prime/tools/elf2rel.c

#include <assert.h>
#include <errno.h>
#include <stdarg.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#ifndef MAX
#define MAX(a, b) (((a) > (b)) ? (a) : (b))
#endif

#ifndef MIN
#define MIN(a, b) (((a) < (b)) ? (a) : (b))
#endif

#define ARRAY_COUNT(arr) (sizeof(arr) / sizeof((arr)[0]))

#define IS_LITTLE_ENDIAN (((char*)((uint32_t[]){1}))[0] == 1)

// Relevant portions of elf.h

typedef uint32_t Elf32_Addr;
typedef uint32_t Elf32_Off;
typedef uint32_t Elf32_Word;
typedef int32_t Elf32_Sword;
typedef uint16_t Elf32_Half;
typedef uint16_t Elf32_Section;

#define EI_NIDENT (16)

typedef struct {
  unsigned char e_ident[EI_NIDENT]; /* Magic number and other info */
  Elf32_Half e_type;                /* Object file type */
  Elf32_Half e_machine;             /* Architecture */
  Elf32_Word e_version;             /* Object file version */
  Elf32_Addr e_entry;               /* Entry point virtual address */
  Elf32_Off e_phoff;                /* Program header table file offset */
  Elf32_Off e_shoff;                /* Section header table file offset */
  Elf32_Word e_flags;               /* Processor-specific flags */
  Elf32_Half e_ehsize;              /* ELF header size in bytes */
  Elf32_Half e_phentsize;           /* Program header table entry size */
  Elf32_Half e_phnum;               /* Program header table entry count */
  Elf32_Half e_shentsize;           /* Section header table entry size */
  Elf32_Half e_shnum;               /* Section header table entry count */
  Elf32_Half e_shstrndx;            /* Section header string table index */
} Elf32_Ehdr;

#define EI_CLASS 4   /* File class byte index */
#define ELFCLASS32 1 /* 32-bit objects */

#define EI_DATA 5     /* Data encoding byte index */
#define ELFDATA2MSB 2 /* 2's complement, big endian */

#define EM_PPC 20 /* PowerPC */

typedef struct {
  Elf32_Word sh_name;      /* Section name (string tbl index) */
  Elf32_Word sh_type;      /* Section type */
  Elf32_Word sh_flags;     /* Section flags */
  Elf32_Addr sh_addr;      /* Section virtual addr at execution */
  Elf32_Off sh_offset;     /* Section file offset */
  Elf32_Word sh_size;      /* Section size in bytes */
  Elf32_Word sh_link;      /* Link to another section */
  Elf32_Word sh_info;      /* Additional section information */
  Elf32_Word sh_addralign; /* Section alignment */
  Elf32_Word sh_entsize;   /* Entry size if section holds table */
} Elf32_Shdr;

#define SHT_PROGBITS 1 /* Program data */
#define SHT_SYMTAB 2   /* Symbol table */
#define SHT_STRTAB 3   /* String table */
#define SHT_RELA 4     /* Relocation entries with addends */
#define SHT_NOBITS 8   /* Program space with no data (bss) */

#define SHF_ALLOC (1 << 1)     /* Occupies memory during execution */
#define SHF_EXECINSTR (1 << 2) /* Executable */

typedef struct {
  Elf32_Word st_name;     /* Symbol name (string tbl index) */
  Elf32_Addr st_value;    /* Symbol value */
  Elf32_Word st_size;     /* Symbol size */
  unsigned char st_info;  /* Symbol type and binding */
  unsigned char st_other; /* Symbol visibility */
  Elf32_Section st_shndx; /* Section index */
} Elf32_Sym;

#define ELF32_ST_TYPE(val) ((val)&0xf)

/* Legal values for ST_TYPE subfield of st_info (symbol type).  */

#define STT_NOTYPE 0 /* Symbol type is unspecified */
#define STT_FUNC 2   /* Symbol is a code object */

typedef struct {
  Elf32_Addr r_offset;  /* Address */
  Elf32_Word r_info;    /* Relocation type and symbol index */
  Elf32_Sword r_addend; /* Addend */
} Elf32_Rela;

