improve: drop soinfo records of loaded modules

In Bionic linker, the `soinfo` structure has a field `next`, which points to the next loaded library in a linked list consisting of all loaded libraries.
Hence, an injected process can easily find all loaded libraries.
Previously in ReZygisk, module library records are hidden by setting the `pathname` field to be empty, which is futile but easier to detect.
Current idea of dropping record can be found in the following commit:
5d635e8c66

loader/src/include/solist.hpp

@@ -5,6 +5,7 @@
 
 #include <string>
 #include "elf_util.h"
+#include "logging.h"
 
 namespace SoList  {
   class SoInfo {
@@ -36,23 +37,59 @@ namespace SoList  {
         return ((std::string *) ((uintptr_t) this + solist_realpath_offset - sizeof(void *)))->c_str();
       }
 
-      void nullify_name() {
-        const char **name = (const char**)get_soname_sym(this);
-
-        static const char *empty_string = "";
-        *name = reinterpret_cast<const char *>(&empty_string);
-      }
-
-      void nullify_path() {
-        const char **name = (const char**)get_realpath_sym(this);
-
-        static const char *empty_string = "";
-        *name = reinterpret_cast<const char *>(&empty_string);
+      void set_next(SoInfo *si) {
+        *(SoInfo **) ((uintptr_t) this + solist_next_offset) = si;
       }
   };
 
+  class ProtectedDataGuard {
+    public:
+      ProtectedDataGuard() {
+        if (ctor != nullptr)
+          (this->*ctor)();
+      }
+
+      ~ProtectedDataGuard() {
+        if (dtor != nullptr)
+          (this->*dtor)();
+      }
+
+      static bool setup(const SandHook::ElfImg &linker) {
+        ctor = MemFunc{.data = {.p = reinterpret_cast<void *>(linker.getSymbAddress(
+                                    "__dl__ZN18ProtectedDataGuardC2Ev")), .adj = 0}}.f;
+        dtor = MemFunc{.data = {.p = reinterpret_cast<void *>(linker.getSymbAddress(
+                                    "__dl__ZN18ProtectedDataGuardD2Ev")), .adj = 0}}.f;
+        return ctor != nullptr && dtor != nullptr;
+      }
+
+      ProtectedDataGuard(const ProtectedDataGuard &) = delete;
+
+      void operator=(const ProtectedDataGuard &) = delete;
+
+    private:
+      using FuncType = void (ProtectedDataGuard::*)();
+
+      static FuncType ctor;
+      static FuncType dtor;
+
+      union MemFunc {
+        FuncType f;
+
+        struct {
+          void *p;
+          std::ptrdiff_t adj;
+        } data;
+      };
+  };
+
+
   static SoInfo *solist = NULL;
   static SoInfo *somain = NULL;
+  static SoInfo **sonext = NULL;
+  ProtectedDataGuard::FuncType ProtectedDataGuard::ctor = NULL;
+  ProtectedDataGuard::FuncType ProtectedDataGuard::dtor = NULL;
+
+  static bool Initialize();
 
   template<typename T>
   inline T *getStaticPointer(const SandHook::ElfImg &linker, const char *name) {
@@ -61,25 +98,28 @@ namespace SoList  {
     return addr == NULL ? NULL : *addr;
   }
 
-  static void NullifySoName(const char* target_name) {
-    for (auto *iter = solist; iter; iter = iter->get_next()) {
-      if (iter->get_name() && iter->get_path() && strstr(iter->get_path(), target_name)) {
-        iter->nullify_path();
-        LOGI("Cleared SOList entry for %s", target_name);
+  static void DropSoPath(const char* target_path) {
+    if (solist == NULL && !Initialize()) {
+      LOGE("Failed to initialize solist");
+      return;
     }
+    SoInfo *prev = NULL;
+    for (auto iter = solist; iter; iter = iter->get_next()) {
+      if (prev != NULL && iter->get_name() && iter->get_path() && strstr(iter->get_path(), target_path)) {
+        SoList::ProtectedDataGuard guard;
+        prev->set_next(iter->get_next());
+        if (iter == *sonext) {
+            *sonext = prev;
         }
-
-    for (auto *iter = somain; iter; iter = iter->get_next()) {
-      if (iter->get_name() && iter->get_path() && strstr(iter->get_path(), target_name)) {
-        iter->nullify_path();
-
-        break;
+        LOGI("Dropped solist record for %s loaded at %s", iter->get_name(), iter->get_path());
       }
+      prev = iter;
     }
   }
 
   static bool Initialize() {
     SandHook::ElfImg linker("/linker");
+    if (!ProtectedDataGuard::setup(linker)) return false;
 
     /* INFO: Since Android 15, the symbol names for the linker have a suffix,
                 this makes it impossible to hardcode the symbol names. To allow
@@ -107,13 +147,20 @@ namespace SoList  {
     char somain_sym_name[sizeof("__dl__ZL6somain") + sizeof(llvm_sufix)];
     snprintf(somain_sym_name, sizeof(somain_sym_name), "__dl__ZL6somain%s", llvm_sufix);
 
+    char sonext_sym_name[sizeof("__dl__ZL6sonext") + sizeof(llvm_sufix)];
+    snprintf(sonext_sym_name, sizeof(somain_sym_name), "__dl__ZL6sonext%s", llvm_sufix);
+
     char vsdo_sym_name[sizeof("__dl__ZL4vdso") + sizeof(llvm_sufix)];
     snprintf(vsdo_sym_name, sizeof(vsdo_sym_name), "__dl__ZL4vdso%s", llvm_sufix);
 
     somain = getStaticPointer<SoInfo>(linker, somain_sym_name);
     if (somain == NULL) return false;
 
-    auto vsdo = getStaticPointer<SoInfo>(linker, vsdo_sym_name);
+    sonext = linker.getSymbAddress<SoInfo **>(sonext_sym_name);
+    if (sonext == NULL) return false;
+
+    SoInfo *vsdo = getStaticPointer<SoInfo>(linker, vsdo_sym_name);
+    if (vsdo == NULL) return false;
 
     SoInfo::get_realpath_sym = reinterpret_cast<decltype(SoInfo::get_realpath_sym)>(linker.getSymbAddress("__dl__ZNK6soinfo12get_realpathEv"));
     SoInfo::get_soname_sym = reinterpret_cast<decltype(SoInfo::get_soname_sym)>(linker.getSymbAddress("__dl__ZNK6soinfo10get_sonameEv"));

loader/src/injector/hook.cpp

@@ -582,13 +582,7 @@ void ZygiskContext::run_modules_post() {
         m.tryUnload();
     }
 
-    // Remove from SoList to avoid detection
-    bool solist_res = SoList::Initialize();
-    if (!solist_res) {
-        LOGE("Failed to initialize SoList");
-    } else {
-        SoList::NullifySoName("jit-cache");
-    }
+    SoList::DropSoPath("jit-cache");
 
     // Remap as well to avoid checking of /memfd:jit-cache
     for (auto &info : lsplt::MapInfo::Scan()) {