Add a simple ELF loader

.gitignore

@@ -3,4 +3,5 @@ toolchain/
 .vscode/
 build/
 initrd/boot/moon
-env-local.sh
+env-local.sh
+initrd/bin/**

CMakeLists.txt

@@ -14,6 +14,8 @@ set(CMAKE_CXX_COMPILER x86_64-luna-g++)
 
 set(CMAKE_ASM_NASM_OBJECT_FORMAT elf64)
 
+set(CMAKE_ASM_NASM_LINK_EXECUTABLE "x86_64-luna-ld <FLAGS> <CMAKE_ASM_NASM_LINK_FLAGS> <LINK_FLAGS> <OBJECTS>  -o <TARGET> <LINK_LIBRARIES>")
+
 set(CMAKE_FIND_ROOT_PATH ${LUNA_ROOT}/toolchain/x86-64-luna)
 
 set(ARCH $ENV{ARCH})
@@ -25,4 +27,5 @@ endif()
 message(STATUS "Configuring Luna for ${ARCH}")
 
 add_subdirectory(luna)
-add_subdirectory(kernel)
+add_subdirectory(kernel)
+add_subdirectory(apps)

apps/CMakeLists.txt

@@ -0,0 +1,6 @@
+function(luna_app SOURCE_FILE APP_NAME)
+    add_executable(${APP_NAME} ${SOURCE_FILE})
+    install(FILES "${CMAKE_CURRENT_BINARY_DIR}/${APP_NAME}" DESTINATION ${LUNA_ROOT}/initrd/bin)
+endfunction()
+
+luna_app(app.asm app)

apps/app.asm

@@ -0,0 +1,18 @@
+section .text
+global _start
+_start:
+    mov eax, ecx
+    push rdx
+    mov eax, 1
+    mov edi, hello_world
+    mov esi, 14
+    int 42h
+    nop
+
+section .rodata
+hello_world:
+    db 'Hello, world!', 0xa, 0
+
+section .bss
+array:
+    resb 10

kernel/CMakeLists.txt

@@ -16,6 +16,7 @@ set(SOURCES
     src/thread/Thread.cpp
     src/thread/Scheduler.cpp
     src/InitRD.cpp
+    src/ELF.cpp
 )
 
 if("${ARCH}" MATCHES "x86_64")

