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binfmt_misc: document registering an entry disabled

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Describe the 'D' flag and what it changes about a registration:

- that the entry has to be enabled before it dispatches anything
- and that the flag is not read back

Scope the bpf section's "carries no flags" rule to invocation flags now
that 'D' composes with 'B'.

Link: https://patch.msgid.link/20260730-work-binfmt_misc-preopen-v1-4-4a0b0da71f16@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
This commit is contained in:
Christian Brauner
2026-07-30 15:34:06 +02:00
parent 6bd0c7aba6
commit d5ae8a7c4d
1 changed files with 13 additions and 2 deletions
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15
Documentation/admin-guide/binfmt-misc.rst
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@@ -107,6 +107,15 @@ Here is what the fields mean:
``PT_INTERP``. See the "Loader substitution" section
below. ``L`` rejects ``T``, ``P``, ``O`` and ``C``;
``F`` composes.
``D`` - registered disabled
The entry is created disabled instead of being matchable at
once, and has to be enabled by writing ``1`` to its file
before it dispatches anything. This splits a registration
into creating the entry and activating it, leaving room to
configure it in between - which is what a ``B`` entry that
binds interpreters needs; see the bpf section below. The flag
is spent on the registration and is not read back: what an
entry file reports afterwards is whether it is enabled.
There are some restrictions:
@@ -224,8 +233,10 @@ handler can decide them differently for each binary it handles:
``PT_INTERP`` and runs the binary as a fully native exec (the ``L``
flag). It excludes the other flags and a staged interpreter argument.
Because these are program choices, a ``B`` entry carries no flags in the
register string; ``F`` (pre-open a fixed interpreter) has no meaning for it.
Because these are program choices, a ``B`` entry carries no invocation
flags in the register string; ``F`` (pre-open a fixed interpreter) has no
meaning for it. The registration directive ``D`` is the exception: it
decides how the entry starts out, not how the interpreter is invoked.
A handler is looked up only in the user namespace the struct_ops map was
registered in. Handlers are not inherited, so an entry can only reference a