Merge branch 'bpf-token-support-in-libbpf-s-bpf-object'

Andrii Nakryiko says:

====================
BPF token support in libbpf's BPF object

Add fuller support for BPF token in high-level BPF object APIs. This is the
most frequently used way to work with BPF using libbpf, so supporting BPF
token there is critical.

Patch #1 is improving kernel-side BPF_TOKEN_CREATE behavior by rejecting to
create "empty" BPF token with no delegation. This seems like saner behavior
which also makes libbpf's caching better overall. If we ever want to create
BPF token with no delegate_xxx options set on BPF FS, we can use a new flag to
enable that.

Patches #2-#5 refactor libbpf internals, mostly feature detection code, to
prepare it from BPF token FD.

Patch #6 adds options to pass BPF token into BPF object open options. It also
adds implicit BPF token creation logic to BPF object load step, even without
any explicit involvement of the user. If the environment is setup properly,
BPF token will be created transparently and used implicitly. This allows for
all existing application to gain BPF token support by just linking with
latest version of libbpf library. No source code modifications are required.
All that under assumption that privileged container management agent properly
set up default BPF FS instance at /sys/bpf/fs to allow BPF token creation.

Patches #7-#8 adds more selftests, validating BPF object APIs work as expected
under unprivileged user namespaced conditions in the presence of BPF token.

Patch #9 extends libbpf with LIBBPF_BPF_TOKEN_PATH envvar knowledge, which can
be used to override custom BPF FS location used for implicit BPF token
creation logic without needing to adjust application code. This allows admins
or container managers to mount BPF token-enabled BPF FS at non-standard
location without the need to coordinate with applications.
LIBBPF_BPF_TOKEN_PATH can also be used to disable BPF token implicit creation
by setting it to an empty value. Patch #10 tests this new envvar functionality.

v2->v3:
  - move some stray feature cache refactorings into patch #4 (Alexei);
  - add LIBBPF_BPF_TOKEN_PATH envvar support (Alexei);
v1->v2:
  - remove minor code redundancies (Eduard, John);
  - add acks and rebase.
====================

Link: https://lore.kernel.org/r/20231213190842.3844987-1-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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