The ufr2 driver relies on black-box binaries from Canon, with baked-in FHS assumptions. Previously, a LD_PRELOAD shim has been used to adapt calls to some libc functions and correct the path arguments passed. Unfortunately, this hack doesn't work for all printers; some of the Canon libraries rely on invoking syscalls directly, and can't be mollified with ld.so tricks.

And so blacker arts were needed. Introduce a wrapper that uses ptrace to intercept all syscalls performed by the binary; when a supported syscall is identified, read its path argument from the binary's address space, see if it's a broken FHS path, and if necessary inject a corrected path into the process's address space before allowing the fixed-up syscall to proceed.

This has some pros and cons. The cons:

• ptrace is sometimes restricted (it's allowed under the default NixOS config though)
• ptrace is only available on Linux (but I think installing cups drivers through nix only makes sense on NixOS?)
• there's a minor performance penalty (syscalls cost several times as much)
• it's crazy (this binary doesn't seem to do any of the things that might break it, though)

The pros:

• got my D530 working (just in time to print that shipping label...)
• it's a general solution; it should work for any printer that uses this driver, and it should work for future versions of the driver

Fixes #44330

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.