[compiler-rt] Introduce the notion of an interceptor trampoline

[melver]

Summary

To make the interceptor implementation more flexible, allowing for 2
levels of indirection instead of just 1 in the current scheme (where the
intercepted function aliases the interceptor implementation), introduce
the notion of an interceptor "trampoline".

A trampoline may be a real function (and not just an alias, where
aliases of aliases do not work), which will simply forward to the
interceptor implementation; the intercepted function will then alias the
trampoline:

func -[alias]-> trampoline -[call]-> interceptor

Make the necessary changes to prepare for introducing real trampolines.

This change does not yet introduce any real trampolines, and so
trampoline == interceptor, and we currently still just have:

func -[alias]-> interceptor

NFC.

compiler-rt/lib/interception/interception.h

 #define INTERPOSER_2(func_name, wrapper_name) __attribute__((used)) \
 const interpose_substitution substitution_##func_name[] \
     __attribute__((section("__DATA, __interpose"))) = { \
     { reinterpret_cast<const uptr>(wrapper_name), \
       reinterpret_cast<const uptr>(func_name) } \
 }
 
 # define WRAP(x) wrap_##x
+# define TRAMPOLINE(x) WRAP(x)
 # define INTERCEPTOR_ATTRIBUTE
 # define DECLARE_WRAPPER(ret_type, func, ...)
 
 #elif SANITIZER_WINDOWS
 # define WRAP(x) __asan_wrap_##x
+# define TRAMPOLINE(x) WRAP(x)
 # define INTERCEPTOR_ATTRIBUTE __declspec(dllexport)
 # define DECLARE_WRAPPER(ret_type, func, ...) \
     extern "C" ret_type func(__VA_ARGS__);
 # define DECLARE_WRAPPER_WINAPI(ret_type, func, ...) \
     extern "C" __declspec(dllimport) ret_type __stdcall func(__VA_ARGS__);
 #elif SANITIZER_FREEBSD || SANITIZER_NETBSD
 # define WRAP(x) __interceptor_ ## x
+# define TRAMPOLINE(x) WRAP(x)
 # define INTERCEPTOR_ATTRIBUTE __attribute__((visibility("default")))
 // FreeBSD's dynamic linker (incompliantly) gives non-weak symbols higher
 // priority than weak ones so weak aliases won't work for indirect calls
 // in position-independent (-fPIC / -fPIE) mode.
 # define DECLARE_WRAPPER(ret_type, func, ...) \
      extern "C" ret_type func(__VA_ARGS__) \
      __attribute__((alias("__interceptor_" #func), visibility("default")));
 #elif !SANITIZER_FUCHSIA
 # define WRAP(x) __interceptor_ ## x
+# define TRAMPOLINE(x) WRAP(x)
 # define INTERCEPTOR_ATTRIBUTE __attribute__((visibility("default")))
 # define DECLARE_WRAPPER(ret_type, func, ...) \
     extern "C" ret_type func(__VA_ARGS__) \
     __attribute__((weak, alias("__interceptor_" #func), visibility("default")));
 #endif
 
 #if SANITIZER_FUCHSIA
 // There is no general interception at all on Fuchsia.

 #else  // SANITIZER_APPLE
 # define REAL(x) x
 # define DECLARE_REAL(ret_type, func, ...) \
     extern "C" ret_type func(__VA_ARGS__);
 # define ASSIGN_REAL(x, y)
 #endif  // SANITIZER_APPLE
 
 #if !SANITIZER_FUCHSIA
-#  define DECLARE_REAL_AND_INTERCEPTOR(ret_type, func, ...) \
+# define DECLARE_REAL_AND_INTERCEPTOR(ret_type, func, ...)  \
     DECLARE_REAL(ret_type, func, __VA_ARGS__)               \
+    extern "C" ret_type TRAMPOLINE(func)(__VA_ARGS__);      \
     extern "C" ret_type WRAP(func)(__VA_ARGS__);
 // Declare an interceptor and its wrapper defined in a different translation
 // unit (ex. asm).
-# define DECLARE_EXTERN_INTERCEPTOR_AND_WRAPPER(ret_type, func, ...)    \
-  extern "C" ret_type WRAP(func)(__VA_ARGS__); \
+# define DECLARE_EXTERN_INTERCEPTOR_AND_WRAPPER(ret_type, func, ...)  \
+    extern "C" ret_type TRAMPOLINE(func)(__VA_ARGS__);                \
+    extern "C" ret_type WRAP(func)(__VA_ARGS__);                      \
     extern "C" ret_type func(__VA_ARGS__);
 #else
 # define DECLARE_REAL_AND_INTERCEPTOR(ret_type, func, ...)
 # define DECLARE_EXTERN_INTERCEPTOR_AND_WRAPPER(ret_type, func, ...)
 #endif
 
