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Date:   Thu, 18 Apr 2019 13:10:49 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To:     Szabolcs Nagy <Szabolcs.Nagy@arm.com>
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Subject: Re: [PATCH 1/5] glibc: Perform rseq(2) registration at C startup
 and thread creation (v8)
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Thread-Topic: glibc: Perform rseq(2) registration at C startup and thread creation (v8)
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----- On Apr 18, 2019, at 12:07 PM, Szabolcs Nagy Szabolcs.Nagy@arm.com wrote:

> On 18/04/2019 16:41, Mathieu Desnoyers wrote:
>> ----- On Apr 18, 2019, at 11:33 AM, Szabolcs Nagy Szabolcs.Nagy@arm.com wrote:
>> 
>>> On 18/04/2019 14:17, Mathieu Desnoyers wrote:
>>>> ----- On Apr 17, 2019, at 3:56 PM, Mathieu Desnoyers
>>>> mathieu.desnoyers@efficios.com wrote:
>>>>> ----- On Apr 17, 2019, at 12:17 PM, Joseph Myers joseph@codesourcery.com wrote:
>>>>>> On Wed, 17 Apr 2019, Mathieu Desnoyers wrote:
>>>>>>
>>>>>>>> +/* RSEQ_SIG is a signature required before each abort handler code.
>>>>>>>> +
>>>>>>>> +   It is a 32-bit value that maps to actual architecture code compiled
>>>>>>>> +   into applications and libraries. It needs to be defined for each
>>>>>>>> +   architecture. When choosing this value, it needs to be taken into
>>>>>>>> +   account that generating invalid instructions may have ill effects on
>>>>>>>> +   tools like objdump, and may also have impact on the CPU speculative
>>>>>>>> +   execution efficiency in some cases.  */
>>>>>>>> +
>>>>>>>> +#define RSEQ_SIG 0xd428bc00	/* BRK #0x45E0.  */
>>>>>>>
>>>>>>> After further investigation, we should probably do the following
>>>>>>> to handle compiling with -mbig-endian on aarch64, which generates
>>>>>>> binaries with mixed code vs data endianness (little endian code,
>>>>>>> big endian data):
>>>>>>
>>>>>> First, the comment on RSEQ_SIG should specify whether it is to be
>>>>>> interpreted in the code or the data endianness.
>>>>>
>>>>> Right. The signature passed as argument to the rseq registration
>>>>> system call needs to be in data endianness (currently exposed kernel
>>>>> ABI).
>>>>>
>>>>> Ideally for userspace, we want to define a signature in code endianness
>>>>> that happens to nicely match specific code patterns.
>>> ...
>>>> For aarch64, I think we can simply do:
>>>>
>>>> /*
>>>>  * aarch64 -mbig-endian generates mixed endianness code vs data:
>>>>  * little-endian code and big-endian data. Ensure the RSEQ_SIG signature
>>>>  * matches code endianness.
>>>>  */
>>>> #define RSEQ_SIG_CODE   0xd428bc00      /* BRK #0x45E0.  */
>>>>
>>>> #ifdef __ARM_BIG_ENDIAN
>>>> #define RSEQ_SIG_DATA   0x00bc28d4      /* BRK #0x45E0.  */
>>>> #else
>>>> #define RSEQ_SIG_DATA   RSEQ_SIG_CODE
>>>> #endif
>>>>
>>>> #define RSEQ_SIG        RSEQ_SIG_DATA
>>>>
>>>> Feedback is most welcome,
>>>
>>> so the RSEQ_SIG value is supposed to be used with .word
>>> in asm instead of .inst?
>> 
>> We want a .inst so it translates into a valid trap instruction.
>> It's better to trap in case program execution reaches this
>> by mistake (makes debugging easier).
> 
> that does not make sense to me.
> 
> ".inst" is an asm directive that requires a the value to
> be the same on BE and LE (normal insn encoding).
> 
> ".word" is an asm directive that requires the value to
> use swapped encoding on BE (if it's used in the instruction
> stream it will create a data mapping symbol and disasm to
> .word value instead of the instruction mnemonics).
> 
> so which one is it?

We declare the signature with ".inst" in assembler.

However, we also need to pass that 32-bit signature value as
argument to the rseq system call when registering rseq.

The signature comparison is performed by the kernel before
moving the instruction pointer to the abort handler. It compares
the signature received as parameter by sys_rseq (data) to the
4-byte signature preceding the abort IP.

On aarch64 big endian, AFAIU the signature in the code is in
little endian, and the signature value passed as argument to
the rseq system call is in big endian. One way to handle this
is to swap the byte order of the signature "data" representation
passed as argument to sys_rseq.

> 
>>> i don't think we use __ARM_* in public headers currently,
>>> but hopefully aarch64_be compilers implement it.
>> 
>> Can I use #if __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__  then ?
> 
> hm, i'd either use #ifdef __AARCH64EB__ (since we already use it)
> or the portable #include endian.h and __BYTE_ORDER == __BIG_ENDIAN

I'll use #ifdef __AARCH64EB__ given this header is specific to aarch64.

Thanks,

Mathieu


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