Kernel/ProcFS: Allow a process directory to have a null Process pointer

In case we are about to delete the PID directory, we clear the Process
pointer. If someone still holds a reference to the PID directory (by
opening it), we still need to delete the process, but we can't delete
the directory, so we will keep it alive, but any operation on it will
fail by propogating the error to userspace about that the Process was
deleted and therefore there's no meaning to trying to do operations on
the directory.

Fixes #8576.

4 files changed, 87 insertions, 24 deletions

diff --git a/Kernel/FileSystem/ProcFS.cpp b/Kernel/FileSystem/ProcFS.cpp
index 2575024246..bed114a1aa 100644
--- a/Kernel/FileSystem/ProcFS.cpp
+++ b/Kernel/FileSystem/ProcFS.cpp
@@ -59,8 +59,6 @@ void ProcFSComponentRegistry::unregister_process(Process& deleted_process)
     process_folder->prepare_for_deletion();
     process_folder->m_list_node.remove();
     dbgln_if(PROCFS_DEBUG, "ProcFSExposedDirectory ref_count now: {}", process_folder->ref_count());
-    // Note: Let's ensure we are the last holder of the ProcFSProcessDirectory object before it can be deleted for good
-    VERIFY(process_folder->ref_count() == 1);
 }
 
 RefPtr<ProcFS> ProcFS::create()
diff --git a/Kernel/ProcessExposed.cpp b/Kernel/ProcessExposed.cpp
index 9d2679d650..ba32ab8fdd 100644
--- a/Kernel/ProcessExposed.cpp
+++ b/Kernel/ProcessExposed.cpp
@@ -160,7 +160,9 @@ KResult ProcFSProcessInformation::refresh_data(FileDescription& description) con
     auto parent_folder = const_cast<ProcFSProcessInformation&>(*this).m_parent_folder.strong_ref();
     if (parent_folder.is_null())
         return KResult(EINVAL);
-    auto process = parent_folder->m_associated_process;
+    auto process = parent_folder->associated_process();
+    if (!process)
+        return KResult(ESRCH);
     process->ptrace_lock().lock();
     if (!process->is_dumpable()) {
         process->ptrace_lock().unlock();
diff --git a/Kernel/ProcessExposed.h b/Kernel/ProcessExposed.h
index 2c739978e6..c5a1a2afb7 100644
--- a/Kernel/ProcessExposed.h
+++ b/Kernel/ProcessExposed.h
@@ -142,19 +142,21 @@ class ProcFSProcessDirectory final
 
 public:
     static NonnullRefPtr<ProcFSProcessDirectory> create(const Process&);
-    NonnullRefPtr<Process> associated_process() { return m_associated_process; }
+    RefPtr<Process> associated_process() { return m_associated_process; }
 
-    virtual uid_t owner_user() const override { return m_associated_process->uid(); }
-    virtual gid_t owner_group() const override { return m_associated_process->gid(); }
+    virtual uid_t owner_user() const override { return m_associated_process ? m_associated_process->uid() : 0; }
+    virtual gid_t owner_group() const override { return m_associated_process ? m_associated_process->gid() : 0; }
     virtual KResult refresh_data(FileDescription&) const override;
     virtual RefPtr<ProcFSExposedComponent> lookup(StringView name) override;
 
+    virtual void prepare_for_deletion() override;
+
 private:
     void on_attach();
     IntrusiveListNode<ProcFSProcessDirectory, RefPtr<ProcFSProcessDirectory>> m_list_node;
 
     explicit ProcFSProcessDirectory(const Process&);
-    NonnullRefPtr<Process> m_associated_process;
+    RefPtr<Process> m_associated_process;
 };
 
 class ProcFSBusDirectory : public ProcFSExposedDirectory {
@@ -227,8 +229,26 @@ public:
 
     virtual KResultOr<size_t> read_bytes(off_t offset, size_t count, UserOrKernelBuffer& buffer, FileDescription* description) const override;
 
