RFR: 8371368: SIGSEGV in JfrVframeStream::next_vframe() on arm64 [v2]

[Markus Grönlund]

On Wed, 19 Nov 2025 11:16:51 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> Markus Grönlund has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   ppc and s390 do not have frame::sender_sp_offset defined
>
> src/hotspot/share/jfr/periodic/sampling/jfrThreadSampling.cpp line 220:
> 
>> 218:           if (is_valid(pc_desc)) {
>> 219:             intptr_t* const synthetic_sp = sender_sp - sampled_nm->frame_size();
>> 220:             top_frame = frame(synthetic_sp, synthetic_sp, sender_sp - 2, pc_desc->real_pc(sampled_nm), sampled_nm);
> 
> Hold on. I am looking at relevant constructor:
> 
> 
> inline frame::frame(intptr_t* sp)
>   : frame(sp, sp,
>           *(intptr_t**)(sp - frame::sender_sp_offset),
>           pauth_strip_verifiable(*(address*)(sp - 1))) {}
> 
> 
> ...and:
> 
> 
> inline intptr_t* frame::fp(const intptr_t* sp) {
>   assert(sp != nullptr, "invariant");
>   return reinterpret_cast<intptr_t*>(sp[-2]);
> }
> 
> 
> So `sender_sp - 2` (which I think is `sp - frame::sender_sp_offset`?) is the _location_ for the FP, not the FP itself?

sender_sp - 2 is the calculated synthetic fp, just like synthetic sp is the calculated sp (from sender sp - cb->frame_size()) for the frame that we are reconstructing.

We are stackwalking backwards, if that helps with the conceptual model.

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PR Review Comment: https://git.openjdk.org/jdk/pull/28373#discussion_r2542128620