Kernel: Implement read functionality for MemoryDevice

So far we only had mmap(2) functionality on the /dev/mem device, but now
we can also do read(2) on it.

The test unit was updated to check we are doing it safely.

Kernel/Devices/MemoryDevice.cpp

@@ -10,6 +10,7 @@
 #include <Kernel/Devices/MemoryDevice.h>
 #include <Kernel/Firmware/BIOS.h>
 #include <Kernel/Memory/AnonymousVMObject.h>
+#include <Kernel/Memory/TypedMapping.h>
 #include <Kernel/Sections.h>
 
 namespace Kernel {
@@ -31,9 +32,17 @@ UNMAP_AFTER_INIT MemoryDevice::~MemoryDevice()
 {
 }
 
-ErrorOr<size_t> MemoryDevice::read(OpenFileDescription&, u64, UserOrKernelBuffer&, size_t)
+ErrorOr<size_t> MemoryDevice::read(OpenFileDescription&, u64 offset, UserOrKernelBuffer& buffer, size_t length)
 {
-    TODO();
+    if (!MM.is_allowed_to_read_physical_memory_for_userspace(PhysicalAddress(offset), length)) {
+        dbgln("MemoryDevice: Trying to read physical memory at {} for range of {} bytes failed due to violation of access", PhysicalAddress(offset), length);
+        return EINVAL;
+    }
+    auto mapping = Memory::map_typed<u8>(PhysicalAddress(offset), length);
+
+    auto bytes = ReadonlyBytes { mapping.ptr(), length };
+    TRY(buffer.write(bytes));
+    return length;
 }
 
 ErrorOr<Memory::Region*> MemoryDevice::mmap(Process& process, OpenFileDescription&, Memory::VirtualRange const& range, u64 offset, int prot, bool shared)
@@ -41,7 +50,7 @@ ErrorOr<Memory::Region*> MemoryDevice::mmap(Process& process, OpenFileDescriptio
     auto viewed_address = PhysicalAddress(offset);
 
     dbgln("MemoryDevice: Trying to mmap physical memory at {} for range of {} bytes", viewed_address, range.size());
-    if (!MM.is_allowed_to_mmap_physical_memory_to_userspace(viewed_address, range)) {
+    if (!MM.is_allowed_to_read_physical_memory_for_userspace(viewed_address, range.size())) {
         dbgln("MemoryDevice: Trying to mmap physical memory at {} for range of {} bytes failed due to violation of access", viewed_address, range.size());
         return EINVAL;
     }

Kernel/Memory/MemoryManager.cpp

@@ -209,13 +209,13 @@ UNMAP_AFTER_INIT void MemoryManager::register_reserved_ranges()
     m_reserved_memory_ranges.append(ContiguousReservedMemoryRange { range.start, m_physical_memory_ranges.last().start.get() + m_physical_memory_ranges.last().length - range.start.get() });
 }
 
-bool MemoryManager::is_allowed_to_mmap_physical_memory_to_userspace(PhysicalAddress start_address, VirtualRange const& range) const
+bool MemoryManager::is_allowed_to_read_physical_memory_for_userspace(PhysicalAddress start_address, size_t read_length) const
 {
     // Note: Guard against overflow in case someone tries to mmap on the edge of
     // the RAM
-    if (start_address.offset_addition_would_overflow(range.size()))
+    if (start_address.offset_addition_would_overflow(read_length))
         return false;
-    auto end_address = start_address.offset(range.size());
+    auto end_address = start_address.offset(read_length);
     for (auto& current_range : m_reserved_memory_ranges) {
         if (current_range.start > start_address)
             continue;

Kernel/Memory/MemoryManager.h

@@ -230,7 +230,7 @@ public:
     PageDirectory& kernel_page_directory() { return *m_kernel_page_directory; }
 
     Vector<UsedMemoryRange> const& used_memory_ranges() { return m_used_memory_ranges; }
-    bool is_allowed_to_mmap_physical_memory_to_userspace(PhysicalAddress, VirtualRange const&) const;
+    bool is_allowed_to_read_physical_memory_for_userspace(PhysicalAddress, size_t read_length) const;
 
     PhysicalPageEntry& get_physical_page_entry(PhysicalAddress);
     PhysicalAddress get_physical_address(PhysicalPage const&);

Tests/Kernel/TestMemoryDeviceMmap.cpp

@@ -9,12 +9,16 @@
 #include <assert.h>
 #include <errno.h>
 #include <fcntl.h>
+#include <inttypes.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <sys/mman.h>
+#include <sys/stat.h>
 #include <unistd.h>
 