-    virtual uid_t owner_user() const override { return m_parent_folder.strong_ref()->m_associated_process->uid(); }
-    virtual gid_t owner_group() const override { return m_parent_folder.strong_ref()->m_associated_process->gid(); }
+    virtual uid_t owner_user() const override
+    {
+        auto parent_folder = m_parent_folder.strong_ref();
+        if (!parent_folder)
+            return false;
+        auto process = parent_folder->associated_process();
+        if (!process)
+            return false;
+        return process->uid();
+    }
+    virtual gid_t owner_group() const override
+    {
+        auto parent_folder = m_parent_folder.strong_ref();
+        if (!parent_folder)
+            return false;
+        auto process = parent_folder->associated_process();
+        if (!process)
+            return false;
+        return process->gid();
+    }
 
 protected:
     ProcFSProcessInformation(StringView name, const ProcFSProcessDirectory& process_folder)
diff --git a/Kernel/ProcessSpecificExposed.cpp b/Kernel/ProcessSpecificExposed.cpp
index 81e1e14be9..ad26f1940b 100644
--- a/Kernel/ProcessSpecificExposed.cpp
+++ b/Kernel/ProcessSpecificExposed.cpp
@@ -93,7 +93,10 @@ KResultOr<size_t> ProcFSProcessStacks::entries_count() const
     auto parent_folder = m_process_folder.strong_ref();
     if (parent_folder.is_null())
         return KResult(EINVAL);
-    return parent_folder->m_associated_process->thread_count();
+    auto process = parent_folder->associated_process();
+    if (process.is_null())
+        return KResult(ESRCH);
+    return process->thread_count();
 }
 
 KResult ProcFSProcessStacks::traverse_as_directory(unsigned fsid, Function<bool(FileSystem::DirectoryEntryView const&)> callback) const
@@ -105,7 +108,9 @@ KResult ProcFSProcessStacks::traverse_as_directory(unsigned fsid, Function<bool(
     callback({ ".", { fsid, component_index() }, 0 });
     callback({ "..", { fsid, parent_folder->component_index() }, 0 });
 
-    auto process = parent_folder->m_associated_process;
+    auto process = parent_folder->associated_process();
+    if (process.is_null())
+        return KResult(ESRCH);
     process->for_each_thread([&](const Thread& thread) {
         int tid = thread.tid().value();
         InodeIdentifier identifier = { fsid, thread.global_procfs_inode_index() };
@@ -120,7 +125,9 @@ RefPtr<ProcFSExposedComponent> ProcFSProcessStacks::lookup(StringView name)
     auto parent_folder = m_process_folder.strong_ref();
     if (parent_folder.is_null())
         return nullptr;
-    auto process = parent_folder->m_associated_process;
+    auto process = parent_folder->associated_process();
+    if (process.is_null())
+        return nullptr;
     RefPtr<ProcFSThreadStack> procfd_stack;
     // FIXME: Try to exit the loop earlier
     process->for_each_thread([&](const Thread& thread) {
@@ -195,7 +202,10 @@ KResultOr<size_t> ProcFSProcessFileDescriptions::entries_count() const
     auto parent_folder = m_process_folder.strong_ref();
     if (parent_folder.is_null())
         return KResult(EINVAL);
-    return parent_folder->m_associated_process->fds().open_count();
+    auto process = parent_folder->associated_process();
+    if (process.is_null())
+        return KResult(ESRCH);
+    return process->fds().open_count();
 }
 KResult ProcFSProcessFileDescriptions::traverse_as_directory(unsigned fsid, Function<bool(FileSystem::DirectoryEntryView const&)> callback) const
 {
@@ -206,7 +216,9 @@ KResult ProcFSProcessFileDescriptions::traverse_as_directory(unsigned fsid, Func
     callback({ ".", { fsid, component_index() }, 0 });
     callback({ "..", { fsid, parent_folder->component_index() }, 0 });
 