/* How to extract and insert information held in the r_info field.  */

#define ELF32_R_SYM(val) ((val) >> 8)
#define ELF32_R_TYPE(val) ((val)&0xff)

#define R_PPC_NONE 0
#define R_PPC_ADDR32 1    /* 32bit absolute address */
#define R_PPC_ADDR24 2    /* 26bit address, 2 bits ignored.  */
#define R_PPC_ADDR16 3    /* 16bit absolute address */
#define R_PPC_ADDR16_LO 4 /* lower 16bit of absolute address */
#define R_PPC_ADDR16_HI 5 /* high 16bit of absolute address */
#define R_PPC_ADDR16_HA 6 /* adjusted high 16bit */
#define R_PPC_ADDR14 7    /* 16bit address, 2 bits ignored */
#define R_PPC_ADDR14_BRTAKEN 8
#define R_PPC_ADDR14_BRNTAKEN 9
#define R_PPC_REL24 10 /* PC relative 26 bit */
#define R_PPC_REL14 11 /* PC relative 16 bit */

#define R_DOLPHIN_SECTION 202
#define R_DOLPHIN_END 203

// end elf.h

struct RelHeader {
  uint32_t moduleId;           // unique module identifier
  uint32_t nextModule;         // always 0; filled in at runtime
  uint32_t prevModule;         // always 0; filled in at runtime
  uint32_t sectionCount;       // number of sections in the section table
  uint32_t sectionTableOffset; // file position of section table
  uint32_t moduleNameOffset;   // offset of the module name in the string table
                               // (not in this file)
  uint32_t moduleNameSize; // size of the module name in the string table (not
                           // in this file)
  uint32_t formatVersion;  // REL format version
  uint32_t bssSize;        // size of the BSS section
  uint32_t relocationTableOffset; // file position of relocation entries
  uint32_t importTableOffset;     // file position of import table
  uint32_t importTableSize;       // size of import table
  uint8_t prologSection; // section in which the _prolog function is in, or 0 if
                         // absent
  uint8_t epilogSection; // section in which the _epilog function is in, or 0 if
                         // absent
  uint8_t unresolvedSection; // section in which the _unresolved function is in,
                             // or 0 if absent
  uint8_t pad33;
  uint32_t prologOffset; // offset of the _prolog function in its section
  uint32_t epilogOffset; // offset of the _epilog function in its section
  uint32_t
      unresolvedOffset; // offset of the _unresolved function in its section
};

struct RelRelocEntry {
  int type;             // relocation type
  int patchSection;     // section which relocation patch applies to
  uint32_t patchOffset; // offset where this relocation patch applies
  int symbolSection;    // section of symbol
  uint32_t symbolAddr;  // for dol symbols, absolute address. for rel symbols,
                        // section offset
};

struct RelImportEntry {
  int moduleId; // ID of module from which the relocation symbols are imported
                // from
  struct RelRelocEntry* relocs; // relocation entries
  int relocsCount;              // number of relocation entries
  size_t relocsOffset;          // file offset to relocation entries
};

struct Module {
  int moduleId; // unique identifier of the module; the id of the DOL is always
                // 0
  const char* filename; // name of the module's ELF file
  FILE* file;           // ELF file
  Elf32_Ehdr ehdr;      // ELF header
  Elf32_Sym* symtab;    // ELF symbol table entries
  int symtabCount;      // number of ELF symbol table entries
  char* strtab;         // ELF string table
  size_t strtabSize;    // size of ELF string table
};

static struct Module dolModule;
static struct Module relModule;
static struct RelImportEntry* imports;
static int importsCount = 0;
static int minSectionCount = 0;
static int undefinedSymError = 0;

static void fatal_error(const char* msg, ...) {
  va_list args;

