[PATCH bpf-next v3 0/4] Add support to emit verifier warnings

Kumar Kartikeya Dwivedi
2026-04-18T17:17:03Z

Currently, there are only two ways of communicating information to the user
when a program is verified, success or failure with a verbose verifier log.
Some information is meant to be more discretionary, e.g. warning about use of
kfuncs that are deprecated, and may be removed in future kernel releases.

An example is shown below.

$ ./test_progs -t kfunc_implicit_args/test_kfunc_implicit_arg_legacy_impl -v
...
2: (b7) r3 = 0 ; R3=0
3: (85) call bpf_kfunc_implicit_arg_legacy_impl#114683
kfunc_implicit_args.c:40 (insn #3) uses deprecated kfunc bpf_kfunc_implicit_arg_legacy_impl(), which will be removed.
Switch to kfunc bpf_kfunc_implicit_arg_legacy() instead.
For older kernels, choose the correct kfunc using bpf_ksym_exists().
4: R0=scalar()
4: (95) exit
...

With verbose logging, the deprecation warnings are printed inline in the
verifier log. When no log level is chosen, the warning is printed in a
block, as follows:

libbpf: prog 'test_kfunc_implicit_arg_legacy_impl': -- BEGIN PROG LOAD WARNINGS --
kfunc_implicit_args.c:40 (insn #3) uses deprecated kfunc bpf_kfunc_implicit_arg_legacy_impl(), which will be removed.
Switch to kfunc bpf_kfunc_implicit_arg_legacy() instead.
For older kernels, choose the correct kfunc using bpf_ksym_exists().
-- END PROG LOAD WARNINGS --

Changelog:
----------
v2 -> v3
v2: https://lore.kernel.org/bpf/20260408021359.3786905-1-memxor@gmail.com

* Use log_level BPF_LOG_LEVEL_WARN bit to request warning logs.
* Add a selftest.
* Drop KF_DEPRECATED.

v1 -> v2
v1: https://lore.kernel.org/bpf/20260329212534.3270005-1-memxor@gmail.com

* Use BTF tags and kfunc flags for deprecation warnings.
* Reuse verifier log to deliver log messages. (Alexei)
* Make the mechanism declarative without kfunc lists in verifier.c.

Kumar Kartikeya Dwivedi (4):
bpf: Add support for verifier warning messages
bpf: Introduce __bpf_kfunc_mark_deprecated annotation
libbpf: Request verifier warnings for object loads
selftests/bpf: Test verifier warning logging

Documentation/bpf/kfuncs.rst | 40 ++++----
include/linux/bpf_verifier.h | 12 ++-
include/linux/btf.h | 5 +
include/linux/compiler_types.h | 14 ++-
kernel/bpf/helpers.c | 8 ++
kernel/bpf/log.c | 2 +-
kernel/bpf/verifier.c | 62 ++++++++++++-
tools/lib/bpf/features.c | 56 +++++++++++
tools/lib/bpf/libbpf.c | 25 +++--
tools/lib/bpf/libbpf.h | 7 +-
tools/lib/bpf/libbpf_internal.h | 2 +
.../bpf/prog_tests/kfunc_implicit_args.c | 92 +++++++++++++++++++
.../selftests/bpf/test_kmods/bpf_testmod.c | 1 +
13 files changed, 291 insertions(+), 35 deletions(-)

base-commit: eb0d6d97c27c29cd7392c8fd74f46edf7dff7ec2
--
2.52.0