alt_saver: some bugfixes for user-reported issues (#46)

* alt_saver: port fix for segment shifting breaking some systems

In certain circumstances, in particular with older kernels, some executables could fail to run after shifting. This was never seen on modern Ubuntu but was seen on RHEL.

NixOS/patchelf@8d2cb4f

* alt_saver: fix crash on invalid symbol section

This was an error in porting and thus a patchelf.rb specific issue.

lib/patchelf/alt_saver.rb

@@ -451,10 +451,10 @@ def rewrite_headers(phdr_address)
 
           begin
             new_index = new_section_idx(old_shndx)
-            next unless new_index
           rescue ArgumentError
             Logger.warn "entry #{entry} in symbol table refers to a non existing section, skipping"
           end
+          next unless new_index
 
           sym[pack[:st_shndx]] = new_index
 
@@ -561,11 +561,12 @@ def rewrite_sections_executable
       if needed_space > start_offset
         needed_space += seg_num_bytes # new load segment is required
 
-        needed_pages = Helper.alignup(needed_space - start_offset, page_size) / page_size
+        extra_bytes = needed_space - start_offset
+        needed_pages = Helper.alignup(extra_bytes, page_size) / page_size
         Logger.debug "needed pages is #{needed_pages}"
         raise PatchError, 'virtual address space underrun' if needed_pages * page_size > first_page
 
-        shift_file(needed_pages, start_offset)
+        shift_file(needed_pages, start_offset, extra_bytes)
 
         first_page -= needed_pages * page_size
         start_offset += needed_pages * page_size
@@ -776,7 +777,7 @@ def shift_segments(shift, start_offset)
     end
     # rubocop:enable Metrics/PerceivedComplexity
 
-    def shift_file(extra_pages, start_offset)
+    def shift_file(extra_pages, start_offset, extra_bytes)
       raise PatchError, "start_offset(#{start_offset}) < ehdr.num_bytes" if start_offset < ehdr.num_bytes
 
       oldsz = @buffer.size
@@ -799,8 +800,8 @@ def shift_file(extra_pages, start_offset)
         p_offset: split_phdr.p_offset - split_shift - shift,
         p_vaddr: split_phdr.p_vaddr - split_shift - shift,
         p_paddr: split_phdr.p_paddr - split_shift - shift,
-        p_filesz: split_shift + shift,
-        p_memsz: split_shift + shift,
+        p_filesz: split_shift + extra_bytes,
+        p_memsz: split_shift + extra_bytes,
         p_flags: ELFTools::Constants::PF_R | ELFTools::Constants::PF_W,
         p_align: page_size
       )