  va_start(args, msg);
  vfprintf(stderr, msg, args);
  va_end(args);
  exit(1);
}

// Swaps byte order if the system is little endian

static void bswap32(uint32_t* ptr) {
  if (IS_LITTLE_ENDIAN)
    *ptr = (((*ptr >> 24) & 0xFF) << 0) | (((*ptr >> 16) & 0xFF) << 8) |
           (((*ptr >> 8) & 0xFF) << 16) | (((*ptr >> 0) & 0xFF) << 24);
}

static void bswap16(uint16_t* ptr) {
  if (IS_LITTLE_ENDIAN)
    *ptr = (((*ptr >> 8) & 0xFF) << 0) | (((*ptr >> 0) & 0xFF) << 8);
}

static int read_checked(FILE* f, size_t offset, void* out, size_t size) {
  return fseek(f, offset, SEEK_SET) == 0 && fread(out, size, 1, f) == 1;
}

static int write_checked(FILE* f, size_t offset, const void* in, size_t size) {
  return fseek(f, offset, SEEK_SET) == 0 && fwrite(in, size, 1, f) == 1;
}

static uint32_t align(uint32_t n, unsigned int alignment) {
  if (alignment == 0 || n % alignment == 0)
    return n;
  return n + alignment - (n % alignment);
}

static int is_supported_reloc_type(int type) {
  switch (type) {
  case R_PPC_ADDR32:
  case R_PPC_ADDR24:
  case R_PPC_ADDR16_LO:
  case R_PPC_ADDR16_HA:
  case R_PPC_REL24:
  case R_PPC_REL14:
    return 1;
  }
  return 0;
}

static const char* symbol_name(const struct Module* module,
                               const Elf32_Sym* sym) {
  if (sym->st_name >= module->strtabSize)
    return NULL;
  return module->strtab + sym->st_name;
}

static int get_symtab_entry(const struct Module* module, int index,
                            Elf32_Sym* sym) {
  if (index >= module->symtabCount)
    return 0;
  *sym = module->symtab[index];
  bswap32(&sym->st_name);
  bswap32(&sym->st_value);
  bswap32(&sym->st_size);
  bswap16(&sym->st_shndx);
  return 1;
}

static int lookup_symbol_by_name(const struct Module* module, const char* name,
                                 Elf32_Sym* sym) {
  int i;

  for (i = 0; i < module->symtabCount; i++) {
    get_symtab_entry(module, i, sym);
    const char* n = symbol_name(module, sym);
    if (n != NULL && strcmp(name, n) == 0)
      return 1;
  }
  return 0;
}

static struct RelImportEntry* get_import_for_module_id(int moduleId) {
  int i;

  for (i = 0; i < importsCount; i++) {
    if (imports[i].moduleId == moduleId)
      return &imports[i];
  }
  return NULL;
}

static void add_reloc_entry(const struct Module* module,
                            const Elf32_Rela* reloc, int relocSection) {
  Elf32_Sym sym;
  int symIdx = ELF32_R_SYM(reloc->r_info);
  int relocType = ELF32_R_TYPE(reloc->r_info);
  struct RelRelocEntry rentry;
  struct RelImportEntry* import;
  int moduleId; // module containing the symbol

  if (!is_supported_reloc_type(relocType))
    fatal_error("relocation type %i not supported\n", relocType);

  rentry.patchSection = relocSection;
  rentry.patchOffset = reloc->r_offset;
  rentry.type = relocType;