 // Generally, you don't need to use DEFINE_REAL by itself, as INTERCEPTOR
 // macros does its job. In exceptional cases you may need to call REAL(foo)
 // without defining INTERCEPTOR(..., foo, ...). For example, if you override

compiler-rt/lib/interception/interception_linux.h

 # error "interception_linux.h should be included from interception library only"
 #endif
 
 #ifndef INTERCEPTION_LINUX_H
 #define INTERCEPTION_LINUX_H
 
 namespace __interception {
 bool InterceptFunction(const char *name, uptr *ptr_to_real, uptr func,
-                       uptr wrapper);
+                       uptr trampoline);
 bool InterceptFunction(const char *name, const char *ver, uptr *ptr_to_real,
-                       uptr func, uptr wrapper);
+                       uptr func, uptr trampoline);
 }  // namespace __interception
 
 #define INTERCEPT_FUNCTION_LINUX_OR_FREEBSD(func) \
   ::__interception::InterceptFunction(            \
       #func,                                      \
       (::__interception::uptr *) & REAL(func),    \
       (::__interception::uptr) & (func),          \
-      (::__interception::uptr) & WRAP(func))
+      (::__interception::uptr) & TRAMPOLINE(func))
 
 // dlvsym is a GNU extension supported by some other platforms.
 #if SANITIZER_GLIBC || SANITIZER_FREEBSD || SANITIZER_NETBSD
 #define INTERCEPT_FUNCTION_VER_LINUX_OR_FREEBSD(func, symver) \
   ::__interception::InterceptFunction(                        \
       #func, symver,                                          \
       (::__interception::uptr *) & REAL(func),                \
       (::__interception::uptr) & (func),                      \
-      (::__interception::uptr) & WRAP(func))
+      (::__interception::uptr) & TRAMPOLINE(func))
 #else
 #define INTERCEPT_FUNCTION_VER_LINUX_OR_FREEBSD(func, symver) \
   INTERCEPT_FUNCTION_LINUX_OR_FREEBSD(func)
 #endif  // SANITIZER_GLIBC || SANITIZER_FREEBSD || SANITIZER_NETBSD
 
 #endif  // INTERCEPTION_LINUX_H
 #endif  // SANITIZER_LINUX || SANITIZER_FREEBSD || SANITIZER_NETBSD ||
         // SANITIZER_SOLARIS

compiler-rt/lib/interception/interception_linux.cpp

while (true) {
     if (*s1 == 0)
       return true;
     s1++;
     s2++;
   }
 }
 #endif
 
-static void *GetFuncAddr(const char *name, uptr wrapper_addr) {
+static void *GetFuncAddr(const char *name, uptr trampoline) {
 #if SANITIZER_NETBSD
   // FIXME: Find a better way to handle renames
   if (StrCmp(name, "sigaction"))
     name = "__sigaction14";
 #endif
   void *addr = dlsym(RTLD_NEXT, name);
   if (!addr) {
     // If the lookup using RTLD_NEXT failed, the sanitizer runtime library is
     // later in the library search order than the DSO that we are trying to
     // intercept, which means that we cannot intercept this function. We still
     // want the address of the real definition, though, so look it up using
     // RTLD_DEFAULT.
     addr = dlsym(RTLD_DEFAULT, name);
 
     // In case `name' is not loaded, dlsym ends up finding the actual wrapper.
     // We don't want to intercept the wrapper and have it point to itself.
-    if ((uptr)addr == wrapper_addr)
+    if ((uptr)addr == trampoline)
       addr = nullptr;
   }
   return addr;
 }
 
 bool InterceptFunction(const char *name, uptr *ptr_to_real, uptr func,
-                       uptr wrapper) {
-  void *addr = GetFuncAddr(name, wrapper);
+                       uptr trampoline) {
+  void *addr = GetFuncAddr(name, trampoline);
   *ptr_to_real = (uptr)addr;
-  return addr && (func == wrapper);
+  return addr && (func == trampoline);
 }
 
 // dlvsym is a GNU extension supported by some other platforms.
 #if SANITIZER_GLIBC || SANITIZER_FREEBSD || SANITIZER_NETBSD
 static void *GetFuncAddr(const char *name, const char *ver) {
   return dlvsym(RTLD_NEXT, name, ver);
 }
 
 bool InterceptFunction(const char *name, const char *ver, uptr *ptr_to_real,
-                       uptr func, uptr wrapper) {
+                       uptr func, uptr trampoline) {
   void *addr = GetFuncAddr(name, ver);
   *ptr_to_real = (uptr)addr;
-  return addr && (func == wrapper);
+  return addr && (func == trampoline);
 }
-#endif  // SANITIZER_GLIBC || SANITIZER_FREEBSD || SANITIZER_NETBSD
+#  endif  // SANITIZER_GLIBC || SANITIZER_FREEBSD || SANITIZER_NETBSD
 
 }  // namespace __interception
 
 #endif  // SANITIZER_LINUX || SANITIZER_FREEBSD || SANITIZER_NETBSD ||
         // SANITIZER_SOLARIS