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macho_edit

Command line utility for modifying Mach-O binaries in various ways.

Supported functionality

  • Extracting and removing archs from a fat binary.
  • Inserting an arch from another binary into a fat binary.
  • Making a fat binary thin or a thin binary fat.
  • Moving around and removing load commands.
  • Inserting new load commands. Currently only LC_LOAD_DYLIB, LC_LOAD_WEAK_DYLIB and LC_RPATH is supported.
  • Removing code signature (LC_CODE_SIGNATURE).

Removing code signature

To remove the code signature it is enough to delete the LC_CODE_SIGNATURE load command and fixup the mach header's ncmds and sizeofcmds, assuming it is the last load command.

However if you just do this codesign_allocate (used by codesign and ldid) will fail with the error:

.../codesign_allocate: file not in an order that can be processed (link edit information does not fill the __LINKEDIT segment):

To fix this macho_edit assumes that the code signature that LC_CODE_SIGNATURE is in the end of the __LINKEDIT segment and the that the segment is in the end of the architectures slice.

It then truncate that slice to remove the code signature part of the __LINKEDIT segment. It also updates the LC_SEGMENT (or LC_SEGMENT64) load command for the __LINKEDIT segment from the new file size. If the binary is fat we also update the size and we might also move the slice and so the offset should also be updated.

After removing the code signature from the __LINKEDIT segment, the last thing in that segment is typically the string table. As the code signature seems to be aligned by 0x10, and so after removing the code signature, nothing points to the padding at the end of the segment, which codesign_allocate doesn't like either. To fix this we just trim the file so the string table in the LC_SYMTAB command is at the end of the slice.

Todo

  • Option to modify mach header flags