binfmt_misc: add the 'L' loader substitution flag

Add the first activation of the PT_INTERP substitution machinery. A
static entry registered with the new 'L' flag no longer runs the
registered interpreter with the binary as payload. It stashes the
interpreter as bprm->loader and declines the match with -ENOEXEC. The
format search continues in the same round. binfmt_elf claims the binary
as a fully native exec and substitutes the stashed file for the binary's
PT_INTERP.

'L' rejects every classic-dispatch flag at registration. 'T', 'P' and
'O' have nothing to act on (no argv splice, no execfd) and 'C' is
subsumed (credentials derive from the binary natively). 'F' composes and
is valuable: with it the substitute is pre-opened at registration time
and immune to mount namespace changes. Without it the substitute is
opened at exec time in the exec'ing task's context, so 'L' joins 'C' in
the requirement that the interpreter be named by an absolute path. As
with 'C', only trusted interpreters should be registered. The
substituted loader runs with credentials derived from the binary.

Like the other flag characters 'L' cannot be used as the field
delimiter. The flag scan would run off the registration buffer.

The interpreter open is shared with the classic path via the
entry_open_interpreter() helper. An open error fails the exec. Map
-ENOEXEC to -EACCES to avoid letting the binary run with its own
PT_INTERP.

Link: https://patch.msgid.link/20260721-work-bpf-binfmt_misc-ptinterp-v2-18-e57866e4ae0f@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

0 files changed, 0 insertions, 0 deletions