  // look for symbol in this module
  if (!get_symtab_entry(module, symIdx, &sym))
    fatal_error("couldn't find symbol index %i\n", symIdx);
  if (sym.st_shndx != 0) // symbol is in this module
  {
    rentry.symbolSection = sym.st_shndx;
    rentry.symbolAddr = sym.st_value + reloc->r_addend;
    moduleId = module->moduleId;
  } else // symbol is in another module (the DOL)
  {
    const char* name = symbol_name(&relModule, &sym);
    if (!lookup_symbol_by_name(&dolModule, name, &sym)) {
      undefinedSymError = 1;
      fprintf(stderr, "could not find symbol '%s' in any module\n", name);
      return;
    }
    if (sym.st_shndx >= dolModule.ehdr.e_shnum)
      fatal_error("bad section index %i\n", sym.st_shndx);

    rentry.symbolSection = sym.st_shndx;
    rentry.symbolAddr = sym.st_value + reloc->r_addend;
    moduleId = dolModule.moduleId;
  }

  import = get_import_for_module_id(moduleId);
  // add import entry if it does not exist.
  if (import == NULL) {
    imports = realloc(imports, (importsCount + 1) * sizeof(*imports));
    import = &imports[importsCount++];
    import->moduleId = moduleId;
    import->relocs = NULL;
    import->relocsCount = 0;
  }

  // add relocation entry
  import->relocs = realloc(import->relocs,
                           (import->relocsCount + 1) * sizeof(*import->relocs));
  import->relocs[import->relocsCount++] = rentry;
}

static void module_get_section_header(const struct Module* module, int secNum,
                                      Elf32_Shdr* shdr) {
  size_t offset = module->ehdr.e_shoff + secNum * module->ehdr.e_shentsize;

  if (secNum >= module->ehdr.e_shnum)
    fatal_error("no such section index %i\n", secNum);
  if (!read_checked(module->file, offset, shdr, sizeof(*shdr)))
    fatal_error("error reading section header\n");

  // convert from big endian
  bswap32(&shdr->sh_name);
  bswap32(&shdr->sh_type);
  bswap32(&shdr->sh_flags);
  bswap32(&shdr->sh_addr);
  bswap32(&shdr->sh_offset);
  bswap32(&shdr->sh_size);
  bswap32(&shdr->sh_link);
  bswap32(&shdr->sh_info);
  bswap32(&shdr->sh_addralign);
  bswap32(&shdr->sh_entsize);
}

static void module_read_relocs(struct Module* module) {
  int i;
  int j;

  undefinedSymError = 0;

  for (i = 0; i < (int)module->ehdr.e_shnum; i++) {
    Elf32_Shdr shdr;
    Elf32_Shdr forSection;

    module_get_section_header(module, i, &shdr);
    if (shdr.sh_type == SHT_RELA) {
      module_get_section_header(module, shdr.sh_info, &forSection);
      if (!(forSection.sh_flags & SHF_ALLOC))
        continue;
      for (j = 0; j < shdr.sh_size / sizeof(Elf32_Rela); j++) {
        Elf32_Rela reloc;

        read_checked(module->file, shdr.sh_offset + j * sizeof(Elf32_Rela),
                     &reloc, sizeof(reloc));
        // convert from big endian
        bswap32(&reloc.r_offset);
        bswap32(&reloc.r_info);
        bswap32((uint32_t*)&reloc.r_addend);

        add_reloc_entry(&relModule, &reloc, shdr.sh_info);
      }
    }
  }

  if (undefinedSymError)
    exit(1);
}

static int open_module(struct Module* module) {
  int i;

  // open file
  module->file = fopen(module->filename, "rb");
  if (module->file == NULL)
    fatal_error("could not open %s for reading: %s\n", module->filename,
                strerror(errno));

  // read and verify ELF header
  if (!read_checked(module->file, 0, &module->ehdr, sizeof(module->ehdr)))
    fatal_error("error reading ELF header\n");
  if (memcmp(module->ehdr.e_ident,
             "\x7F"
             "ELF",
             4) != 0)
    fatal_error("%s is not a valid ELF file\n", module->filename);

