QT Applications - Replacing embedded resources

Question

Is it possible to replace embedded resources [e.g. styles, images, text] in a Linux [ELF] binary?

I noticed that I can change text but if I type more text or if I remove text, then the segmentation faults start coming up. I have not gone through the ELF spec yet but I am wondering if it is possible.

I managed to extract the images from the binary using the mediaextract project but I need to do just the opposite without breaking the binary structure.

Answer 1

This answer is specific for Qt's resource system (.qrc, rcc).

From the docs:

Currently, Qt always stores the data directly in the executable, even on Windows, macOS, and iOS, where the operating system provides native support for resources. This might change in a future Qt release.

So yes, the Qt resources are contained in the binary.

rcc'ing a .qrc file yields a .cpp file containing (mainly) simple char arrays which represent resource data, the resource names and some other metadata.

Compiling such a .cpp file creates byte fields in the binary.

You can alter such resources within a binary, but only in very limited ways.

• For starters, if the binary contains any kind of self-check (like hashing the data section and comparing it to some pre-calculated hash), you will not be able to change the data in a reasonable way.

• If your data doesn't have the same byte length as the original data, you can't simply replace it because it would alter the internal layout of the binary and invalidate relative addresses.

• In case of replacing with shorter strings you might get away with zero-padding at the end.

Resources are compressed by default (in the ZIP format). It is possible to turn off compression.

• If compression was turned on during compilation (which you don't control, as it seems), you'd need to create new data which compresses to the same length as the original.