Chris Mason
99aff23783
apply_syscall_filter() unconditionally inserts the write() syscall
into c->syscall_filter when exec_fd or handoff_timestamp_fd is in
use, so the parent can receive the exec status / handoff timestamp
from the child. When the unit configured a positive
SystemCallFilter= allow-list that deliberately omits write(), the
resulting widening of the operator's policy happens silently with
no trace in the journal.
Emit a log_debug() before the seccomp_filter_set_add_by_name() call
when syscall_allow_list is true, so the widening is at least
observable to operators inspecting the unit's debug log.
While here, document that mutating c->syscall_filter through a
'const ExecContext *c' is intentional: apply_syscall_filter() runs
only in the post-fork child, which owns a private copy of the
address space, so the hashmap change is never observed by the
manager.
No functional change for the allow-list itself; write() is still
added exactly as before.
Fixes: 84b79215cc ("core: do not filter out write() if required in the very late stage")
Assisted-by: kres (claude-opus-4-7)
Signed-off-by: Chris Mason <clm@meta.com>
System and Service Manager
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