Now that we have the __bpf_kfunc tag, we should use add it to all
existing kfuncs to ensure that they'll never be elided in LTO builds.

Signed-off-by: David Vernet <void@manifault.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/bpf/20230201173016.342758-4-void@manifault.com

The first element of a struct bpf_cpumask is a cpumask_t. This is done
to allow struct bpf_cpumask to be cast to a struct cpumask. If this
element were ever moved to another field, any BPF program passing a
struct bpf_cpumask * to a kfunc expecting a const struct cpumask * would
immediately fail to load. Add a build-time assertion so this is
assumption is captured and verified.

Signed-off-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20230128141537.100777-1-void@manifault.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Now that we've added a series of new cpumask kfuncs, we should document
them so users can easily use them. This patch adds a new cpumasks.rst
file to document them.

Signed-off-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20230125143816.721952-6-void@manifault.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Certain programs may wish to be able to query cpumasks. For example, if
a program that is tracing percpu operations wishes to track which tasks
end up running on which CPUs, it could be useful to associate that with
the tasks' cpumasks. Similarly, programs tracking NUMA allocations, CPU
scheduling domains, etc, could potentially benefit from being able to
see which CPUs a task could be migrated to.

This patch enables these types of use cases by introducing a series of
bpf_cpumask_* kfuncs. Amongst these kfuncs, there are two separate
"classes" of operations:

1. kfuncs which allow the caller to allocate and mutate their own
   cpumask kptrs in the form of a struct bpf_cpumask * object. Such
   kfuncs include e.g. bpf_cpumask_create() to allocate the cpumask, and
   bpf_cpumask_or() to mutate it. "Regular" cpumasks such as p->cpus_ptr
   may not be passed to these kfuncs, and the verifier will ensure this
   is the case by comparing BTF IDs.

2. Read-only operations which operate on const struct cpumask *
   arguments. For example, bpf_cpumask_test_cpu(), which tests whether a
   CPU is set in the cpumask. Any trusted struct cpumask * or struct
   bpf_cpumask * may be passed to these kfuncs. The verifier allows
   struct bpf_cpumask * even though the kfunc is defined with struct
   cpumask * because the first element of a struct bpf_cpumask is a
   cpumask_t, so it is safe to cast.

A follow-on patch will add selftests which validate these kfuncs, and
another will document them.

Signed-off-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20230125143816.721952-3-void@manifault.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>