  // convert from big endian
  bswap16(&module->ehdr.e_type);
  bswap16(&module->ehdr.e_machine);
  bswap32(&module->ehdr.e_version);
  bswap32(&module->ehdr.e_entry);
  bswap32(&module->ehdr.e_phoff);
  bswap32(&module->ehdr.e_shoff);
  bswap32(&module->ehdr.e_flags);
  bswap16(&module->ehdr.e_ehsize);
  bswap16(&module->ehdr.e_phentsize);
  bswap16(&module->ehdr.e_phnum);
  bswap16(&module->ehdr.e_shentsize);
  bswap16(&module->ehdr.e_shnum);
  bswap16(&module->ehdr.e_shstrndx);

  if (module->ehdr.e_shentsize < sizeof(Elf32_Shdr))
    fatal_error("invalid section header size");

  // Verify architecture
  if (module->ehdr.e_ident[EI_CLASS] != ELFCLASS32 ||
      module->ehdr.e_ident[EI_DATA] != ELFDATA2MSB ||
      module->ehdr.e_machine != EM_PPC)
    fatal_error("%s: wrong architecture. expected PowerPC 32-bit big endian.\n",
                module->filename);

  // Read symbol table and string table
  for (i = 0; i < (int)module->ehdr.e_shnum; i++) {
    Elf32_Shdr shdr;

    module_get_section_header(module, i, &shdr);
    if (shdr.sh_type == SHT_SYMTAB && module->symtab == NULL) {
      module->symtabCount = shdr.sh_size / sizeof(Elf32_Sym);
      module->symtab = malloc(shdr.sh_size);
      if (!read_checked(module->file, shdr.sh_offset, module->symtab,
                        shdr.sh_size))
        fatal_error("error reading symbol table\n");
    } else if (shdr.sh_type == SHT_STRTAB && i != module->ehdr.e_shstrndx &&
               module->strtab == NULL) {
      module->strtabSize = shdr.sh_size;
      module->strtab = malloc(shdr.sh_size);
      if (!read_checked(module->file, shdr.sh_offset, module->strtab,
                        shdr.sh_size))
        fatal_error("error reading string table\n");
    }
  }
  if (module->symtab == NULL)
    fatal_error("%s does not have a symbol table.\n", module->filename);
  if (module->strtab == NULL)
    fatal_error("%s does not have a string table.\n", module->filename);

  return 1;
}

// searches for the special functions "_prolog", "_epliog", and "_unresolved"
static void find_rel_entry_functions(const struct Module* module,
                                     struct RelHeader* relHdr) {
  int i;

  // puts("finding entry points");
  for (i = 0; i < module->symtabCount; i++) {
    Elf32_Sym sym;
    Elf32_Shdr shdr;
    const char* name;

    get_symtab_entry(module, i, &sym);
    name = symbol_name(module, &sym);
    if (name == NULL)
      continue;
    if (strcmp(name, "_prolog") == 0) {
      module_get_section_header(module, sym.st_shndx, &shdr);
      relHdr->prologSection = sym.st_shndx;
      relHdr->prologOffset = sym.st_value - shdr.sh_addr;
    } else if (strcmp(name, "_epilog") == 0) {
      module_get_section_header(module, sym.st_shndx, &shdr);
      relHdr->epilogSection = sym.st_shndx;
      relHdr->epilogOffset = sym.st_value - shdr.sh_addr;
    } else if (strcmp(name, "_unresolved") == 0) {
      module_get_section_header(module, sym.st_shndx, &shdr);
      relHdr->unresolvedSection = sym.st_shndx;
      relHdr->unresolvedOffset = sym.st_value - shdr.sh_addr;
    }
  }
}

// patches the bl instruction at insnp to jump to offset
static void patch_rel24_branch_offset(uint8_t* insnp, int32_t branchOffset) {
  const uint32_t offsetMask =
      0x03FFFFFC; // bits of instruction that contain the offset
  uint32_t insn = (insnp[0] << 24) // read instruction
                  | (insnp[1] << 16) | (insnp[2] << 8) | (insnp[3] << 0);

  assert(((insn >> 26) & 0x3F) ==
         18); // TODO: do other instructions besides bl use R_PPC_REL24?
  insn =
      (insn & ~offsetMask) | (branchOffset & offsetMask); // patch instruction
  // write instruction
  insnp[0] = (insn >> 24) & 0xFF;
  insnp[1] = (insn >> 16) & 0xFF;
  insnp[2] = (insn >> 8) & 0xFF;
  insnp[3] = (insn >> 0) & 0xFF;
}

static void patch_code_relocs(struct RelHeader* relHdr, int sectionId,
                              uint8_t* code, size_t size) {
  struct RelImportEntry* import;
  struct RelRelocEntry* reloc;
  int i;