kernel/src/ELF.cpp

@@ -0,0 +1,137 @@
+#include "ELF.h"
+#include "Log.h"
+#include "arch/CPU.h"
+#include "arch/MMU.h"
+#include "memory/MemoryManager.h"
+#include <luna/Alignment.h>
+#include <luna/Alloc.h>
+#include <luna/CString.h>
+#include <luna/ScopeGuard.h>
+
+static bool can_execute_segment(u32 flags)
+{
+    return flags & 1;
+}
+
+static bool can_write_segment(u32 flags)
+{
+    return flags & 2;
+}
+
+/*static bool can_write_and_execute_segment(u32 flags)
+{
+    return can_write_segment(flags) && can_execute_segment(flags);
+}*/
+
+ELFSegment::ELFSegment(u64 base, usize size) : m_base(base), m_size(size)
+{
+}
+
+namespace ELFLoader
+{
+    // FIXME: Check that all calls to read_contents() read the proper amount of bytes.
+    Result<ELFData> load(const TarStream::Entry& elf_entry, const TarStream& stream)
+    {
+        LinkedList<ELFSegment> segments;
+
+        auto guard = make_scope_guard([&] { segments.consume([](ELFSegment* segment) { delete segment; }); });
+
+        Elf64_Ehdr elf_header;
+        usize nread = stream.read_contents(elf_entry, &elf_header, 0, sizeof elf_header);
+        if (nread < sizeof elf_header)
+        {
+            kdbgln("Error while loading ELF: ELF header does not fit in entry");
+            return err(ENOEXEC);
+        }
+
+        if (memcmp(elf_header.e_ident, ELFMAG, SELFMAG) != 0)
+        {
+            kdbgln("Error while loading ELF: ELF header has no valid magic");
+            return err(ENOEXEC);
+        }
+
+        if (elf_header.e_ident[EI_CLASS] != ELFCLASS64)
+        {
+            kdbgln("Error while loading ELF: ELF object is not 64-bit");
+            return err(ENOEXEC);
+        }
+
+        if (elf_header.e_ident[EI_DATA] != ELFDATA2LSB)
+        {
+            kdbgln("Error while loading ELF: ELF object is not 2's complement little-endian");
+            return err(ENOEXEC);
+        }
+
+        if (elf_header.e_type != ET_EXEC)
+        {
+            kdbgln("Error while loading ELF: ELF object is not an executable");
+            return err(ENOEXEC);
+        }
+
+        if (elf_header.e_machine != EM_MACH)
+        {
+            kdbgln("Error while loading ELF: ELF object's target architecture does not match the current one (%s)",
+                   CPU::platform_string());
+            return err(ENOEXEC);
+        }
+
+        if (elf_header.e_phnum == 0)
+        {
+            kdbgln("Error while loading ELF: ELF object has no program headers");
+            return err(ENOEXEC);
+        }
+
+        kdbgln("ELF: Loading ELF with entry=%#.16lx", elf_header.e_entry);
+
+        usize i;
+        Elf64_Phdr program_header;
+
+        for (stream.read_contents(elf_entry, &program_header, elf_header.e_phoff, sizeof program_header), i = 0;
+             i < elf_header.e_phnum;
+             i++, stream.read_contents(elf_entry, &program_header, elf_header.e_phoff + (i * elf_header.e_phentsize),
+                                       sizeof program_header))
+        {
+            if (program_header.p_type == PT_LOAD)
+            {
+                kdbgln("ELF: Loading segment (offset=%zu, base=%#.16lx, filesize=%zu, memsize=%zu)",
+                       program_header.p_offset, program_header.p_vaddr, program_header.p_filesz,
+                       program_header.p_memsz);
+
+                check(is_aligned<ARCH_PAGE_SIZE>(program_header.p_vaddr));
+                /*expect(!can_write_and_execute_segment(program_header.p_flags),
+                       "Segment is both writable and executable");*/
+
+                ELFSegment* segment = TRY(make<ELFSegment>(program_header.p_vaddr, program_header.p_memsz));
+                segments.append(segment);
+
+                int flags = MMU::User | MMU::NoExecute;
+                if (can_write_segment(program_header.p_flags)) flags |= MMU::ReadWrite;
+                else if (can_execute_segment(program_header.p_flags))
+                    flags &= ~MMU::NoExecute;
+
+                // Allocate physical memory for the segment
+                TRY(MemoryManager::alloc_at(program_header.p_vaddr,
+                                            get_blocks_from_size(program_header.p_memsz, ARCH_PAGE_SIZE), flags));
+
+                // Load the file section of the segment
+                stream.read_contents(elf_entry, (void*)program_header.p_vaddr, program_header.p_offset,
+                                     program_header.p_filesz);
+
+                // Fill out the rest of the segment with 0s
+                memset((void*)(program_header.p_vaddr + program_header.p_filesz), 0,
+                       program_header.p_memsz - program_header.p_filesz);
+            }
+            else { kdbgln("ELF: Encountered non-loadable program header, skipping"); }
+        }
+
+        if (segments.count() == 0)
+        {
+            kdbgln("Error while loading ELF: No loadable segments");
+            return err(ENOEXEC);
+        }
+
+        guard.deactivate();
+
+        return ELFData { segments, elf_header.e_entry };
+    }
+}

