With default_permissions (the mount default) the kernel enforces unix
permission bits from the getattr/lookup attributes before it ever calls
Open, Create, or Mknod. The mount was re-checking permissions in
AcquireHandle and createRegularFile anyway, which duplicated the kernel's
work and kept the supplementary-group lookup on the per-file hot path.
Gate only the mode-bit access check on default_permissions being off, so
a non-root copy does no permission work on open/create. createRegularFile
still loads the parent to validate it exists, since the create RPC skips
the filer-side parent check. With default_permissions off the mount
remains the sole enforcer, so the full check still runs.
Chris Lu
5112da98a2