patchShebangs doesn't select correct /bin/sh for cross builds

[elitak]

Issue description

pkgs/build-support/setup-hooks/patch-shebangs.sh uses command -v to determine the substitute for the shebang path. This selects the wrong path during cross-builds and results in x86_64 binaries getting pulled in as dependencies to cross-built binaries that can't possibly use them at runtime.

I don't know if we can reliably pick the correct /bin/sh without it being in the PATH envvar. I presume patchShebangs works fine for binaries that don't overlap with stdenv's and are included in buildInputs.

Steps to reproduce

If you cross-build some packages to ARM platforms, patchShebangs runs in the fixup phase and run head -n1 /nix/store/*-glibc-2.24-arm*-bin/bin/sotruss, you'll see it points to the native /bin/sh rather than the cross one. I was considering a PR to just add dontPatchShebangs=true; to the cross build of glibc, but figured it'd be better if the root issue were fixed instead.

Technical details

• System: "16.03pre75806.77f8f35"
• Nix version: nix-env (Nix) 1.12pre1234_abcdef
• Nixpkgs version: 17.03.git.01febe0 (Gorilla)

[bjornfor]

/participate. (I've stumbled across this too.)

[elitak]

Is it safe to assume /bin/{ba,a}sh are the only paths that will ever realistically have this problem? If so, how and where would we pull in bash.crossDrv as a buildInput without it clobbering the native /bin/sh used by stdenv to run the builder? Once we're already executing patchShebangs, it's too late to find any .crossDrvs.

This problem overlaps with another, #21191, which is that crossDrvs clobber stdenv's binaries in the PATH.

The best solution I can come up with is to manually include bash in buildInputs of e.g. glibc, add logic in pks/stdenv/generic/setup.sh to not add ${bash.crossDrv}/bin to the PATH when crossbuilding, based on some new derivation attr like excludeBuildInputsFromPath = [ bash ];, then search $crossPkgs/bin during patchShebangs, independently of what's set in the PATH.

Or we could just make setup.sh smarter about importing /bin/ PATHs that contain elements of the stdenv, namely: coreutils bash busybox.

[Ericson2314]

NixOS/nix#1080 is essential to fix this without greatly complicating bootstrapping.

[Ericson2314]

@matthewbauer don't forget to close. :)

[matthewbauer]

Addressed in #43833