On 18/04/2019 14:17, Mathieu Desnoyers wrote: > ----- On Apr 17, 2019, at 3:56 PM, Mathieu Desnoyers mathieu.desnoyers@xxxxxxxxxxxx wrote: >> ----- On Apr 17, 2019, at 12:17 PM, Joseph Myers joseph@xxxxxxxxxxxxxxxx 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)