  // Remove redundant R_PPC_REL24 relocations for calls to functions within
  // the same module
  import = get_import_for_module_id(relModule.moduleId);
  assert(import != NULL);
  for (i = 0, reloc = &import->relocs[0]; i < import->relocsCount;
       i++, reloc++) {
    if (reloc->patchSection == sectionId && reloc->type == R_PPC_REL24) {
      assert(reloc->patchOffset < size);
      patch_rel24_branch_offset(code + reloc->patchOffset,
                                reloc->symbolAddr - reloc->patchOffset);
      // remove the relocation
      reloc->type = R_PPC_NONE;
    }
  }

  // Patch all calls to functions outside this module to jump to _unresolved
  // by default.
  if (relHdr->unresolvedSection == 0)
    return;
  import = get_import_for_module_id(0);
  assert(import != NULL);
  for (i = 0, reloc = &import->relocs[0]; i < import->relocsCount;
       i++, reloc++) {
    if (reloc->patchSection == sectionId && reloc->type == R_PPC_REL24) {
      assert(reloc->patchOffset < size);
      patch_rel24_branch_offset(code + reloc->patchOffset,
                                relHdr->unresolvedOffset - reloc->patchOffset);
    }
  }
}

static int compare_relocs(const void* a, const void* b) {
  const struct RelRelocEntry* relocA = a;
  const struct RelRelocEntry* relocB = b;

  // Sort by sections to which these relocations apply
  if (relocA->patchSection != relocB->patchSection)
    return relocA->patchSection - relocB->patchSection;
  // Sort by patch offset
  if (relocA->patchOffset != relocB->patchOffset)
    return relocA->patchOffset - relocB->patchOffset;
  // Otherwise, leave the order alone
  return (uintptr_t)a - (uintptr_t)b;
}

static int compare_imports(const void* a, const void* b) {
  const struct RelImportEntry* impA = a;
  const struct RelImportEntry* impB = b;

  return impA->moduleId - impB->moduleId;
}

static void write_rel_file(struct Module* module, struct RelHeader* relHdr,
                           const char* filename) {
  int i, j;
  size_t filePos = sizeof(struct RelHeader); // skip over header for now
  struct RelImportEntry* imp;
  FILE* fout = fopen(filename, "wb");

  if (fout == NULL)
    fatal_error("could not open %s for writing: %s\n", filename,
                strerror(errno));

  relHdr->moduleId = module->moduleId;
  relHdr->formatVersion = 1;

  find_rel_entry_functions(module, relHdr);

  // 1. Write section table and section contents

  relHdr->sectionTableOffset = filePos;
  relHdr->sectionCount = MAX(module->ehdr.e_shnum, minSectionCount);
  // section contents follow section info table
  filePos = relHdr->sectionTableOffset + relHdr->sectionCount * 8;
  relHdr->bssSize = 0;
  for (i = 0; i < module->ehdr.e_shnum; i++) {
    Elf32_Shdr shdr;
    struct {
      uint32_t offset;
      uint32_t size;
    } secEntry = {0};

    module_get_section_header(module, i, &shdr);
    // write section contents
    if (shdr.sh_type == SHT_PROGBITS && (shdr.sh_flags & SHF_ALLOC)) {
      size_t sizeAligned = align(shdr.sh_size, 4);
      uint32_t execflg = (shdr.sh_flags & SHF_EXECINSTR) ? 1 : 0;

