landlock: Add landlock_deny_access_fs and landlock_deny_access_net

Add per-type tracepoints emitted from landlock_log_denial() when an access is denied: landlock_deny_access_fs for filesystem denials and landlock_deny_access_net for network denials. They use the "deny_" prefix (rather than "check_") to mark that they fire only on a denial, and they complement the check_rule events by making the denial-by-absence case explicit (when no rule matches, no check_rule event fires). Unlike the audit records, these events fire regardless of the audit configuration and the domain's log flags: the user's "disable logging" intent applies to audit records, not to kernel tracing. The logged field records whether the domain's log policy would submit the denial to audit; it is the decision computed once by landlock_log_denial() and passed to both the audit and the tracing emitter, so a stateless ftrace filter can select the audit-visible denials with logged==1. TP_PROTO passes the denying hierarchy node, not the task's current domain, so domain_id reports the specific node that blocked the access, matching audit record semantics. (check_rule instead passes the current domain, which it needs to size its per-layer array.) same_exec is also passed explicitly because it is computed from the credential bitmask and is not derivable from the hierarchy pointer alone. The denial field is named blockers to match the audit record field. The filesystem path comes from the request's audit data. Its type selects which union member holds the object, exactly as dump_common_audit_data() selects it (a path, a file's path, an ioctl op's path, or a bare dentry); reading the wrong member would dereference garbage, so every reachable type has an explicit case and an unexpected one is flagged with WARN_ONCE() instead of misread. Path-backed types resolve via d_absolute_path() (as landlock_add_rule_fs does) and the bare-dentry case via dentry_path_raw(). The inode number is read defensively. A filesystem denial can carry a negative dentry (no backing inode), for example a denied creation, so the event mirrors the guard in dump_common_audit_data() and reports inode 0 rather than dereferencing a NULL inode. The sibling fs tracepoints do not need the guard: a dentry that matches a rule during an access check, or one opened to add a rule, always has a backing inode. Landlock tracepoints are reachable by unprivileged sandboxees, so a denial on a negative dentry with the event enabled must not fault the kernel.

MSMickaël Salaün committed 3 months ago