From a6db2f985a4ee635fa01cf1e649a05a6188888de Mon Sep 17 00:00:00 2001
From: Brian Gianforcaro <bgianf@serenityos.org>
Date: Sun, 1 Aug 2021 04:33:06 -0700
Subject: [PATCH] Kernel: Handle OOM in DiskCache when mounting Ext2
 filesystems

Create the disk cache up front, so we can verify it succeeds.
Make the KBuffer allocation fail-able, so we can properly handle
failure when the user asks up to mount a Ext2 filesystem under
OOM conditions.
---
 Kernel/FileSystem/BlockBasedFileSystem.cpp | 41 +++++++++++++++-------
 Kernel/FileSystem/BlockBasedFileSystem.h   |  1 +
 Kernel/FileSystem/Ext2FileSystem.cpp       |  6 ++++
 3 files changed, 36 insertions(+), 12 deletions(-)

diff --git a/Kernel/FileSystem/BlockBasedFileSystem.cpp b/Kernel/FileSystem/BlockBasedFileSystem.cpp
index baa0b422483..0a0c29c6a19 100644
--- a/Kernel/FileSystem/BlockBasedFileSystem.cpp
+++ b/Kernel/FileSystem/BlockBasedFileSystem.cpp
@@ -20,13 +20,14 @@ struct CacheEntry {
 
 class DiskCache {
 public:
-    explicit DiskCache(BlockBasedFileSystem& fs)
+    static constexpr size_t EntryCount = 10000;
+    explicit DiskCache(BlockBasedFileSystem& fs, NonnullOwnPtr<KBuffer> cached_block_data, NonnullOwnPtr<KBuffer> entries_buffer)
         : m_fs(fs)
-        , m_cached_block_data(KBuffer::create_with_size(m_entry_count * m_fs.block_size()))
-        , m_entries(KBuffer::create_with_size(m_entry_count * sizeof(CacheEntry)))
+        , m_cached_block_data(move(cached_block_data))
+        , m_entries(move(entries_buffer))
     {
-        for (size_t i = 0; i < m_entry_count; ++i) {
-            entries()[i].data = m_cached_block_data.data() + i * m_fs.block_size();
+        for (size_t i = 0; i < EntryCount; ++i) {
+            entries()[i].data = m_cached_block_data->data() + i * m_fs.block_size();
             m_clean_list.append(entries()[i]);
         }
     }
@@ -83,8 +84,8 @@ public:
         return new_entry;
     }
 
-    const CacheEntry* entries() const { return (const CacheEntry*)m_entries.data(); }
-    CacheEntry* entries() { return (CacheEntry*)m_entries.data(); }
+    const CacheEntry* entries() const { return (const CacheEntry*)m_entries->data(); }
+    CacheEntry* entries() { return (CacheEntry*)m_entries->data(); }
 
     template<typename Callback>
     void for_each_dirty_entry(Callback callback)
@@ -95,12 +96,11 @@ public:
 
 private:
     BlockBasedFileSystem& m_fs;
-    size_t m_entry_count { 10000 };
     mutable HashMap<BlockBasedFileSystem::BlockIndex, CacheEntry*> m_hash;
     mutable IntrusiveList<CacheEntry, RawPtr<CacheEntry>, &CacheEntry::list_node> m_clean_list;
     mutable IntrusiveList<CacheEntry, RawPtr<CacheEntry>, &CacheEntry::list_node> m_dirty_list;
-    KBuffer m_cached_block_data;
-    KBuffer m_entries;
+    NonnullOwnPtr<KBuffer> m_cached_block_data;
+    NonnullOwnPtr<KBuffer> m_entries;
     bool m_dirty { false };
 };
 
@@ -114,6 +114,25 @@ BlockBasedFileSystem::~BlockBasedFileSystem()
 {
 }
 
+bool BlockBasedFileSystem::initialize()
+{
+    VERIFY(block_size() != 0);
+    auto cached_block_data = KBuffer::try_create_with_size(DiskCache::EntryCount * block_size());
+    if (!cached_block_data)
+        return false;
+
+    auto entries_data = KBuffer::try_create_with_size(DiskCache::EntryCount * sizeof(CacheEntry));
+    if (!entries_data)
+        return false;
+
+    auto disk_cache = adopt_own_if_nonnull(new (nothrow) DiskCache(*this, cached_block_data.release_nonnull(), entries_data.release_nonnull()));
+    if (!disk_cache)
+        return false;
+
+    m_cache = move(disk_cache);
+    return true;
+}
+
 KResult BlockBasedFileSystem::write_block(BlockIndex index, const UserOrKernelBuffer& data, size_t count, size_t offset, bool allow_cache)
 {
     VERIFY(m_logical_block_size);
@@ -293,8 +312,6 @@ void BlockBasedFileSystem::flush_writes()
 
 DiskCache& BlockBasedFileSystem::cache() const
 {
-    if (!m_cache)
-        m_cache = make<DiskCache>(const_cast<BlockBasedFileSystem&>(*this));
     return *m_cache;
 }
 
diff --git a/Kernel/FileSystem/BlockBasedFileSystem.h b/Kernel/FileSystem/BlockBasedFileSystem.h
index e78bdcfcce8..0e78e9ef10d 100644
--- a/Kernel/FileSystem/BlockBasedFileSystem.h
+++ b/Kernel/FileSystem/BlockBasedFileSystem.h
@@ -15,6 +15,7 @@ public:
     TYPEDEF_DISTINCT_ORDERED_ID(u64, BlockIndex);
 
     virtual ~BlockBasedFileSystem() override;
+    virtual bool initialize() override;
 
     u64 logical_block_size() const { return m_logical_block_size; };
 
diff --git a/Kernel/FileSystem/Ext2FileSystem.cpp b/Kernel/FileSystem/Ext2FileSystem.cpp
index 579f5e2f290..2bd8ddb7cea 100644
--- a/Kernel/FileSystem/Ext2FileSystem.cpp
+++ b/Kernel/FileSystem/Ext2FileSystem.cpp
@@ -89,6 +89,7 @@ const ext2_group_desc& Ext2FS::group_descriptor(GroupIndex group_index) const
 bool Ext2FS::initialize()
 {
     MutexLocker locker(m_lock);
+
     VERIFY((sizeof(ext2_super_block) % logical_block_size()) == 0);
     auto super_block_buffer = UserOrKernelBuffer::for_kernel_buffer((u8*)&m_super_block);
     bool success = raw_read_blocks(2, (sizeof(ext2_super_block) / logical_block_size()), super_block_buffer);
@@ -115,6 +116,11 @@ bool Ext2FS::initialize()
     set_block_size(EXT2_BLOCK_SIZE(&super_block));
     set_fragment_size(EXT2_FRAG_SIZE(&super_block));
 
+    // Note: This depends on the block size being available.
+    auto baseclass_result = BlockBasedFileSystem::initialize();
+    if (!baseclass_result)
+        return baseclass_result;
+
     VERIFY(block_size() <= (int)max_block_size);
 
     m_block_group_count = ceil_div(super_block.s_blocks_count, super_block.s_blocks_per_group);