Ask a load segment where it is, rather than reading its header (#461)

Every one of these is a LOAD segment, and LoadSegment already answers for
what they were taking out of the header: mem_head, file_head, and mem_tail
for the p_vaddr + p_memsz the harness was adding up itself.

The gadget corpus is byte-identical at levels 0, 1 and 2, and the address
the harness steers writes to is unchanged for every fixture.

Left alone: helper.rb reads e_machine, for which there is no accessor --
ELFFile#machine answers with a display string ("Advanced Micro Devices
X86-64 processor"), which is no basis for choosing a fetcher.

aletheia/lib/aletheia/runner.rb

@@ -70,7 +70,7 @@ module Aletheia
     def spare_writable(target)
       end_of_data = File.open(target) do |f|
         ELFTools::ELFFile.new(f).segments_by_type(:load).select(&:writable?)
-                         .map { |s| s.header.p_vaddr.to_i + s.header.p_memsz.to_i }.max
+                         .map(&:mem_tail).max
       end
       return nil unless end_of_data
 

lib/one_gadget/fetchers/disassembly.rb

@@ -139,7 +139,7 @@ module OneGadget
           record_instruction_sets(elf)
           @objdump.read_raw(machine: OneGadget::Helper.objdump_arch(OneGadget::Helper.architecture(file)),
                             endian: elf.endian,
-                            vma: seg.header.p_vaddr.to_i - seg.header.p_offset.to_i)
+                            vma: seg.mem_head - seg.file_head)
         end
       end
 
@@ -187,7 +187,7 @@ module OneGadget
           seg = executable_segment(elf)
           return nil if seg.nil?
 
-          scan_calls(seg.header.p_vaddr.to_i, seg.data, targets)
+          scan_calls(seg.mem_head, seg.data, targets)
         end
       rescue ELFTools::ELFError
         nil # not something we can scan; disassemble everything
@@ -197,7 +197,7 @@ module OneGadget
       # what raw disassembly is taken from. It holds a little besides the code, and
       # is used in place of a section because a stripped file has none.
       # @param [ELFTools::ELFFile] elf
-      # @return [ELFTools::Segments::Segment, nil]
+      # @return [ELFTools::Segments::LoadSegment, nil]
       def executable_segment(elf)
         elf.segments_by_type(:load).find(&:executable?)
       end

lib/one_gadget/fetchers/mips.rb

@@ -169,7 +169,7 @@ module OneGadget
         @loaded_code = File.open(file) do |fd|
           elf = ELFTools::ELFFile.new(fd)
           segment = executable_segment(elf)
-          segment && { base: segment.header.p_vaddr.to_i,
+          segment && { base: segment.mem_head,
                        words: segment.data.unpack(elf.endian == :big ? 'N*' : 'V*') }
         end
       end