From 9d3f2b289563cb7c845c8a35cb0e7553b21f85e2 Mon Sep 17 00:00:00 2001
From: Roland Reichwein <mail@reichwein.it>
Date: Sun, 29 Mar 2020 15:01:38 +0200
Subject: Fix ELF

---
 elf.cpp | 69 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 64 insertions(+), 5 deletions(-)

(limited to 'elf.cpp')

diff --git a/elf.cpp b/elf.cpp
index 9a835ff..2923392 100644
--- a/elf.cpp
+++ b/elf.cpp
@@ -2,17 +2,25 @@
 
 #include "file.h"
 
+// TODO
 //#include <boost/endian/conversion.hpp>
 
 #include <cstring>
 
+// We need at least 4096 bytes aligment. Otherwise we get segfaults at startup
+#define PAD 0x1000
+
 // Helper Functions
 namespace {
  void AddFileHeader(std::vector<uint8_t>& data, const std::vector<uint8_t>& code)
  {
   Elf::FileHeader fh;
 
-  fh.e_shoff = sizeof(fh) + sizeof(Elf::ProgramHeader) + code.size() + 16;
+#ifdef PAD
+  fh.e_shoff = PAD + code.size() + 17;
+#else
+  fh.e_shoff = sizeof(fh) + sizeof(Elf::ProgramHeader) + code.size() + 17;
+#endif
 
   size_t old_size {data.size()};
   data.resize(old_size + sizeof(fh));
@@ -23,7 +31,12 @@ namespace {
  {
   Elf::ProgramHeader ph;
 
+#ifdef PAD
+  ph.p_offset = PAD;
+  ph.p_align = PAD;
+#else
   ph.p_offset = sizeof(Elf::FileHeader) + sizeof(Elf::ProgramHeader);
+#endif
   ph.p_filesz = code.size();
   ph.p_memsz = code.size();
 
@@ -32,13 +45,35 @@ namespace {
   std::memcpy(data.data() + old_size, reinterpret_cast<void*>(&ph), sizeof(ph));
  }
 
- void AddSectionHeaderText(std::vector<uint8_t>& data, const std::vector<uint8_t>& code)
+ void AddSectionHeaderNull(std::vector<uint8_t>& data, const std::vector<uint8_t>& code)
  {
   Elf::SectionHeader sh;
 
   sh.sh_name = 0; // offset in section
+  sh.sh_size = 0;
+  sh.sh_type = 0; // <null>
+  sh.sh_flags = 0;
+  sh.sh_addr = 0;
+  sh.sh_offset = 0;
+
+  size_t old_size {data.size()};
+  data.resize(old_size + sizeof(sh));
+  std::memcpy(data.data() + old_size, reinterpret_cast<void*>(&sh), sizeof(sh));
+ }
+ 
+ void AddSectionHeaderText(std::vector<uint8_t>& data, const std::vector<uint8_t>& code)
+ {
+  Elf::SectionHeader sh;
+
+  sh.sh_name = 1; // offset in section
   sh.sh_size = code.size();
   sh.sh_type = 1; // program
+#ifdef PAD
+  sh.sh_offset = PAD;
+#else
+  sh.sh_offset = sizeof(Elf::FileHeader) + sizeof(Elf::ProgramHeader);
+#endif
+  sh.sh_addralign = 1;
 
   size_t old_size {data.size()};
   data.resize(old_size + sizeof(sh));
@@ -49,15 +84,30 @@ namespace {
  {
   Elf::SectionHeader sh;
 
-  sh.sh_name = 0; // offset in section
-  sh.sh_size = 16;
+  sh.sh_name = 7; // offset in section
+  sh.sh_size = 17;
   sh.sh_type = 3; // section names
   sh.sh_flags = 0;
+  sh.sh_addr = 0;
+#ifdef PAD
+  sh.sh_offset = PAD + code.size();
+#else
+  sh.sh_offset = sizeof(Elf::FileHeader) + sizeof(Elf::ProgramHeader) + code.size();
+#endif
+  sh.sh_addralign = 1;
 
   size_t old_size {data.size()};
   data.resize(old_size + sizeof(sh));
   std::memcpy(data.data() + old_size, reinterpret_cast<void*>(&sh), sizeof(sh));
  }
+
+ void PadUpTo(std::vector<uint8_t>& data, size_t size)
+ {
+  if (data.size() < size) {
+   data.resize(size);
+  } else
+   throw std::runtime_error("Padding not possible. Too many bytes already.");
+ }
 }
 
 void Elf::Write(const std::filesystem::path& path, const std::vector<uint8_t>& code)
@@ -67,13 +117,21 @@ void Elf::Write(const std::filesystem::path& path, const std::vector<uint8_t>& c
  AddFileHeader(data, code);
  AddProgramHeader(data, code);
 
+#ifdef PAD
+ PadUpTo(data, PAD);
+#endif
+
  data.insert(data.end(), code.begin(), code.end());
  
- std::string section_names(".text\0.shstrtab\0", 16);
+ std::string section_names("\0.text\0.shstrtab\0", 17);
+ if (section_names.size() != 17)
+  throw std::runtime_error("Bad size");
+
  size_t old_size {data.size()};
  data.resize(old_size + section_names.size());
  std::memcpy(data.data() + old_size, section_names.data(), section_names.size());
 
+ AddSectionHeaderNull(data, code);
  AddSectionHeaderText(data, code);
  AddSectionHeaderSectionNames(data, code);
 
@@ -88,3 +146,4 @@ std::vector<uint8_t> Elf::Read(const std::filesystem::path& path)
 
  return result;
 }
+
-- 
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