thneed: lookup dlsym offset at compile time (#20019)

old-commit-hash: 1744076514

selfdrive/modeld/SConscript

@@ -3,6 +3,15 @@ lenv = env.Clone()
 
 libs = [cereal, messaging, common, 'OpenCL', 'SNPE', 'symphony-cpu', 'capnp', 'zmq', 'kj', 'yuv', gpucommon, visionipc]
 
+def get_dlsym_offset():
+  """Returns the offset between dlopen and dlsym in libdl.so"""
+  import ctypes
+  libdl = ctypes.PyDLL('libdl.so')
+  dlopen = ctypes.cast(libdl.dlopen, ctypes.c_void_p).value
+  dlsym = ctypes.cast(libdl.dlsym, ctypes.c_void_p).value
+  return dlsym - dlopen
+
+
 common_src = [
   "models/commonmodel.cc",
   "runners/snpemodel.cc",
@@ -21,8 +30,9 @@ if arch == "aarch64" or arch == "larch64":
   libs += ['gnustl_shared'] if arch == "aarch64" else ['pthread', 'dl']
 
   common_src += thneed_src
-  lenv['CFLAGS'].append("-DUSE_THNEED")
+  dlsym_offset = get_dlsym_offset()
   lenv['CXXFLAGS'].append("-DUSE_THNEED")
+  lenv['CXXFLAGS'].append(f"-DDLSYM_OFFSET={dlsym_offset}")
 else:
   libs += ['pthread']
 
@@ -45,6 +55,7 @@ else:
 
 common_model = lenv.Object(common_src)
 
+
 # build thneed model
 if arch == "aarch64" or arch == "larch64":
   compiler = lenv.Program('thneed/compile', ["thneed/compile.cc" ]+common_model, LIBS=libs)

selfdrive/modeld/thneed/thneed.cc

@@ -433,11 +433,8 @@ cl_program thneed_clCreateProgramWithSource(cl_context context, cl_uint count, c
 }
 
 void *dlsym(void *handle, const char *symbol) {
-  // TODO: Find dlsym in a better way. Currently this is hand looked up in libdl.so
-#if defined QCOM
-  void *(*my_dlsym)(void *handle, const char *symbol) = (void *(*)(void *handle, const char *symbol))((uintptr_t)dlopen-0x2d4);
-#elif defined QCOM2
-  void *(*my_dlsym)(void *handle, const char *symbol) = (void *(*)(void *handle, const char *symbol))((uintptr_t)dlopen+0x138);
+#if defined(QCOM) || defined(QCOM2)
+  void *(*my_dlsym)(void *handle, const char *symbol) = (void *(*)(void *handle, const char *symbol))((uintptr_t)dlopen + DLSYM_OFFSET);
 #else
   #error "Unsupported platform for thneed"
 #endif