Correctly find TLS block in aarch64.

Previously, we were just hardcoding the assumption that the TLS block
for the main binary starts 16 bytes after the thread control block.

This is _usually_ true, but not always, so let's follow the spec more
carefully by actually chasing the required pointers to get the correct address.

Author: umanwizard

interpreter/customlabels/customlabels.go

@@ -80,7 +80,7 @@ func Loader(_ interpreter.EbpfHandler, info *interpreter.LoaderInfo) (interprete
 			return nil, err
 		}
 		if ef.Machine == elf.EM_AARCH64 {
-			tlsAddr = libpf.Address(tlsSym.Address + 16)
+			tlsAddr = libpf.Address(tlsSym.Address)
 		} else if ef.Machine == elf.EM_X86_64 {
 			// Symbol addresses are relative to the start of the
 			// thread-local storage image, but the thread pointer points to the _end_

support/ebpf/interpreter_dispatcher.ebpf.c

@@ -212,12 +212,33 @@ get_native_custom_labels(PerCPURecord *record, NativeCustomLabelsProcInfo *proc)
     return false;
   }
 
-  u64 offset = tsd_base + proc->tls_offset;
-  DEBUG_PRINT("cl: native custom labels data at 0x%llx", offset);
+  int err;
+#if defined(__aarch64__)
+  // ELF Handling For Thread-Local Storage, p.5.
+  // The thread register points to a "TCB" (Thread Control Block)
+  // whose first element is a pointer to a "DTV"  (Dynamic Thread Vector)...
+  u64 dtv_addr;
+  if ((err = bpf_probe_read_user(&dtv_addr, sizeof(void *), (void *)(tsd_base)))) {
+    increment_metric(metricID_UnwindNativeCustomLabelsErrReadData);
+    DEBUG_PRINT("Failed to read TLS DTV addr: %d", err);
+    return false;
+  }
+  // ... and at offsite 16 in the DTV, there is a pointer to the TLS block.
+  u64 addr;
+  if ((err = bpf_probe_read_user(&addr, sizeof(void *), (void *)(dtv_addr + 16)))) {
+    increment_metric(metricID_UnwindNativeCustomLabelsErrReadData);
+    DEBUG_PRINT("Failed to read main TLS block addr: %d", err);
+    return false;
+  }
+  addr += proc->tls_offset;
+#else
+  u64 addr = tsd_base + proc->tls_offset;
+#endif
+
+  DEBUG_PRINT("cl: native custom labels data at 0x%llx", addr);
 
   NativeCustomLabelsSet *p_current_set;
-  int err;
-  if ((err = bpf_probe_read_user(&p_current_set, sizeof(void *), (void *)(offset)))) {
+  if ((err = bpf_probe_read_user(&p_current_set, sizeof(void *), (void *)(addr)))) {
     increment_metric(metricID_UnwindNativeCustomLabelsErrReadData);
     DEBUG_PRINT("Failed to read custom labels current set pointer: %d", err);
     return false;