kernel/src/ELF.h

@@ -0,0 +1,78 @@
+#pragma once
+#include <luna/LinkedList.h>
+#include <luna/TarStream.h>
+#include <luna/Types.h>
+
+#define ELFMAG "\177ELF"
+#define SELFMAG 4
+#define EI_CLASS 4    /* File class byte index */
+#define ELFCLASS64 2  /* 64-bit objects */
+#define EI_DATA 5     /* Data encoding byte index */
+#define ELFDATA2LSB 1 /* 2's complement, little endian */
+#define ET_EXEC 2     /* Executable file */
+#define PT_LOAD 1     /* Loadable program segment */
+#ifdef ARCH_X86_64
+#define EM_MACH 62 /* AMD x86-64 architecture */
+#else
+#error "Unknown architecture."
+#endif
+
+typedef struct
+{
+    u8 e_ident[16];  /* Magic number and other info */
+    u16 e_type;      /* Object file type */
+    u16 e_machine;   /* Architecture */
+    u32 e_version;   /* Object file version */
+    u64 e_entry;     /* Entry point virtual address */
+    u64 e_phoff;     /* Program header table file offset */
+    u64 e_shoff;     /* Section header table file offset */
+    u32 e_flags;     /* Processor-specific flags */
+    u16 e_ehsize;    /* ELF header size in bytes */
+    u16 e_phentsize; /* Program header table entry size */
+    u16 e_phnum;     /* Program header table entry count */
+    u16 e_shentsize; /* Section header table entry size */
+    u16 e_shnum;     /* Section header table entry count */
+    u16 e_shstrndx;  /* Section header string table index */
+} Elf64_Ehdr;
+
+typedef struct
+{
+    u32 p_type;   /* Segment type */
+    u32 p_flags;  /* Segment flags */
+    u64 p_offset; /* Segment file offset */
+    u64 p_vaddr;  /* Segment virtual address */
+    u64 p_paddr;  /* Segment physical address */
+    u64 p_filesz; /* Segment size in file */
+    u64 p_memsz;  /* Segment size in memory */
+    u64 p_align;  /* Segment alignment */
+} Elf64_Phdr;
+
+struct ELFSegment : public LinkedListNode<ELFSegment>
+{
+    u64 base() const
+    {
+        return m_base;
+    }
+
+    usize size() const
+    {
+        return m_size;
+    }
+
+    ELFSegment(u64 base, usize size);
+
+  private:
+    u64 m_base;
+    usize m_size;
+};
+
+struct ELFData
+{
+    LinkedList<ELFSegment> segments;
+    u64 entry;
+};
+
+namespace ELFLoader
+{
+    Result<ELFData> load(const TarStream::Entry& elf_entry, const TarStream& stream);
+};

kernel/src/main.cpp

@@ -1,3 +1,4 @@
+#include "ELF.h"
 #include "InitRD.h"
 #include "Log.h"
 #include "arch/CPU.h"
@@ -67,6 +68,8 @@ Result<void> init()
     kinfoln("Used memory:     %s", to_dynamic_unit(MemoryManager::used()).release_value().chars());
     kinfoln("Reserved memory: %s", to_dynamic_unit(MemoryManager::reserved()).release_value().chars());
 
+    MMU::unmap(0x400000);
+
     TarStream::Entry entry;
     while (TRY(g_initrd.read_next_entry().try_set_value_with_specific_error(entry, 0)))
     {
@@ -75,11 +78,11 @@ Result<void> init()
             kinfoln("Found file %s in initial ramdisk, of size %s", entry.name,
                     to_dynamic_unit(entry.size).release_value().chars());
 
-            if (!strcmp(entry.name, "sys/config"))
+            if (!strcmp(entry.name, "bin/app"))
             {
-                auto contents = TRY(g_initrd.read_contents_as_string(entry, 0, entry.size));
-
-                kinfoln("%s", contents.chars());
+                auto data = TRY(ELFLoader::load(entry, g_initrd));
+                data.segments.consume([](ELFSegment* segment) { delete segment; });
+                kinfoln("Loaded ELF with entry=%#.16lx", data.entry);
             }
         }
     }