-    auto process = parent_folder->m_associated_process;
+    auto process = parent_folder->associated_process();
+    if (process.is_null())
+        return KResult(ESRCH);
     size_t count = 0;
     process->fds().enumerate([&](auto& file_description_metadata) {
         if (!file_description_metadata.is_valid()) {
@@ -225,7 +237,9 @@ RefPtr<ProcFSExposedComponent> ProcFSProcessFileDescriptions::lookup(StringView
     auto parent_folder = m_process_folder.strong_ref();
     if (parent_folder.is_null())
         return nullptr;
-    auto process = parent_folder->m_associated_process;
+    auto process = parent_folder->associated_process();
+    if (process.is_null())
+        return nullptr;
     RefPtr<ProcFSProcessFileDescription> procfd_fd;
     // FIXME: Try to exit the loop earlier
     size_t count = 0;
@@ -259,7 +273,9 @@ private:
         auto parent_folder = m_parent_folder.strong_ref();
         if (parent_folder.is_null())
             return false;
-        auto process = parent_folder->m_associated_process;
+        auto process = parent_folder->associated_process();
+        if (process.is_null())
+            return false;
         JsonObjectSerializer obj { builder };
 #define __ENUMERATE_PLEDGE_PROMISE(x)       \
     if (process->has_promised(Pledge::x)) { \
@@ -295,8 +311,11 @@ private:
         auto parent_folder = m_parent_folder.strong_ref();
         if (parent_folder.is_null())
             return false;
+        auto process = parent_folder->associated_process();
+        if (process.is_null())
+            return false;
         JsonArraySerializer array { builder };
-        for (auto& unveiled_path : parent_folder->m_associated_process->unveiled_paths()) {
+        for (auto& unveiled_path : process->unveiled_paths()) {
             if (!unveiled_path.was_explicitly_unveiled())
                 continue;
             auto obj = array.add_object();
@@ -337,7 +356,9 @@ private:
         auto parent_folder = m_parent_folder.strong_ref();
         if (parent_folder.is_null())
             return false;
-        auto process = parent_folder->m_associated_process;
+        auto process = parent_folder->associated_process();
+        if (process.is_null())
+            return false;
         if (!process->perf_events()) {
             dbgln("ProcFS: No perf events for {}", process->pid());
             return false;
@@ -364,7 +385,9 @@ private:
         if (parent_folder.is_null())
             return false;
         JsonArraySerializer array { builder };
-        auto process = parent_folder->m_associated_process;
+        auto process = parent_folder->associated_process();
+        if (process.is_null())
+            return false;
         if (process->fds().open_count() == 0) {
             array.finish();
             return true;
@@ -414,7 +437,10 @@ private:
         auto parent_folder = m_parent_process_directory.strong_ref();
         if (parent_folder.is_null())
             return false;
-        builder.append_bytes(parent_folder->m_associated_process->root_directory_relative_to_global_root().absolute_path().to_byte_buffer());
+        auto process = parent_folder->associated_process();
+        if (process.is_null())
+            return false;
+        builder.append_bytes(process->root_directory_relative_to_global_root().absolute_path().to_byte_buffer());
         return true;
     }
     WeakPtr<ProcFSProcessDirectory> m_parent_process_directory;
@@ -437,7 +463,9 @@ private:
         auto parent_folder = m_parent_folder.strong_ref();
         if (parent_folder.is_null())
             return false;
-        auto process = parent_folder->m_associated_process;
+        auto process = parent_folder->associated_process();
+        if (process.is_null())
+            return false;
         JsonArraySerializer array { builder };
         {
             ScopedSpinLock lock(process->space().get_lock());
@@ -500,7 +528,10 @@ private:
         auto parent_folder = m_parent_process_directory.strong_ref();
         if (parent_folder.is_null())
             return false;
-        builder.append_bytes(parent_folder->m_associated_process->current_directory().absolute_path().bytes());
+        auto process = parent_folder->associated_process();
+        if (process.is_null())
+            return false;
+        builder.append_bytes(process->current_directory().absolute_path().bytes());
         return true;
     }
 
@@ -519,7 +550,10 @@ public:
         auto parent_folder = m_parent_process_directory.strong_ref();
         if (parent_folder.is_null())
             return false;
-        if (!parent_folder->m_associated_process->executable())
+        auto process = parent_folder->associated_process();
+        if (process.is_null())
+            return false;
+        if (!process->executable())
             return 0;
         return ProcFSExposedComponent::required_mode();
     }
@@ -535,7 +569,10 @@ private:
         auto parent_folder = m_parent_process_directory.strong_ref();
         if (parent_folder.is_null())
             return false;
-        auto* custody = parent_folder->m_associated_process->executable();
+        auto process = parent_folder->associated_process();
+        if (process.is_null())
+            return false;
+        auto* custody = process->executable();
         if (!custody)
             return false;
         builder.append(custody->absolute_path().bytes());
@@ -582,6 +619,12 @@ NonnullRefPtr<ProcFSProcessDirectory> ProcFSProcessDirectory::create(const Proce
     return adopt_ref_if_nonnull(new (nothrow) ProcFSProcessDirectory(process)).release_nonnull();
 }
 
+void ProcFSProcessDirectory::prepare_for_deletion()
+{
+    ProcFSExposedDirectory::prepare_for_deletion();
+    m_associated_process.clear();
+}
+
 ProcFSProcessDirectory::ProcFSProcessDirectory(const Process& process)
     : ProcFSExposedDirectory(String::formatted("{:d}", process.pid().value()), ProcFSComponentRegistry::the().root_folder())
     , m_associated_process(process)