From f816abcbad4903a815612986fd9d8172bcb752c3 Mon Sep 17 00:00:00 2001
From: Brian Gianforcaro <bgianf@serenityos.org>
Date: Sun, 1 Aug 2021 01:40:07 -0700
Subject: [PATCH] Kernel: Add DoubleBuffer::try_create() factory method for OOM
 hardening

We need to expose the ability for DoubleBuffer creation to expose
failure, as DoubleBuffer depends on KBuffer, which also has to be able
to expose failure during OOM.

We will remove the non OOM API once all users have been converted.
---
 Kernel/DoubleBuffer.cpp | 28 +++++++++++++++++++++-------
 Kernel/DoubleBuffer.h   |  5 +++--
 2 files changed, 24 insertions(+), 9 deletions(-)

diff --git a/Kernel/DoubleBuffer.cpp b/Kernel/DoubleBuffer.cpp
index 269a82d544..b549b8a047 100644
--- a/Kernel/DoubleBuffer.cpp
+++ b/Kernel/DoubleBuffer.cpp
@@ -17,22 +17,36 @@ inline void DoubleBuffer::compute_lockfree_metadata()
     m_space_for_writing = m_capacity - m_write_buffer->size;
 }
 
+OwnPtr<DoubleBuffer> DoubleBuffer::try_create(size_t capacity)
+{
+    auto storage = KBuffer::try_create_with_size(capacity * 2, Region::Access::Read | Region::Access::Write, "DoubleBuffer");
+    if (!storage)
+        return {};
+
+    return adopt_own_if_nonnull(new (nothrow) DoubleBuffer(capacity, storage.release_nonnull()));
+}
+
 DoubleBuffer::DoubleBuffer(size_t capacity)
+    : DoubleBuffer(capacity, KBuffer::try_create_with_size(capacity * 2, Region::Access::Read | Region::Access::Write, "DoubleBuffer").release_nonnull())
+{
+}
+
+DoubleBuffer::DoubleBuffer(size_t capacity, NonnullOwnPtr<KBuffer> storage)
     : m_write_buffer(&m_buffer1)
     , m_read_buffer(&m_buffer2)
-    , m_storage(KBuffer::create_with_size(capacity * 2, Region::Access::Read | Region::Access::Write, "DoubleBuffer"))
+    , m_storage(move(storage))
     , m_capacity(capacity)
 {
-    m_buffer1.data = m_storage.data();
+    m_buffer1.data = m_storage->data();
     m_buffer1.size = 0;
-    m_buffer2.data = m_storage.data() + capacity;
+    m_buffer2.data = m_storage->data() + capacity;
     m_buffer2.size = 0;
     m_space_for_writing = capacity;
 }
 
 void DoubleBuffer::flip()
 {
-    if (m_storage.is_null())
+    if (m_storage->is_null())
         return;
     VERIFY(m_read_buffer_index == m_read_buffer->size);
     swap(m_read_buffer, m_write_buffer);
@@ -43,7 +57,7 @@ void DoubleBuffer::flip()
 
 KResultOr<size_t> DoubleBuffer::write(const UserOrKernelBuffer& data, size_t size)
 {
-    if (!size || m_storage.is_null())
+    if (!size || m_storage->is_null())
         return 0;
     MutexLocker locker(m_lock);
     size_t bytes_to_write = min(size, m_space_for_writing);
@@ -59,7 +73,7 @@ KResultOr<size_t> DoubleBuffer::write(const UserOrKernelBuffer& data, size_t siz
 
 KResultOr<size_t> DoubleBuffer::read(UserOrKernelBuffer& data, size_t size)
 {
-    if (!size || m_storage.is_null())
+    if (!size || m_storage->is_null())
         return 0;
     MutexLocker locker(m_lock);
     if (m_read_buffer_index >= m_read_buffer->size && m_write_buffer->size != 0)
@@ -78,7 +92,7 @@ KResultOr<size_t> DoubleBuffer::read(UserOrKernelBuffer& data, size_t size)
 
 KResultOr<size_t> DoubleBuffer::peek(UserOrKernelBuffer& data, size_t size)
 {
-    if (!size || m_storage.is_null())
+    if (!size || m_storage->is_null())
         return 0;
     MutexLocker locker(m_lock);
     if (m_read_buffer_index >= m_read_buffer->size && m_write_buffer->size != 0) {
diff --git a/Kernel/DoubleBuffer.h b/Kernel/DoubleBuffer.h
index c99ef5da40..f130bb592d 100644
--- a/Kernel/DoubleBuffer.h
+++ b/Kernel/DoubleBuffer.h
@@ -16,8 +16,8 @@ namespace Kernel {
 
 class DoubleBuffer {
 public:
+    [[nodiscard]] static OwnPtr<DoubleBuffer> try_create(size_t capacity = 65536);
     explicit DoubleBuffer(size_t capacity = 65536);
-
     [[nodiscard]] KResultOr<size_t> write(const UserOrKernelBuffer&, size_t);
     [[nodiscard]] KResultOr<size_t> write(const u8* data, size_t size)
     {
@@ -47,6 +47,7 @@ public:
     }
 
 private:
+    explicit DoubleBuffer(size_t capacity, NonnullOwnPtr<KBuffer> storage);
     void flip();
     void compute_lockfree_metadata();
 
@@ -60,7 +61,7 @@ private:
     InnerBuffer m_buffer1;
     InnerBuffer m_buffer2;
 
-    KBuffer m_storage;
+    NonnullOwnPtr<KBuffer> m_storage;
     Function<void()> m_unblock_callback;
     size_t m_capacity { 0 };
     size_t m_read_buffer_index { 0 };