LibCore: Make MappedFile a Stream

The internal reuse of FixedMemoryStream makes this straightforward.
There alread is one user of the new API, demonstrating the need for this
change beyond what I said out to use it for :^)

Userland/Libraries/LibCore/MappedFile.cpp

@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2018-2021, Andreas Kling <kling@serenityos.org>
+ * Copyright (c) 2023, kleines Filmröllchen <filmroellchen@serenityos.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -14,10 +15,11 @@
 
 namespace Core {
 
-ErrorOr<NonnullOwnPtr<MappedFile>> MappedFile::map(StringView path)
+ErrorOr<NonnullOwnPtr<MappedFile>> MappedFile::map(StringView path, OpenMode mode)
 {
-    auto fd = TRY(Core::System::open(path, O_RDONLY | O_CLOEXEC, 0));
-    return map_from_fd_and_close(fd, path);
+    auto const file_mode = mode == OpenMode::ReadOnly ? O_RDONLY : O_RDWR;
+    auto fd = TRY(Core::System::open(path, file_mode | O_CLOEXEC, 0));
+    return map_from_fd_and_close(fd, path, mode);
 }
 
 ErrorOr<NonnullOwnPtr<MappedFile>> MappedFile::map_from_file(NonnullOwnPtr<Core::File> stream, StringView path)
@@ -25,24 +27,39 @@ ErrorOr<NonnullOwnPtr<MappedFile>> MappedFile::map_from_file(NonnullOwnPtr<Core:
     return map_from_fd_and_close(stream->leak_fd(Badge<MappedFile> {}), path);
 }
 
-ErrorOr<NonnullOwnPtr<MappedFile>> MappedFile::map_from_fd_and_close(int fd, [[maybe_unused]] StringView path)
+ErrorOr<NonnullOwnPtr<MappedFile>> MappedFile::map_from_fd_and_close(int fd, [[maybe_unused]] StringView path, OpenMode mode)
 {
     TRY(Core::System::fcntl(fd, F_SETFD, FD_CLOEXEC));
 
     ScopeGuard fd_close_guard = [fd] {
-        close(fd);
+        ::close(fd);
     };
 
     auto stat = TRY(Core::System::fstat(fd));
     auto size = stat.st_size;
 
-    auto* ptr = TRY(Core::System::mmap(nullptr, size, PROT_READ, MAP_SHARED, fd, 0, 0, path));
+    int protection;
+    int flags;
+    switch (mode) {
+    case OpenMode::ReadOnly:
+        protection = PROT_READ;
+        flags = MAP_SHARED;
+        break;
+    case OpenMode::ReadWrite:
+        protection = PROT_READ | PROT_WRITE;
+        // Don't map a read-write mapping shared as a precaution.
+        flags = MAP_PRIVATE;
+        break;
+    }
 
-    return adopt_own(*new MappedFile(ptr, size));
+    auto* ptr = TRY(Core::System::mmap(nullptr, size, protection, flags, fd, 0, 0, path));
+
+    return adopt_own(*new MappedFile(ptr, size, mode));
 }
 
-MappedFile::MappedFile(void* ptr, size_t size)
-    : m_data(ptr)
+MappedFile::MappedFile(void* ptr, size_t size, OpenMode mode)
+    : FixedMemoryStream(Bytes { ptr, size }, mode == OpenMode::ReadWrite)
+    , m_data(ptr)
     , m_size(size)
 {
 }

Userland/Libraries/LibCore/MappedFile.h

@@ -7,7 +7,9 @@
 
 #pragma once
 
+#include <AK/ByteBuffer.h>
 #include <AK/Error.h>
+#include <AK/MemoryStream.h>
 #include <AK/Noncopyable.h>
 #include <AK/NonnullOwnPtr.h>
 #include <AK/RefCounted.h>
@@ -15,25 +17,29 @@
 
 namespace Core {
 
-class MappedFile {
+class MappedFile : public FixedMemoryStream {
     AK_MAKE_NONCOPYABLE(MappedFile);
     AK_MAKE_NONMOVABLE(MappedFile);
 
 public:
-    static ErrorOr<NonnullOwnPtr<MappedFile>> map(StringView path);
-    static ErrorOr<NonnullOwnPtr<MappedFile>> map_from_file(NonnullOwnPtr<Core::File>, StringView path);
-    static ErrorOr<NonnullOwnPtr<MappedFile>> map_from_fd_and_close(int fd, StringView path);
-    ~MappedFile();
+    // Reflects a simplified version of mmap protection and flags.
+    enum class OpenMode {
+        ReadOnly,
+        ReadWrite,
+    };
 
+    static ErrorOr<NonnullOwnPtr<MappedFile>> map(StringView path, OpenMode mode = OpenMode::ReadOnly);
+    static ErrorOr<NonnullOwnPtr<MappedFile>> map_from_file(NonnullOwnPtr<Core::File>, StringView path);
+    static ErrorOr<NonnullOwnPtr<MappedFile>> map_from_fd_and_close(int fd, StringView path, OpenMode mode = OpenMode::ReadOnly);
+    virtual ~MappedFile();
+
+    // Non-stream APIs for using MappedFile as a simple POSIX API wrapper.
     void* data() { return m_data; }
     void const* data() const { return m_data; }
-
-    size_t size() const { return m_size; }
-
     ReadonlyBytes bytes() const { return { m_data, m_size }; }
 
 private:
-    explicit MappedFile(void*, size_t);
+    explicit MappedFile(void*, size_t, OpenMode);
 
     void* m_data { nullptr };
     size_t m_size { 0 };

Userland/Libraries/LibGUI/FileIconProvider.cpp

@@ -174,7 +174,7 @@ Icon FileIconProvider::icon_for_executable(DeprecatedString const& path)
 
     auto& mapped_file = file_or_error.value();
 
-    if (mapped_file->size() < SELFMAG) {
+    if (mapped_file->size().release_value() < SELFMAG) {
         app_icon_cache.set(path, s_executable_icon);
         return s_executable_icon;
     }
@@ -184,7 +184,7 @@ Icon FileIconProvider::icon_for_executable(DeprecatedString const& path)
         return s_executable_icon;
     }
 
-    auto image = ELF::Image((u8 const*)mapped_file->data(), mapped_file->size());
+    auto image = ELF::Image((u8 const*)mapped_file->data(), mapped_file->size().release_value());
     if (!image.is_valid()) {
         app_icon_cache.set(path, s_executable_icon);
         return s_executable_icon;