binfmt_misc: let a register string create an entry disabled

An entry is matchable as soon as it is registered. create_entry() sets
the enabled bit for every type and add_entry() links it straight into the
instance, so everything an entry needs has to fit in the write that
creates it.

Add a 'D' flag. The entry is created disabled and has to be enabled by
writing '1' to its entry file before it can match anything. That splits a
registration into create and activate, which a later patch uses to
configure an entry beyond what one register string can carry. It is
useful on its own too. Entries can be staged without dispatching the
moment they are written.

A staged entry stays out of the search list entirely. add_entry() only
hashes an entry that is born matchable, and the first '1' written to
the entry file hashes a staged one, which takes its place in the search
order at that point. The rcu insertion publishes the fully configured
entry, so the exec side keeps the plain enabled test it always had.

Removal cannot rely on the search list anymore. Whether an entry was
already removed is now decided by its dentry, '-1' to the status file
walks the directory instead of the list so staged entries do not
survive it, and a '1' through a file handle held across a removal
publishes nothing.

'D' is consumed at registration and not recorded. What matters afterwards
is whether the entry is enabled, and the entry file already reports that.

A 'B' entry's flags field had to be empty so far because every flag it
could name shaped the invocation, which a bpf handler picks per exec with
bpf_binprm_set_flags(). 'D' shapes the registration instead. So the rule
becomes what it always meant: a 'B' entry carries no invocation flags,
and 'D' composes.

Link: https://patch.msgid.link/20260730-work-binfmt_misc-preopen-v1-1-4a0b0da71f16@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

0 files changed, 0 insertions, 0 deletions