binfmt_misc: document loader substitution

Describe the L mode next to the transparent one.

Link: https://patch.msgid.link/20260721-work-bpf-binfmt_misc-ptinterp-v2-21-e57866e4ae0f@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

Documentation/admin-guide/binfmt-misc.rst

@@ -260,6 +260,53 @@ direct execution is the address space layout. The interpreter occupies
 the main-image position and the program lives in the mmap region.
 
 
+Loader substitution
+-------------------
+
+The ``L`` flag turns the execution model around. Instead of running the
+registered interpreter with the binary as its payload the kernel loads
+the matched binary itself as the main image and substitutes the registered
+interpreter for the loader named in the binary's ``PT_INTERP``.
+
+Because the exec is native, there is no dispatch identity to
+reconstruct and no contract the substitute has to implement. A stock
+dynamic loader works unchanged. The argument vector is untouched,
+credentials and ``AT_SECURE`` derive from the binary, there is no
+``AT_EXECFD`` and no marker in the aux vector, the binary sits in the
+main-image slot with the native brk placement so ``/proc/pid/maps``,
+core dumps and perf mmap records have the native shape, and the
+identity is already complete when ``PTRACE_EVENT_EXEC`` stops the
+tracee. So launching under a debugger works, not just attaching. ``L``
+entries are for ELF binaries of a native architecture. Foreign-arch
+emulation and non-ELF payloads remain the domain of the classic and
+transparent modes.
+
+The override applies when the format that finally claims the file is
+ELF with a ``PT_INTERP``. A matched binary without one or an
+interpreter-less ``ET_DYN`` drops the override and runs natively. A file
+claimed by another format - a ``#!`` script, say - is handled by that
+format as if the entry had not matched. ``L`` is therefore not an
+enforcement mechanism: it decides how a binary that asks for a loader is
+run, it does not guarantee that everything matching the entry runs under
+the substitute. A format that cannot consume the override at all instead
+refuses the exec with ``ENOEXEC`` before the point of no return.
+
+A wrong-architecture ELF fails the whole exec with ``ENOEXEC`` exactly
+as if no entry had matched. A substitute that is not ELF of the right
+architecture fails with ``ELIBBAD``. The usual ``PT_INTERP`` sanity
+checks on the binary still apply. But the segment's content is otherwise
+irrelevant.
+
+``L`` rejects the classic-dispatch flags ``T``, ``P``, ``O`` and ``C``
+at registration. ``F`` composes and is valuable: with it the substitute
+is opened at registration time, so later mount namespace or path changes
+cannot redirect it. Without it the substitute is opened when the binary
+is executed, and the path is resolved in the mount namespace and root of
+whoever runs the binary, which is why it has to be absolute. As with
+``C``, register only trusted interpreters. The substituted loader runs
+with credentials derived from the binary.
+
+
 Hints
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