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Date:   Mon, 27 Apr 2020 16:27:37 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
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Subject: Re: [PATCH glibc 5/9] glibc: Perform rseq(2) registration at C
 startup and thread creation (v17)
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Thread-Topic: glibc: Perform rseq(2) registration at C startup and thread creation (v17)
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----- On Apr 27, 2020, at 1:26 PM, Mathieu Desnoyers mathieu.desnoyers@efficios.com wrote:
[...]
>>> We don't mind that user-space uses that pointer, but we never want the kernel
>>> to touch that pointer rather than the 32/64-bit-aware fields. One possibility
>>> would be to do:
>>>
>>>     union
>>>       {
>>>         uint64_t ptr64;
>>> #ifdef __LP64__
>>>         uint64_t ptr;
>>> #else
>>>         struct
>>>           {
>>> #if (defined (__BYTE_ORDER) && (__BYTE_ORDER == __BIG_ENDIAN)) || defined
>>> (__BIG_ENDIAN)
>>>             uint32_t padding; /* Initialized to zero.  */
>>>             uint32_t ptr32;
>>> #else /* LITTLE */
>>>             uint32_t ptr32;
>>>             uint32_t padding; /* Initialized to zero.  */
>>> #endif /* ENDIAN */
>>>           } ptr;
>>> #endif
>>>
>>> #ifndef __KERNEL__
>>>      const struct rseq_cs *uptr;
>>> #endif
>>>       } rseq_cs;
>>>
>>> in the union, so only user-space can see that field. Thoughts ?
>> 
>> I think this depends on where the x32 question lands.
> 
> x32 should not be an issue as explained above, so I'm very open to
> add this "uptr" for user-space only.

Actually, the snippet above is broken on 32-bit. It needs to be:

    union
      {
        uint64_t ptr64;
#ifdef __LP64__
        uint64_t ptr;
# ifndef __KERNEL__
        const struct rseq_cs *uptr;
# endif
#else   
        struct
          {
#if (defined (__BYTE_ORDER) && (__BYTE_ORDER == __BIG_ENDIAN)) || defined (__BIG_ENDIAN)
            uint32_t padding; /* Initialized to zero.  */
            uint32_t ptr32;
#else /* LITTLE */
            uint32_t ptr32;
            uint32_t padding; /* Initialized to zero.  */
#endif /* ENDIAN */
          } ptr;
# ifndef __KERNEL__
        struct
          {
#  if (defined (__BYTE_ORDER) && (__BYTE_ORDER == __BIG_ENDIAN)) || defined (__BIG_ENDIAN)
            uint32_t padding; /* Initialized to zero.  */
            const struct rseq_cs *uptr32;
#  else /* LITTLE */
            const struct rseq_cs *uptr32;
            uint32_t padding; /* Initialized to zero.  */
#  endif /* ENDIAN */
          } uptr;
# endif
#endif
      } rseq_cs;

I'll leave this out of the patchset for now as we'd need more feedback on its
usefulness.

Thanks,

Mathieu

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