getdeps: keep $LDFLAGS on the shared library links under --shared-lib

build/fbcode_builder/getdeps/shared_lib.py

@@ -12,6 +12,7 @@
 
 from __future__ import annotations
 
+import os
 import typing
 
 if typing.TYPE_CHECKING:
@@ -36,14 +37,20 @@ def apply_shared_lib_top_level_cmake_defines(
     - On Linux: append -Wl,--exclude-libs=ALL to the shared/module linker
       flags so the deps' symbols stay out of the produced .so's dynamic
       export table (avoids ODR clashes if the host process loads another
-      copy of any dep).
+      copy of any dep). Defining CMAKE_*_LINKER_FLAGS on the command line
+      stops CMake from seeding them from $LDFLAGS, so start from $LDFLAGS
+      (as CMake would) to keep the caller's linker flags, e.g. a distro's
+      hardening flags, on the shared library links.
     """
     if not build_opts.shared_lib:
         return
     defines.setdefault("BUILD_SHARED_LIBS", "ON")
     if build_opts.is_linux():
+        ldflags = os.environ.get("LDFLAGS", "")
         for var in ("CMAKE_SHARED_LINKER_FLAGS", "CMAKE_MODULE_LINKER_FLAGS"):
-            defines[var] = _append_token(defines.get(var, ""), "-Wl,--exclude-libs=ALL")
+            defines[var] = _append_token(
+                defines.get(var, ldflags), "-Wl,--exclude-libs=ALL"
+            )
 
 
 def apply_shared_lib_dep_env(

build/fbcode_builder/getdeps/test/shared_lib_test.py

@@ -4,8 +4,9 @@
 # LICENSE file in the root directory of this source tree.
 
 
+import os
 import unittest
-from unittest.mock import MagicMock
+from unittest.mock import MagicMock, patch
 
 from ..envfuncs import Env
 from ..shared_lib import (
@@ -79,6 +80,31 @@ def test_idempotent(self) -> None:
         apply_shared_lib_top_level_cmake_defines(defines, opts)
         self.assertEqual(defines["CMAKE_SHARED_LINKER_FLAGS"], "-Wl,--exclude-libs=ALL")
 
+    def test_env_ldflags_seed_the_linker_flags(self) -> None:
+        defines = {}
+        with patch.dict(os.environ, {"LDFLAGS": "-Wl,-z,relro -Wl,-z,now"}):
+            apply_shared_lib_top_level_cmake_defines(
+                defines, make_build_opts(is_linux=True)
+            )
+        for var in ("CMAKE_SHARED_LINKER_FLAGS", "CMAKE_MODULE_LINKER_FLAGS"):
+            self.assertEqual(
+                defines[var], "-Wl,-z,relro -Wl,-z,now -Wl,--exclude-libs=ALL"
+            )
+
+    def test_explicit_linker_flags_win_over_env_ldflags(self) -> None:
+        defines = {"CMAKE_SHARED_LINKER_FLAGS": "-Wl,--as-needed"}
+        with patch.dict(os.environ, {"LDFLAGS": "-Wl,-z,now"}):
+            apply_shared_lib_top_level_cmake_defines(
+                defines, make_build_opts(is_linux=True)
+            )
+        self.assertEqual(
+            defines["CMAKE_SHARED_LINKER_FLAGS"],
+            "-Wl,--as-needed -Wl,--exclude-libs=ALL",
+        )
+        self.assertEqual(
+            defines["CMAKE_MODULE_LINKER_FLAGS"], "-Wl,-z,now -Wl,--exclude-libs=ALL"
+        )
+
 
 class ApplyDepEnvTest(unittest.TestCase):
     def test_no_op_when_shared_lib_off(self) -> None: