Merge patch series "binfmt_misc: bind interpreters to a bpf-backed entry"

Christian Brauner <brauner@kernel.org> says:

binfmt_misc: bind interpreters to a bpf-backed entry

A 'B' entry's load program hands the kernel an absolute path and
open_exec() resolves it at exec time in the mount namespace of whoever
runs the binary. So the handler names an interpreter but never gets to
say which file that is. Whoever controls the filesystem view of the exec
does.

Static entries have had the answer for a while. 'F' opens the file at
registration and every exec runs a clone of it. I can't just reuse it as
it stands. It pre-opens the one interpreter named in the register string
and a 'B' entry has no fixed interpreter. The program picks per exec,
and a qemu-user shaped handler wants one per guest architecture. So it
may want a whole set of them and that doesn't fit in a register string.

An entry is matchable the moment it is registered, so everything it
needs has to fit in that one write. Patch 1 adds a 'D' flag that creates
the entry disabled and splits a registration into create and activate:

    echo ':qemu:B::::qemu_user:D' > register
    echo '+aarch64 /usr/bin/qemu-aarch64' > qemu
    echo '+arm /usr/bin/qemu-arm' > qemu
    echo 1 > qemu

Each path is opened by its write, with the credentials the entry file was
opened with. Same open_exec() call, same place as 'F'. The program picks
one per exec with bpf_binprm_select_interp() and gets a clone of the
file. Nothing is resolved again, in any namespace.

A 'D' entry simply isn't hashed until that first '1', so the rcu
insertion that publishes the entry also publishes its interpreters and
the exec side needs no barriers. Reading the entry file doesn't take any
locks either. Bindings are rcu-published and the open file already pins
everything the read looks at. We use paths, not fds which makes the
config remain nice and static and can be shipped via /etc/binfmt.d.

* patches from https://patch.msgid.link/20260730-work-binfmt_misc-preopen-v1-0-4a0b0da71f16@kernel.org:
  binfmt_misc: document interpreters bound by a 'B' entry
  selftests/exec: test interpreters bound to a 'B' entry
  binfmt_misc: let a 'B' entry bind its interpreters
  binfmt_misc: carry pre-opened interpreters in struct binfmt_misc_interp
  selftests/exec: share the bpf handler preconditions
  binfmt_misc: document registering an entry disabled
  selftests/exec: test registering an entry disabled
  selftests/exec: let binfmt_flag_supported() return a bool
  binfmt_misc: let a register string create an entry disabled

Link: https://patch.msgid.link/20260730-work-binfmt_misc-preopen-v1-0-4a0b0da71f16@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>