SYSCALL_ERROR_P (ret))
    {
      if (INTERNAL_SYSCALL_ERRNO (ret) == EBUSY)
        __libc_fatal ("rseq already registered for this thread\n");
      __rseq_abi.cpu_id = RSEQ_CPU_ID_REGISTRATION_FAILED;
    }
}

Thanks,

Mathieu

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