      filePos = align(filePos, shdr.sh_addralign);
      if (shdr.sh_size > 0) {
        uint8_t* data = calloc(sizeAligned, 1);

        if (!read_checked(module->file, shdr.sh_offset, data, shdr.sh_size))
          fatal_error("error reading section\n");
        if (execflg)
          patch_code_relocs(relHdr, i, data, sizeAligned);
        if (!write_checked(fout, filePos, data, shdr.sh_size))
          fatal_error("error writing rel section\n");
        free(data);
      }
      secEntry.offset = filePos | execflg;
      filePos += shdr.sh_size;
    }
    if (shdr.sh_flags & SHF_ALLOC)
      secEntry.size = shdr.sh_size;

    // write section table entry
    bswap32(&secEntry.offset);
    bswap32(&secEntry.size);
    write_checked(fout, relHdr->sectionTableOffset + i * 8, &secEntry,
                  sizeof(secEntry));

    // calculate total BSS size
    if ((shdr.sh_flags & SHF_ALLOC) && shdr.sh_type == SHT_NOBITS)
      relHdr->bssSize += shdr.sh_size;
  }

  // 2. Write relocation data

  relHdr->relocationTableOffset = filePos;
  // sort imports by module id
  qsort(imports, importsCount, sizeof(struct RelImportEntry), compare_imports);
  for (i = 0, imp = &imports[0]; i < importsCount; i++, imp++) {
    struct RelRelocEntry* reloc;
    int currSection = -1;
    uint32_t prevOffset = 0;
    struct {
      uint16_t offset;
      uint8_t type;
      uint8_t section;
      uint32_t symaddr;
    } ent;

    imp->relocsOffset = filePos;
    // sort relocation entries by section
    qsort(imp->relocs, imp->relocsCount, sizeof(struct RelRelocEntry),
          compare_relocs);
    for (j = 0, reloc = &imp->relocs[0]; j < imp->relocsCount; j++, reloc++) {
      if (reloc->type == R_PPC_NONE) // ignore null relocations
        continue;

      if (reloc->patchSection != currSection) // section changed
      {
        currSection = reloc->patchSection;

        // write section change
        ent.offset = 0;
        ent.type = R_DOLPHIN_SECTION;
        ent.section = reloc->patchSection;
        ent.symaddr = 0;
        bswap16(&ent.offset);
        bswap32(&ent.symaddr);
        if (!write_checked(fout, filePos, &ent, sizeof(ent)))
          fatal_error("error writing relocation entry");

        filePos += sizeof(ent);
        prevOffset = 0;
      }

      // write relocation
      assert(reloc->patchOffset >= prevOffset);
      ent.offset = reloc->patchOffset - prevOffset;
      ent.type = reloc->type;
      ent.section = reloc->symbolSection;
      ent.symaddr = reloc->symbolAddr;
      bswap16(&ent.offset);
      bswap32(&ent.symaddr);
      if (!write_checked(fout, filePos, &ent, sizeof(ent)))
        fatal_error("error writing relocation entry");

      filePos += sizeof(ent);
      prevOffset = reloc->patchOffset;
    }

    // write end
    ent.offset = 0;
    ent.type = R_DOLPHIN_END;
    ent.section = 0;
    ent.symaddr = 0;
    bswap16(&ent.offset);
    bswap32(&ent.symaddr);
    if (!write_checked(fout, filePos, &ent, sizeof(ent)))
      fatal_error("error writing relocation entry");

    filePos += sizeof(ent);
  }

  // 3. Write module import table

  relHdr->importTableOffset = filePos;
  for (i = 0, imp = &imports[0]; i < importsCount; i++, imp++) {
    // write import table entry
    struct {
      uint32_t moduleId;
      uint32_t relocsOffset;
    } ent;
    ent.moduleId = imp->moduleId;
    ent.relocsOffset = imp->relocsOffset;
    bswap32(&ent.moduleId);
    bswap32(&ent.relocsOffset);
    write_checked(fout, relHdr->importTableOffset + i * 8, &ent, sizeof(ent));
  }
  relHdr->importTableSize = importsCount * 8;

  // 4. Write REL header.

