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

Szabolcs Nagy
2019-04-18T15:34:02Z

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?

i don't think we use __ARM_* in public headers currently,
but hopefully aarch64_be compilers implement it.

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)