How to (legitimately) access files after putting self into chrooted sandbox?

Question

Changing a Linux C++ program which gives the user limited file access. Thus the program chroots itself to a sandbox with the files the user can get at. All worked well.

Now, however, the program needs to access some files for its own needs (not the user's) but they are outside the sandbox. I know chroot allows access to files opened before the chroot but in this case the needed files could a few among many hundreds so it is obviously impractical to open them all just for the couple that might be required.

Is there any way to get at the files?

Answer 1

Copy them into the sandbox or open them all before chrooting. Seriously. If there was a way to do this, there would be a way to suborn it to allow other access and make your protection useless.

The whole point of the sandbox is to prevent exactly what you're trying to achieve.

Answer 2

If the files are all in 1 directory, you could use mount to bind them to a directory inside the sandbox.

mount --bind /path/to/files /sandbox/files

The you can access the files through /sandbox/files/. If you don't want the user to see them, do mount --bind /path/to/files /sandbox/.files so the .files directory is hidden