[musl] [PATCH] x86_64: Generate an eh-frame entry for syscall_cp

[Ivan Trofimov <i.trofimow@yandex.ru>]

Some eBPF-based profilers (open-telemetry/opentelemetry-ebpf-profiler, 
yandex/perforator, parca-dev/parca etc. at github.com) leverage eh-frame 
section to unwind the stacks of native executables in kernel context.

This approach works pretty well most of the time, but sometimes fails to 
unwind through hand-written assembly due to it missing cfi-* directives.
Usually it's not that big of a deal and doesn't change the whole picture 
much, however with musl things are different, since these unwinders fail 
to unwind syscalls back to userspace, as musl doesn't generate an 
eh-frame entry for __syscall_cp function.

Imagine a flamegraph, where all (or most) of the syscalls are completely 
out of place: this is the problem this patch aims to fix for x86-64.

Signed-off-by: Ivan Trofimov <i.trofimow@yandex.ru>

---
  src/thread/x86_64/syscall_cp.s | 3 +++
  1 file changed, 3 insertions(+)

diff --git a/src/thread/x86_64/syscall_cp.s b/src/thread/x86_64/syscall_cp.s
index 4f101716..296415d7 100644
--- a/src/thread/x86_64/syscall_cp.s
+++ b/src/thread/x86_64/syscall_cp.s
@@ -10,6 +10,7 @@
  .hidden __syscall_cp_asm
  .type   __syscall_cp_asm,@function
  __syscall_cp_asm:
+.cfi_startproc

  __cp_begin:
         mov (%rdi),%eax
@@ -29,3 +30,5 @@ __cp_end:
         ret
  __cp_cancel:
         jmp __cancel
+
+.cfi_endproc
--