  // convert to big endian
  bswap32(&relHdr->moduleId);
  bswap32(&relHdr->sectionCount);
  bswap32(&relHdr->sectionTableOffset);
  bswap32(&relHdr->moduleNameOffset);
  bswap32(&relHdr->moduleNameSize);
  bswap32(&relHdr->formatVersion);
  bswap32(&relHdr->bssSize);
  bswap32(&relHdr->relocationTableOffset);
  bswap32(&relHdr->importTableOffset);
  bswap32(&relHdr->importTableSize);
  bswap32(&relHdr->prologOffset);
  bswap32(&relHdr->epilogOffset);
  bswap32(&relHdr->unresolvedOffset);
  write_checked(fout, 0, relHdr, sizeof(*relHdr));

  fclose(fout);
}

static int parse_number(const char* str, int* n) {
  char* end;
  *n = strtol(str, &end, 0);
  return end > str && *end == 0;
}

int main(int argc, char** argv) {
  int i;
  int moduleId = -1;
  int nameOffset = 0;
  int nameLen = 0;
  const char* relName = NULL;
  struct RelHeader relHdr = {0};

  // Read command-line args
  for (i = 1; i < argc; i++) {
    if (strcmp(argv[i], "-c") == 0 ||
        strcmp(argv[i], "--pad-section-count") == 0) {
      if (i + 1 < argc && parse_number(argv[i + 1], &minSectionCount))
        i++;
      else
        goto usage;
    } else if (strcmp(argv[i], "-i") == 0 ||
               strcmp(argv[i], "--module-id") == 0) {
      if (i + 1 < argc && parse_number(argv[i + 1], &moduleId))
        i++;
      else
        goto usage;
    } else if (strcmp(argv[i], "-o") == 0 ||
               strcmp(argv[i], "--name-offset") == 0) {
      if (i + 1 < argc && parse_number(argv[i + 1], &nameOffset))
        i++;
      else
        goto usage;
    } else if (strcmp(argv[i], "-l") == 0 ||
               strcmp(argv[i], "--name-length") == 0) {
      if (i + 1 < argc && parse_number(argv[i + 1], &nameLen))
        i++;
      else
        goto usage;
    } else {
      if (relModule.filename == NULL)
        relModule.filename = argv[i];
      else if (dolModule.filename == NULL)
        dolModule.filename = argv[i];
      else if (relName == NULL)
        relName = argv[i];
      else
        goto usage;
    }
  }
  if (relModule.filename == NULL || dolModule.filename == NULL ||
      relName == NULL)
    goto usage;

  open_module(&relModule);
  open_module(&dolModule);

  dolModule.moduleId = 0;
  relModule.moduleId = moduleId;

  module_read_relocs(&relModule);

  // TODO: Read this information from string table
  relHdr.moduleNameOffset = nameOffset;
  relHdr.moduleNameSize = nameLen;
  write_rel_file(&relModule, &relHdr, relName);

  fclose(relModule.file);
  fclose(dolModule.file);

  free(dolModule.strtab);
  free(dolModule.symtab);
  for (i = 0; i < importsCount; i++)
    free(imports[i].relocs);
  free(imports);
  return 0;

usage:
  fprintf(stderr,
          "usage: %s reloc_elf static_elf rel_file\n"
          "options:\n"
          "  -i, --module-id <n>          sets the module ID in the rel header "
          "to <n>\n"
          "  -c, --pad-section-count <n>  ensures that the rel will have at "
          "least <n>\n"
          "                               sections\n"
          "  -o, --name-offset <offset>   sets the name offset in the rel "
          "header to\n"
          "                               <offset>\n"
          "  -l, --name-length <len>      sets the name length in the rel "
          "header to <len>\n",
          argv[0]);
  return 1;
}