+static u8 read_buffer[0x100000];
+
 static ALWAYS_INLINE bool mem_chunk(int fd, u64 base, u64 length)
 {
     u64 mmoffset = base % sysconf(_SC_PAGESIZE);
@@ -22,30 +26,80 @@ static ALWAYS_INLINE bool mem_chunk(int fd, u64 base, u64 length)
     return mmp != MAP_FAILED;
 }
 
+enum class ReadResult {
+    SeekFailure,
+    ReadFailure,
+    ReadSuccess,
+};
+
+static ALWAYS_INLINE ReadResult read_chunk(int fd, u64 base, u64 length)
+{
+    VERIFY(length <= sizeof(read_buffer));
+    auto rs = lseek(fd, base, SEEK_SET);
+    if (rs < 0) {
+        fprintf(stderr, "Couldn't seek to offset %" PRIi64 " while verifying: %s\n", base, strerror(errno));
+        return ReadResult::SeekFailure;
+    }
+    if (read(fd, read_buffer, length) < 0)
+        return ReadResult::ReadFailure;
+    return ReadResult::ReadSuccess;
+}
+
+TEST_CASE(test_memory_access_device_read)
+{
+    int rc = geteuid();
+    EXPECT_EQ(rc, 0);
+
+    int fd = open("/dev/mem", O_RDONLY);
+    EXPECT(fd >= 0);
+
+    // FIXME: This is expected to work on QEMU machines (both 440FX and Q35),
+    // however, it will be much nicer to have some sort of a node in the ProcFS
+    // to expose physical memory ranges (e820 memory map).
+
+    auto read_result = read_chunk(fd, 0x0, 0x100000);
+    EXPECT_EQ(read_result, ReadResult::ReadFailure);
+
+    read_result = read_chunk(fd, 0xe0000, 0x100000 - 0xe0000);
+    EXPECT_EQ(read_result, ReadResult::ReadSuccess);
+
+    read_result = read_chunk(fd, 0x100000, 0x200000 - 0x100000);
+    EXPECT_EQ(read_result, ReadResult::ReadFailure);
+
+    read_result = read_chunk(fd, 0xf0000, 70000);
+    EXPECT_EQ(read_result, ReadResult::ReadFailure);
+
+    read_result = read_chunk(fd, 0xfffc0000, 16384);
+    EXPECT_EQ(read_result, ReadResult::ReadSuccess);
+
+    read_result = read_chunk(fd, 0xfffc0000, 0x100000);
+    EXPECT_EQ(read_result, ReadResult::ReadFailure);
+}
+
 TEST_CASE(test_memory_access_device_mmap)
 {
     int rc = geteuid();
     EXPECT_EQ(rc, 0);
 
     int fd = open("/dev/mem", O_RDONLY);
-    EXPECT_EQ(fd < 0, false);
+    EXPECT(fd >= 0);
 
     // FIXME: This is expected to work on QEMU machines (both 440FX and Q35),
     // however, it will be much nicer to have some sort of a node in the ProcFS
     // to expose physical memory ranges (e820 memory map).
 
-    auto result = mem_chunk(fd, 0xe0000, 0x100000 - 0xe0000);
-    EXPECT_EQ(result, true);
+    auto mmap_result = mem_chunk(fd, 0xe0000, 0x100000 - 0xe0000);
+    EXPECT_EQ(mmap_result, true);
 
-    result = mem_chunk(fd, 0x100000, 0x200000 - 0x100000);
-    EXPECT_EQ(result, false);
+    mmap_result = mem_chunk(fd, 0x100000, 0x200000 - 0x100000);
+    EXPECT_EQ(mmap_result, false);
 
-    result = mem_chunk(fd, 0xf0000, 70000);
-    EXPECT_EQ(result, false);
+    mmap_result = mem_chunk(fd, 0xf0000, 70000);
+    EXPECT_EQ(mmap_result, false);
 
-    result = mem_chunk(fd, 0xfffc0000, 16384);
-    EXPECT_EQ(result, true);
+    mmap_result = mem_chunk(fd, 0xfffc0000, 16384);
+    EXPECT_EQ(mmap_result, true);
 
-    result = mem_chunk(fd, 0xfffc0000, 0x100000);
-    EXPECT_EQ(result, false);
+    mmap_result = mem_chunk(fd, 0xfffc0000, 0x100000);
+    EXPECT_EQ(mmap_result, false);
 }