contrib: add stack_trace_call_fault.py

It's difficult to automatically detect calling an invalid, non-NULL
pointer when getting a stack trace. This manually recreates what we do
for calls to NULL since commit 412ce95 ("libdrgn: x86_64: unwind
call when pc is 0"). This was used to debug the issue fixed by "net:
tcp: fix crashes trying to free half-baked MTU probes" [1].

1: https://lore.kernel.org/all/20231010173651.3990234-1-kuba@kernel.org/T/

Signed-off-by: Omar Sandoval <osandov@osandov.com>

contrib/stack_trace_call_fault.py

@@ -0,0 +1,46 @@
+#!/usr/bin/env drgn
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# SPDX-License-Identifier: LGPL-2.1-or-later
+
+"""
+Get a stack trace from a call to an invalid address on x86-64. drgn should do
+this automatically eventually.
+"""
+
+from drgn import Object
+
+
+def pt_regs_members_from_stack_frame(frame):
+    regs = frame.registers()
+    return {
+        "r15": regs.get("r15", 0),
+        "r14": regs.get("r14", 0),
+        "r13": regs.get("r13", 0),
+        "r12": regs.get("r12", 0),
+        "bp": regs.get("rbp", 0),
+        "bx": regs.get("rbx", 0),
+        "r11": regs.get("r11", 0),
+        "r10": regs.get("r10", 0),
+        "r9": regs.get("r9", 0),
+        "r8": regs.get("r8", 0),
+        "ax": regs.get("rax", 0),
+        "cx": regs.get("rcx", 0),
+        "dx": regs.get("rdx", 0),
+        "si": regs.get("rsi", 0),
+        "di": regs.get("rdi", 0),
+        "orig_ax": -1,
+        "ip": regs.get("rip", 0),
+        "cs": regs.get("cs", 0),
+        "flags": regs.get("rflags", 0),
+        "sp": regs.get("rsp", 0),
+        "ss": regs.get("ss", 0),
+    }
+
+
+pt_regs_members = pt_regs_members_from_stack_frame(
+    prog.crashed_thread().stack_trace()[0]
+)
+pt_regs_members["ip"] = prog.read_word(pt_regs_members["sp"]) - 1
+pt_regs_members["sp"] += 8
+trace = prog.stack_trace(Object(prog, "struct pt_regs", pt_regs_members))
+print(trace)