linux-kernel - Re: [PATCH glibc 5/9] glibc: Perform rseq(2) registration at C startup and thread creation (v17)

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Message-ID: <122741024.71528.1588019257574.JavaMail.zimbra@efficios.com>
Date:   Mon, 27 Apr 2020 16:27:37 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Florian Weimer <fw@...eb.enyo.de>
Cc:     Michael Kerrisk <mtk.manpages@...il.com>,
        libc-alpha <libc-alpha@...rceware.org>,
        Carlos O'Donell <carlos@...hat.com>,
        Rich Felker <dalias@...c.org>,
        linux-api <linux-api@...r.kernel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Will Deacon <will.deacon@....com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ben Maurer <bmaurer@...com>, Dave Watson <davejwatson@...com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paul <paulmck@...ux.vnet.ibm.com>, Paul Turner <pjt@...gle.com>,
        Joseph Myers <joseph@...esourcery.com>,
        Szabolcs Nagy <szabolcs.nagy@....com>
Subject: Re: [PATCH glibc 5/9] glibc: Perform rseq(2) registration at C
 startup and thread creation (v17)

----- On Apr 27, 2020, at 1:26 PM, Mathieu Desnoyers mathieu.desnoyers@...icios.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