Merge branch 'libbpf-add-bpf_program__clone-for-individual-program-loading'

Mykyta Yatsenko says:

====================
libbpf: Add bpf_program__clone() for individual program loading

This series adds bpf_program__clone() to libbpf and converts veristat to
use it, replacing the costly per-program object re-opening pattern.

veristat needs to load each BPF program in isolation to collect
per-program verification statistics. Previously it achieved this by
opening a fresh bpf_object for every program, disabling autoload on all
but the target, and loading the whole object. For object files with many
programs this meant repeating ELF parsing and BTF processing N times.

Patch 1 introduces bpf_program__clone(), which loads a single program
from a prepared object into the kernel and returns an fd owned by the
caller. It populates load parameters from the prepared object and lets
callers override any field via bpf_prog_load_opts. Fields written by the
prog_prepare_load_fn callback (expected_attach_type, attach_btf_id,
attach_btf_obj_fd) are seeded from prog/obj defaults before the
callback, then overridden with caller opts after, so explicit values
always win.

Patch 2 converts veristat to prepare the object once and clone each
program individually, eliminating redundant work.

Patch 3 adds a selftest verifying that caller-provided attach_btf_id
overrides are respected by bpf_program__clone().

Performance
Tested on selftests: 918 objects, ~4270 programs:
 - Wall time:   36.88s -> 23.18s (37% faster)
 - User time:   20.80s -> 16.07s (23% faster)
 - Kernel time: 12.07s ->  6.06s (50% faster)

Per-program loading also improves coverage: 83 programs that previously
failed now succeed.

Known regression:
 - Program-containing maps (PROG_ARRAY, DEVMAP, CPUMAP) track owner
   program type. Programs with incompatible attributes loaded against
   a shared map will be rejected. This is expected kernel behavior.

Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
---
Changes in v5:
- Fix overriding of the attach_btf_id, attach_btf_fd, etc: the override
provided by the caller is applied after prog_prepare_load_fn().
- Added selftest to verify attach_btf_id override works as expected.
- Link to v4: https://lore.kernel.org/all/20260316-veristat_prepare-v3-0-94e5691e0494@meta.com/

Changes in v4:
- Replace OPTS_SET() with direct struct assignment for local
  bpf_prog_load_opts in bpf_program__clone() (libbpf.c)
- Remove unnecessary pattr pointer indirection (libbpf.c)
- Separate input and output fields in bpf_program__clone(): input
  fields (prog_flags, fd_array, etc.) are merged from caller opts
  before the callback; output fields (expected_attach_type,
  attach_btf_id, attach_btf_obj_fd) are initialized from prog/obj
  defaults for the callback, then overridden with caller opts after,
  so explicit caller values always win (libbpf.c)
- Add selftest for attach_btf_id override
- Link to v3: https://lore.kernel.org/r/20260206-veristat_prepare-a4a041873c53-v3@meta.com

Changes in v3:
- Clone fd_array_cnt in bpf_object__clone()
- In veristat do not fail if bpf_object__prepare() fails,
continue per-program processing to produce per program output
- Link to v2: https://lore.kernel.org/r/20260220-veristat_prepare-v2-0-15bff49022a7@meta.com

Changes in v2:
 - Removed map cloning entirely (libbpf.c)
 - Renamed bpf_prog_clone() -> bpf_program__clone()
 - Removed unnecessary obj NULL check (libbpf.c)
 - Fixed opts handling — no longer mutates caller's opts (libbpf.c)
 - Link to v1: https://lore.kernel.org/all/20260212-veristat_prepare-v1-0-c351023fb0db@meta.com/

---
====================

Link: https://patch.msgid.link/20260317-veristat_prepare-v4-0-74193d4cc9d9@meta.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>