fix linux unwind info, per x86_64 ABI

Closes #23074
Fixes #35155

Author: vtjnash

src/task.c

@@ -807,10 +807,25 @@ STATIC_OR_JS void NOINLINE JL_NORETURN start_task(void)
 {
 #ifdef _OS_WINDOWS_
 #if defined(_CPU_X86_64_)
-    // install the unhandled exception hanlder at the top of our stack
+    // install the unhandled exception handler at the top of our stack
     // to call directly into our personality handler
     asm volatile ("\t.seh_handler __julia_personality, @except\n\t.text");
 #endif
+#else
+    // wipe out the call-stack unwind capability beyond this function
+    // (we are noreturn, so it is not a total lie)
+#if defined(_CPU_X86_64_)
+    // per nongnu libunwind: "x86_64 ABI specifies that end of call-chain is marked with a NULL RBP or undefined return address"
+    // so we do all 3, to be extra certain of it
+    asm volatile ("\t.cfi_undefined rip");
+    asm volatile ("\t.cfi_undefined rbp");
+    asm volatile ("\t.cfi_return_column rbp");
+#else
+    // per nongnu libunwind: "DWARF spec says undefined return address location means end of stack"
+    // we use whatever happens to be register 1 on this platform for this
+    asm volatile ("\t.cfi_undefined 1");
+    asm volatile ("\t.cfi_return_column 1");
+#endif
 #endif
 
     // this runs the first time we switch to a task