use dlsym to find _r_debug to simplify build configs

Summary:
Rather than customizing the build rules to force `-fPIC` and static linkage on this one source, do everything in-source using `dlsym` and let the build rules be simplified.

The `dlsym` result is cached permanently at module load time. It must happen at load time before any signal handlers are attached, since `dlsym` is not async-signal-safe.

Reviewed By: jpporto

Differential Revision: D71051318

fbshipit-source-id: 955793e7b589ad3376b3804c4c696a20dc311c2b

Author: yfeldblum

third-party/folly/src/folly/debugging/symbolizer/detail/Debug.cpp

@@ -16,28 +16,70 @@
 
 #include <folly/debugging/symbolizer/detail/Debug.h>
 
-#include <folly/CppAttributes.h>
+#include <folly/Portability.h>
+
+#ifndef _WIN32
+#include <dlfcn.h>
+#endif
 
 #if FOLLY_HAVE_ELF
 #include <link.h>
 #endif
 
 #if defined(__APPLE__) && !TARGET_OS_OSX
-#define NO_R_DEBUG
+#define FOLLY_DETAIL_HAS_R_DEBUG 0
 #elif !defined(__linux__) || !FOLLY_HAVE_ELF || !FOLLY_HAVE_DWARF
-#define NO_R_DEBUG
+#define FOLLY_DETAIL_HAS_R_DEBUG 0
+#else
+#define FOLLY_DETAIL_HAS_R_DEBUG 1
 #endif
 
 namespace folly {
 namespace symbolizer {
 namespace detail {
 
+#if FOLLY_DETAIL_HAS_R_DEBUG
+
+/// There is a strong requirement of finding the true _r_debug in ld.so.
+///
+/// The previous code used the declaration of the extern variable as found in
+///
+///   #include <link.h>
+///
+/// The intention of using the extern variable is to get the toolchain to emit a
+/// GOT access, e.g. like:
+///
+///   mov rax, qword ptr [rip + _r_debug@GOTPCREL]
+///
+/// This works in typical conditions. However, in the case of compiling with:
+///
+///   -mcmodel=small -fno-pic
+///
+/// ie with non-pic and small-code-model, there is a different outcome. Here
+/// instead, the toolchain emits a COPY relocation to copy _r_debug into the
+/// executable and emits a direct non-GOT access to the copy like:
+///
+///   mov eax, offset _r_debug
+///
+/// This causes all accesses in ld.so to be redirected to the copy in the main
+/// executable, which is a valid approach.
+///
+/// However, LLDB specifically looks for _r_debug in ld.so. When the debugger
+/// inspects a process, the _r_debug in the executable has all of the current
+/// information about the link and load state, but the debugger looks only at
+/// the _r_debug in ld.so which is empty! This breaks debugging.
+static r_debug* r_debug_cache_;
+[[gnu::constructor(101)]] void r_debug_cache_init_() {
+  r_debug_cache_ = static_cast<r_debug*>(dlsym(RTLD_DEFAULT, "_r_debug"));
+}
+
+#endif
+
 struct r_debug* get_r_debug() {
-#ifdef NO_R_DEBUG
-  return nullptr;
-#else
-  return &_r_debug;
+#if FOLLY_DETAIL_HAS_R_DEBUG
+  return r_debug_cache_;
 #endif
+  return nullptr;
 }
 
 } // namespace detail