Kernel: Migrate hostname locking to ProtectedValue

Author: https://github.com/boricj
Commit: https://github.com/SerenityOS/serenity/commit/626b99ce1c4
Pull-request: https://github.com/SerenityOS/serenity/pull/8851
Reviewed-by: https://github.com/awesomekling ✅
Reviewed-by: https://github.com/bgianfo

Kernel/Process.cpp

@@ -4,6 +4,7 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
+#include <AK/Singleton.h>
 #include <AK/StdLibExtras.h>
 #include <AK/StringBuilder.h>
 #include <AK/Time.h>
@@ -45,11 +46,16 @@ static void create_signal_trampoline();
 RecursiveSpinLock g_processes_lock;
 static Atomic<pid_t> next_pid;
 READONLY_AFTER_INIT Process::List* g_processes;
-READONLY_AFTER_INIT String* g_hostname;
-READONLY_AFTER_INIT Mutex* g_hostname_lock;
 READONLY_AFTER_INIT HashMap<String, OwnPtr<Module>>* g_modules;
 READONLY_AFTER_INIT Memory::Region* g_signal_trampoline_region;
 
+static AK::Singleton<ProtectedValue<String>> s_hostname;
+
+ProtectedValue<String>& hostname()
+{
+    return *s_hostname;
+}
+
 ProcessID Process::allocate_pid()
 {
     // Overflow is UB, and negative PIDs wreck havoc.
@@ -67,8 +73,10 @@ UNMAP_AFTER_INIT void Process::initialize()
     next_pid.store(0, AK::MemoryOrder::memory_order_release);
     g_processes = new Process::List();
     g_process_groups = new ProcessGroup::List();
-    g_hostname = new String("courage");
-    g_hostname_lock = new Mutex;
+
+    hostname().with_exclusive([&](auto& name) {
+        name = "courage";
+    });
 
     create_signal_trampoline();
 }

Kernel/Process.h

@@ -22,6 +22,7 @@
 #include <Kernel/Forward.h>
 #include <Kernel/FutexQueue.h>
 #include <Kernel/Locking/Mutex.h>
+#include <Kernel/Locking/ProtectedValue.h>
 #include <Kernel/Memory/AddressSpace.h>
 #include <Kernel/PerformanceEventBuffer.h>
 #include <Kernel/ProcessGroup.h>
@@ -33,6 +34,7 @@
 
 namespace Kernel {
 
+ProtectedValue<String>& hostname();
 Time kgettimeofday();
 
 #define ENUMERATE_PLEDGE_PROMISES         \

Kernel/Syscalls/hostname.cpp

@@ -8,37 +8,36 @@
 
 namespace Kernel {
 
-extern String* g_hostname;
-extern Mutex* g_hostname_lock;
-
 KResultOr<FlatPtr> Process::sys$gethostname(Userspace<char*> buffer, size_t size)
 {
     VERIFY_NO_PROCESS_BIG_LOCK(this)
     REQUIRE_PROMISE(stdio);
     if (size > NumericLimits<ssize_t>::max())
         return EINVAL;
-    MutexLocker locker(*g_hostname_lock, Mutex::Mode::Shared);
-    if (size < (g_hostname->length() + 1))
-        return ENAMETOOLONG;
-    if (!copy_to_user(buffer, g_hostname->characters(), g_hostname->length() + 1))
-        return EFAULT;
-    return 0;
+    return hostname().with_shared([&](const auto& name) -> KResultOr<FlatPtr> {
+        if (size < (name.length() + 1))
+            return ENAMETOOLONG;
+        if (!copy_to_user(buffer, name.characters(), name.length() + 1))
+            return EFAULT;
+        return 0;
+    });
 }
 
-KResultOr<FlatPtr> Process::sys$sethostname(Userspace<const char*> hostname, size_t length)
+KResultOr<FlatPtr> Process::sys$sethostname(Userspace<const char*> buffer, size_t length)
 {
     VERIFY_NO_PROCESS_BIG_LOCK(this)
     REQUIRE_NO_PROMISES;
     if (!is_superuser())
         return EPERM;
-    MutexLocker locker(*g_hostname_lock, Mutex::Mode::Exclusive);
     if (length > 64)
         return ENAMETOOLONG;
-    auto copied_hostname = copy_string_from_user(hostname, length);
-    if (copied_hostname.is_null())
-        return EFAULT;
-    *g_hostname = move(copied_hostname);
-    return 0;
+    return hostname().with_exclusive([&](auto& name) -> KResultOr<FlatPtr> {
+        auto copied_hostname = copy_string_from_user(buffer, length);
+        if (copied_hostname.is_null())
+            return EFAULT;
+        name = move(copied_hostname);
+        return 0;
+    });
 }
 
 }

Kernel/Syscalls/uname.cpp

@@ -11,15 +11,8 @@ namespace Kernel {
 KResultOr<FlatPtr> Process::sys$uname(Userspace<utsname*> user_buf)
 {
     VERIFY_NO_PROCESS_BIG_LOCK(this)
-    extern String* g_hostname;
-    extern Mutex* g_hostname_lock;
-
     REQUIRE_PROMISE(stdio);
 
-    MutexLocker locker(*g_hostname_lock, Mutex::Mode::Shared);
-    if (g_hostname->length() + 1 > sizeof(utsname::nodename))
-        return ENAMETOOLONG;
-
     utsname buf {};
     memcpy(buf.sysname, "SerenityOS", 11);
     memcpy(buf.release, "1.0-dev", 8);
@@ -30,7 +23,9 @@ KResultOr<FlatPtr> Process::sys$uname(Userspace<utsname*> user_buf)
     memcpy(buf.machine, "x86_64", 7);
 #endif
 
-    memcpy(buf.nodename, g_hostname->characters(), g_hostname->length() + 1);
+    hostname().with_shared([&](const auto& name) {
+        memcpy(buf.nodename, name.characters(), name.length() + 1);
+    });
 
     if (!copy_to_user(user_buf, &buf))
         return EFAULT;