[PATCH 0/2] fs: support $ORIGIN in ELF interpreter paths

[Farid Zakaria <farid.m.zakaria@gmail.com>]

Currently, standard ELF and ELF FDPIC loaders require a fixed, absolute
path to the dynamic linker/interpreter (specified via PT_INTERP). This
creates significant inflexibility for relocatable dynamic interpreters,
where binaries are packaged independent of global system paths.

The primary goal of this patch series is to support relocatable binaries
in Nix, where packages are stored in a read-only store (typically /nix/store).
Allowing the ELF interpreter path to be resolved dynamically relative to
the binary's location via $ORIGIN enables Nix packages to be relocated
without needing patchelf or wrapper scripts.

For details on the design and motivation for this in Nix, see:
https://fzakaria.com/2026/06/21/nix-needs-relocatable-binaries

This series introduces support for resolving the $ORIGIN placeholder in 
the ELF interpreter path, bringing the kernel's binary loading behavior
in line with user-space dynamic linker origin resolution. 

To achieve this cleanly:
- We introduce a shared 'resolve_elf_interpreter()' helper in the VFS
  exec subsystem to avoid code duplication across loader implementations.
- For security, we restrict detection strictly to the prefix string
  "$ORIGIN" to prevent complex parsing exploits in kernel space.

Testing & Verification:
- Added a KUnit test case verifying path resolution logic.
- Added a kselftests integration test checking that a dynamically
  linked binary with its interpreter set to '$ORIGIN/mock_interp' successfully
  loads the mock interpreter (built statically using nolibc to avoid
  glibc TLS setup constraints during interpreter load-time).
- Verified end-to-end correct execution (PASS) using a minimal initramfs
  under QEMU.

Farid Zakaria (2):
  fs: support $ORIGIN in ELF interpreter paths
  selftests/exec: add test suites for $ORIGIN interpreter resolution

 fs/binfmt_elf.c                               | 11 ++++-
 fs/binfmt_elf_fdpic.c                         | 15 ++++++-
 fs/exec.c                                     | 42 +++++++++++++++++++
 fs/tests/exec_kunit.c                         | 26 ++++++++++++
 include/linux/binfmts.h                       |  2 +
 tools/testing/selftests/exec/Makefile         | 12 ++++--
 tools/testing/selftests/exec/mock_interp.c    |  6 +++
 tools/testing/selftests/exec/origin_interp.sh | 16 +++++++
 tools/testing/selftests/exec/test_prog.c      |  5 +++
 9 files changed, 128 insertions(+), 7 deletions(-)
 create mode 100644 tools/testing/selftests/exec/mock_interp.c
 create mode 100755 tools/testing/selftests/exec/origin_interp.sh
 create mode 100644 tools/testing/selftests/exec/test_prog.c

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2.51.2