Don't abort on dlopen/dlsym when dynamic linking is not available (#19527)

Instead simply return failure with a error message in `dlerror`.  This
matches the bahaviour of other libc/syscall functions and allows the
behaviour to be controlled via the existing ALLOW_UNIMPLEMENTED_SYSCALLS
setting.

The only downside (IIUC) is that emscripten is a little less
helpful/magical in advising of what to do if these functions are used
without `-sMAIN_MODULE`.

Fixes: #19411

Author: sbc100

ChangeLog.md

@@ -23,6 +23,11 @@ See docs/process.md for more on how version tagging works.
 - The log message that emcc will sometime print (for example when auto-building
   system libraries) can now be completely supressed by running with
   `EMCC_LOGGING=0`.
+- Runtime dynamic linking symbols such as dlopen and dlsym will no longer cause
+  a linker error when building without `-sMAIN_MODULE`.  Instead stub functions
+  will be included that fail at runtime.  This matches the behaviour of other
+  libc functions that we don't implement.  For those that prefer to get a linker
+  error we have the `-sALLOW_UNIMPLEMENTED_SYSCALLS` settings. (#19527)
 
 3.1.41 - 06/06/23
 -----------------

src/library_dylink.js

@@ -6,10 +6,11 @@
  * Dynamic library loading
  */
 
-var dlopenMissingError = "'To use dlopen, you need enable dynamic linking, see https://emscripten.org/docs/compiling/Dynamic-Linking.html'"
+#if !RELOCATABLE
+#error "library_dylink.js requires RELOCATABLE"
+#endif
 
 var LibraryDylink = {
-#if RELOCATABLE
 #if FILESYSTEM
   $registerWasmPlugin__deps: ['$preloadPlugins'],
   $registerWasmPlugin: function() {
@@ -309,49 +310,7 @@ var LibraryDylink = {
     dbg('done reportUndefinedSymbols');
 #endif
   },
-#endif
 
-#if !MAIN_MODULE
-#if !ALLOW_UNIMPLEMENTED_SYSCALLS
-  _dlopen_js__deps: [function() { error(dlopenMissingError); }],
-  _emscripten_dlopen_js__deps: [function() { error(dlopenMissingError); }],
-  _dlsym_js__deps: [function() { error(dlopenMissingError); }],
-  _dlsym_catchup_js__deps: [function() { error(dlopenMissingError); }],
-  dlopen__deps: [function() { error(dlopenMissingError); }],
-  emscripten_dlopen__deps: [function() { error(dlopenMissingError); }],
-  __dlsym__deps: [function() { error(dlopenMissingError); }],
-#else
-  $dlopenMissingError: `= ${dlopenMissingError}`,
-  _dlopen_js__deps: ['$dlopenMissingError'],
-  _emscripten_dlopen_js__deps: ['$dlopenMissingError'],
-  _dlsym_js__deps: ['$dlopenMissingError'],
-  _dlsym_catchup_js__deps: ['$dlopenMissingError'],
-  dlopen__deps: ['$dlopenMissingError'],
-  emscripten_dlopen__deps: ['$dlopenMissingError'],
-  __dlsym__deps: ['$dlopenMissingError'],
-#endif
-  _dlopen_js: function(handle) {
-    abort(dlopenMissingError);
-  },
-  _emscripten_dlopen_js: function(handle, onsuccess, onerror, user_data) {
-    abort(dlopenMissingError);
-  },
-  _dlsym_js: function(handle, symbol) {
-    abort(dlopenMissingError);
-  },
-  _dlsym_catchup_js: function(handle, symbolIndex) {
-    abort(dlopenMissingError);
-  },
-  dlopen: function(filename, flags) {
-    abort(dlopenMissingError);
-  },
-  emscripten_dlopen: function(handle, onsuccess, onerror, user_data) {
-    abort(dlopenMissingError);
-  },
-  __dlsym: function(handle, symbol, ra) {
-    abort(dlopenMissingError);
-  },
-#else // MAIN_MODULE != 0
   // dynamic linker/loader (a-la ld.so on ELF systems)
   $LDSO__deps: ['$newDSO'],
   $LDSO: {
@@ -1255,7 +1214,6 @@ var LibraryDylink = {
 #endif
     return result;
   },
-#endif // MAIN_MODULE != 0
 };
 
 mergeInto(LibraryManager.library, LibraryDylink);

src/library_sigs.js

@@ -211,7 +211,6 @@ sigs = {
   __asctime_r__sig: 'ppp',
   __assert_fail__sig: 'vppip',
   __call_sighandler__sig: 'vpi',
-  __dlsym__sig: 'pppp',
   __emscripten_init_main_thread_js__sig: 'vp',
   __emscripten_thread_cleanup__sig: 'vp',
   __handle_stack_overflow__sig: 'vp',
@@ -512,7 +511,6 @@ sigs = {
   boxRGBA__sig: 'ipiiiiiiii',
   clock_res_get__sig: 'iip',
   clock_time_get__sig: 'iijp',
-  dlopen__sig: 'ppi',
   eglBindAPI__sig: 'ii',
   eglChooseConfig__sig: 'ipppip',
   eglCreateContext__sig: 'ppppp',
@@ -593,7 +591,6 @@ sigs = {
   emscripten_destroy_audio_context__sig: 'vi',
   emscripten_destroy_web_audio_node__sig: 'vi',
   emscripten_destroy_worker__sig: 'vi',
-  emscripten_dlopen__sig: 'vpippp',
   emscripten_enter_soft_fullscreen__sig: 'ipp',
   emscripten_err__sig: 'vp',
   emscripten_errn__sig: 'vpp',

