[PATCH v2 0/5] binfmt_misc: bpf-backed binary type handlers

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

This is a continuation of Christian's bpf-backed binfmt_misc POC [1], which he offered
to hand off to me. I am carrying it forward. The kernel design is his and is
unchanged. This series rebases it onto his binfmt_misc locking/cleanup series [2]
and adds selftests (+ fixed from the one's shared), and therefore does
not apply to mainline alone.

As for motivation for this whole change, Christian did a great VL;MR;
write-up [1].

TL;DR: binfmt_misc can match a binary and hand it to a fixed interpreter,
but it can't match programmatically or compute the interpreter per binary.
The Nix community would love to support relocatable binaries, where the
correct loader can only be found relative to the binary itself.
This adds a 'B' handler type: a binfmt_misc_ops struct_ops program that
inspects the binary and picks the interpreter with one new kfunc,
bpf_binprm_set_interp(). 

  bpftool struct_ops register nix_origin.bpf.o /sys/fs/bpf
  echo ':nix-origin:B:nix_origin::::' > /proc/sys/fs/binfmt_misc/register

Patch 5's nix_origin.bpf.c is Christian's example program with a small fix
so it passes the verifier. Christian, happy to add a Co-developed-by: and your
Signed-off-by: if you would like the credit in the trailer.

For those that want to validate this functionality on a NixOS machine,
you may find my flake.nix handy [3]. It builds the kernel, compiles the
bpf objects against its BTF, and runs the selftest on boot,

  # be sure to change virtualisation.sharedDirectories in flake.nix to your checkout
  $ nix build .#nixosConfigurations.micro-vm.config.system.build.vm -o /tmp/vm
  $ NIXPKGS_QEMU_KERNEL_micro_vm=$PWD/arch/x86/boot/bzImage \
  $ NIX_DISK_IMAGE=/tmp/vm.qcow2 \
  $ QEMU_KERNEL_PARAMS="binfmt_autotest console=ttyS0" \
  $ QEMU_OPTS="-nographic -no-reboot" \
  /tmp/vm/bin/run-micro-vm-vm

[1] https://lore.kernel.org/r/20260707-work-bpf-binfmt_misc-v1-0-74b995c84ec1@kernel.org
[2] https://lore.kernel.org/all/20260710-work-binfmt_misc-locking-v3-0-a162f7cb58d6@kernel.org/
[3] https://gist.github.com/fzakaria/0155e11a0882bd3d6e63f4070e7fac0c

To: Christian Brauner <brauner@kernel.org>
To: Alexei Starovoitov <ast@kernel.org>
To: Daniel Borkmann <daniel@iogearbox.net>
To: Martin KaFai Lau <martin.lau@linux.dev>
To: Shuah Khan <shuah@kernel.org>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Kees Cook <kees@kernel.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Jan Kara <jack@suse.cz>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Jann Horn <jannh@google.com>
Cc: John Ericson <mail@johnericson.me>
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-mm@kvack.org
Cc: linux-kernel@vger.kernel.org
Cc: bpf@vger.kernel.org
Cc: linux-doc@vger.kernel.org
Cc: linux-kselftest@vger.kernel.org

Signed-off-by: Farid Zakaria <farid.m.zakaria@gmail.com>
---
Changes in v2:
- Patch 5 adds new tests that validate the functionality:
  - a bpf handler matches a synthetic aarch64 header and routes it to a fixed
     interpreter the program chooses;
  - a handler resolves a "$ORIGIN/..."-relative PT_INTERP to an
     interpreter co-located with the binary (i.e, Nix usecase);
- Patch 3 is rebased to fit the new binfmt_misc code style: the 'B' field parsing is now a
  parse_bpf_fields() helper matching the new parse_{magic,extension}_fields()
  split, and load_misc_binary() keeps the bpf retry loop in the
  __free() cleanup style.
- Link to v1: https://lore.kernel.org/r/20260707-work-bpf-binfmt_misc-v1-0-74b995c84ec1@kernel.org

---
Christian Brauner (4):
      exec: stash a bpf-selected interpreter in struct linux_binprm
      binfmt_misc: add binfmt_misc_ops bpf struct_ops
      binfmt_misc: wire up bpf-backed 'B' entries
      bpf: allow fs kfuncs for binfmt_misc_ops programs

Farid Zakaria (1):
      selftests/exec: add binfmt_misc bpf-backed handler test

 Documentation/admin-guide/binfmt-misc.rst        |  40 +++-
 fs/Kconfig.binfmt                                |  14 ++
 fs/Makefile                                      |   1 +
 fs/binfmt_misc.c                                 | 149 +++++++++++-
 fs/binfmt_misc_bpf.c                             | 275 +++++++++++++++++++++++
 fs/bpf_fs_kfuncs.c                               |  23 +-
 fs/exec.c                                        |   1 +
 include/linux/binfmt_misc.h                      |  49 ++++
 include/linux/binfmts.h                          |   1 +
 tools/testing/selftests/exec/Makefile            |  37 +++
 tools/testing/selftests/exec/binfmt_bpf_app.c    |  12 +
 tools/testing/selftests/exec/binfmt_bpf_interp.c |  15 ++
 tools/testing/selftests/exec/binfmt_misc_bpf.c   | 260 +++++++++++++++++++++
 tools/testing/selftests/exec/bpf_interp.bpf.c    |  52 +++++
 tools/testing/selftests/exec/nix_origin.bpf.c    | 179 +++++++++++++++
 15 files changed, 1091 insertions(+), 17 deletions(-)
---
base-commit: 6203a47c1d78f948cd5e9ad1a4c089745e466870
change-id: 20260711-binfmt-misc-bpf-v2-0f5c1f99b9ed

Best regards,
-- 
Farid Zakaria <farid.m.zakaria@gmail.com>