Better lock inversion prevention

Summary:
Saving and accessing dynamic linker structures outside of a `dl_iterate_phdr(3)` call is actually the wrong thing to do - the lock held by `dl_iterate_phdr` is needed to protect `addSharedLib()` from reading invalid memory.

Instead, eliminate the other side of the potential inversion, ensuring that we release our own lock before we call into code that transitively grabs the linker lock.

Reviewed By: ricardorey10

Differential Revision: D14278774

fbshipit-source-id: 6b91e0f2dc853deb299e0f2a82554d8a7e3afe2e

deps/linker/sharedlibs.cpp

@@ -73,8 +73,13 @@ bool ends_with(const char* str, const char* ending) {
 } // namespace (anonymous)
 
 elfSharedLibData sharedLib(char const* libname) {
-  ReaderLock rl(&sharedLibsMutex_);
-  auto lib = sharedLibData().at(basename(libname));
+  auto lib = [&]{
+    // this lambda is to ensure that we only hold the lock for the lookup.
+    // the boolean check outside this block transitively calls dladdr(3), which
+    // would cause a lock inversion with refresh_shared_libs under Bionic
+    ReaderLock rl(&sharedLibsMutex_);
+    return sharedLibData().at(basename(libname));
+  }();
   if (!lib) {
     throw std::out_of_range(libname);
   }
@@ -114,21 +119,12 @@ refresh_shared_libs() {
       return 1;
     }
 
-    std::vector<dl_phdr_info> to_add;
-    dl_iterate_phdr(+[](dl_phdr_info* info, size_t, void* vec) {
-      auto to_add = reinterpret_cast<std::vector<dl_phdr_info>*>(vec);
+    dl_iterate_phdr(+[](dl_phdr_info* info, size_t, void*) {
       if (info->dlpi_name && ends_with(info->dlpi_name, ".so")) {
-        // dl_iterate_phdr holds a global dl_* lock while invoking this and
-        // addSharedLib grabs its own locks. Other functions will grab those
-        // same locks and then call dl_* functions, which would result in a
-        // lock inversion.
-        to_add->push_back(*info);
+        addSharedLib(info->dlpi_name, info);
       }
       return 0;
-    }, &to_add);
-    for (auto info : to_add) {
-      addSharedLib(info.dlpi_name, &info);
-    }
+    }, nullptr);
   } else {
 #ifndef __LP64__ /* prior to android L there were no 64-bit devices anyway */
     soinfo* si = reinterpret_cast<soinfo*>(dlopen(nullptr, RTLD_LOCAL));