UPX packing/unpacking information preservation

Question

If I pack an executable using UPX and then unpack the executable using UPX -d, the executables are not the same. I understand techniques for unpacking executables that are packed using UPX, but I was wondering - what happens to the executable during packing/unpacking? How does this affect the preservation of information inside the executable? (For example, if I compile C code and then decompile it I'll have lost a lot of variable name information.)

EDIT: I've verified that after UPX packing/repacking the binary is not the same. The state machine looks something like:

#1 (Original) ----UPX Pack----> #2 (Packed)

#2 (Packed) -----UPX Unpack----> #3 (Unpacked)

#3 (Unpacked) -----UPX Pack-----> #4 (Repacked)

#4 (Repacked) -----UPX Unpack----> #3 (Unpacked)

Where #1, #2, #3, and #4 all have different hashes.

Answer 1

If you compare the hash values of original/unpacked files, then they are different since upx -d does not restore bit-by-bit of the original file. Indeed, UPX parses the original file and keeps only information so that the packed data, after being unpacked, can be executed exactly the same as the original one, i.e. the original/unpacked files are semantically equivalent but not physically equivalent^^.

That is understandable since there is information that not affect to the execution of the binary, a trivial instance is the data between the end of DOS stub and the begin of PE header. For more detail, you may refer to the function pack of the class, for example, PackW32Pe (in p_w32pe.h/cpp) for PE and the function unpack of the class Packer (in packer.h/cpp).

For example, we can see that UPX modifies the (DOS and PE) header of the unpacked file using the following code (I have renamed some variables for more comprehensible). First, it decompresses and extracts the header:

// decompress
decompress(input_buffer, output_buffer);
upx_byte *extrainfo = output_buffer + get_le32(output_buffer + ph.u_len - 4);

memcpy(&output_header, extrainfo, sizeof (output_header));

then modifies slightly the header:

...
output_header.headersize = rvamin;
output_header.chksum = 0;

//NEW: disable reloc stripping if ASLR is enabled
if(input_header.dllflags & IMAGE_DLL_CHARACTERISTICS_DYNAMIC_BASE)
    opt->win32_pe.strip_relocs = false;

// FIXME: ih.flags is checked here because of a bug in UPX 0.92
if ((opt->win32_pe.strip_relocs && !isdll) || (input_header.flags & RELOCS_STRIPPED))
{
  output_header.flags |= RELOCS_STRIPPED;
  ODADDR(PEDIR_RELOC) = 0;
  ODSIZE(PEDIR_RELOC) = 0;
}

// write decompressed file
if (output_file)
{
...