Re: [PATCH 1/5] glibc: Perform rseq(2) registration at C startup and thread creation (v8)

Mathieu Desnoyers
2019-04-18T15:41:10Z

----- 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).

>
> 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 ?

>
> otherwise this looks ok to me.
>
> (i think a rare palindrome instruction would work too, e.g.
> 0a5f5f0a and w10, w24, wzr, lsr #23 // shifted 0
> 2a5f5f2a orr w10, w25, wzr, lsr #23
> eb9f9feb negs x11, xzr, asr #39
> c83f3fc8 stxp wzr, x8, x15, [x30] // store to LR ignoring success
> d9ffffd9 stz2g x25, [x30, #-16]! // v8.5 tag+zero 2 granules around LR
> etc. it does not need to be a guaranteed trap)

Unfortunately it's not a trap :/

Thanks,

Mathieu

--
Mathieu Desnoyers
EfficiOS Inc.