[PATCH 3/6] seccomp: Implement constant action bitmaps

[David Laight]

From: Jann Horn
> Sent: 24 September 2020 13:29
...
> I think our goal here should be that if a syscall is always allowed,
> seccomp should execute the smallest amount of instructions we can get
> away with, and touch the smallest amount of memory possible (and
> preferably that memory should be shared between threads). The bitmap
> fastpath should probably also avoid populate_seccomp_data().

If most syscalls are expected to be allowed then an initial:
	if (global_mask & (1u << (syscall_number & 63))
test can be used to skip any further lookups.

Although ISTR someone suggesting that the global_mask should
be per-cpu because even shared read-only cache lines were
expensive on some architecture.

	David

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