src/modules.js

@@ -49,7 +49,6 @@ global.LibraryManager = {
       'library_html5.js',
       'library_stack_trace.js',
       'library_wasi.js',
-      'library_dylink.js',
       'library_makeDynCall.js',
       'library_eventloop.js',
       'library_promise.js',
@@ -80,6 +79,10 @@ global.LibraryManager = {
       libraries.push('library_syscall.js');
     }
 
+    if (RELOCATABLE) {
+      libraries.push('library_dylink.js');
+    }
+
     if (FILESYSTEM) {
       libraries.push('library_fs_shared.js');
       if (WASMFS) {

system/lib/libc/emscripten_libc_stubs.c

@@ -213,3 +213,17 @@ weak int sigaltstack(const stack_t *restrict ss, stack_t *restrict old_ss) {
   errno = ENOSYS;
   return -1;
 }
+
+#ifndef __PIC__
+void __dl_seterr(const char*, ...);
+
+weak void *__dlsym(void *restrict p, const char *restrict s, void *restrict ra) {
+  __dl_seterr("dynamic linking not enabled");
+  return NULL;
+}
+
+weak void* dlopen(const char* file, int flags) {
+  __dl_seterr("dynamic linking not enabled");
+  return NULL;
+}
+#endif

test/other/test_dlopen_blocking.c

@@ -5,14 +5,14 @@
 
 int main() {
   void* handle = dlopen("libside.so", RTLD_NOW);
-  printf("handle: %p\n", handle);
   if (!handle) {
-    printf("%s\n", dlerror());
+    printf("dlopen failed: %s\n", dlerror());
     return 1;
   }
+  printf("handle: %p\n", handle);
   int* foo = (int*)dlsym(handle, "foo");
   if (!foo) {
-    printf("%s\n", dlerror());
+    printf("dlsym failed: %s\n", dlerror());
     return 1;
   }
   printf("foo = %d\n", *foo);

test/test_other.py

@@ -12209,13 +12209,13 @@ def test_unimplemented_syscalls_dlopen(self):
     cmd = [EMCC, test_file('other/test_dlopen_blocking.c')] + self.get_emcc_args()
     self.run_process(cmd)
     err = self.run_js('a.out.js', assert_returncode=NON_ZERO)
-    self.assertContained('Aborted(To use dlopen, you need enable dynamic linking, see https://emscripten.org/docs/compiling/Dynamic-Linking.html)', err)
+    self.assertContained('dlopen failed: dynamic linking not enabled', err)
 
     # If we build the same thing with ALLOW_UNIMPLEMENTED_SYSCALLS=0 we
     # expect a link-time failure rather than a runtime one.
     cmd += ['-sALLOW_UNIMPLEMENTED_SYSCALLS=0']
     err = self.expect_fail(cmd)
-    self.assertContained('To use dlopen, you need enable dynamic linking, see https://emscripten.org/docs/compiling/Dynamic-Linking.html', err)
+    self.assertContained('undefined symbol: dlopen', err)
 
   def test_unimplemented_syscalls_dladdr(self):
     create_file('main.c', '''

tools/gen_sig_info.py

@@ -169,7 +169,10 @@ def ignore_symbol(s, cxx):
     return True
   if s.startswith('gl') and any(s.endswith(x) for x in ('NV', 'EXT', 'WEBGL', 'ARB', 'ANGLE')):
     return True
-  if cxx and s in ('__dlsym', '__asctime_r'):
+  if s in ('__stack_base', '__memory_base', '__table_base', '__global_base', '__heap_base',
+           '__stack_pointer', '__stack_high', '__stack_low'):
+    return True
+  if cxx and s in ('__asctime_r',):
     return True
   return False
 
@@ -381,7 +384,7 @@ def main(args):
   extract_sig_info(sig_info, {'LEGACY_GL_EMULATION': 1}, ['-DGLES'])
   extract_sig_info(sig_info, {'USE_GLFW': 2, 'FULL_ES3': 1, 'MAX_WEBGL_VERSION': 2})
   extract_sig_info(sig_info, {'STANDALONE_WASM': 1})
-  extract_sig_info(sig_info, {'MAIN_MODULE': 2, 'USE_WEBGPU': 1})
+  extract_sig_info(sig_info, {'MAIN_MODULE': 2, 'RELOCATABLE': 1, 'USE_WEBGPU': 1})
 
   write_sig_library(args.output, sig_info)
   if args.update: