[PATCH bpf-next v2 0/6] Add support to emit verifier warnings

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

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 --

The set was tested with and without kernel changes applied, so that we
could conclude that libbpf feature detection works correctly and
backwards compatibility is not violated on older kernels. Still, it
would be helpful if other reviewers could recheck all assumptions and
make sure I didn't miss any corner case.

One case I need more opinions on: log_buf.c passes log_buf with
log_level=0 and expects it to be empty, with newer libbpf that won't be
true on older kernels anymore. I don't think this is something that
needs to be addressed, but explicitly noting it here since I saw that
test fail when testing new libbpf on older kernels. The kernel will now
populate such log_buf will "processed N insns ..." log since on failure
libbpf will retry with log_level=1, so unless applications are relying
on log_buf to be empty for correctness it should be ok.

Changelog:
----------
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 (6):
  bpf: Add support for verifier warning messages
  bpf: Extract bpf_get_linfo_file_line
  bpf: Make find_linfo widely available
  bpf: Use KF_DEPRECATED to emit verifier warnings
  bpf: Add __bpf_kfunc_replacement() annotation
  libbpf: Flush verifier warning messages by default

 include/linux/bpf.h                           |  3 +
 include/linux/bpf_verifier.h                  |  1 +
 include/linux/btf.h                           |  6 ++
 include/linux/compiler_types.h                | 14 +++-
 kernel/bpf/core.c                             | 66 ++++++++++++++++---
 kernel/bpf/helpers.c                          | 24 ++++---
 kernel/bpf/log.c                              | 49 +-------------
 kernel/bpf/verifier.c                         | 60 ++++++++++++++++-
 tools/lib/bpf/bpf.c                           |  5 +-
 tools/lib/bpf/features.c                      | 56 ++++++++++++++++
 tools/lib/bpf/libbpf.c                        | 20 ++++--
 tools/lib/bpf/libbpf_internal.h               |  2 +
 .../selftests/bpf/test_kmods/bpf_testmod.c    |  3 +-
 13 files changed, 233 insertions(+), 76 deletions(-)


base-commit: 5c662b1c1789f51f79ee9c648681abc8410dfa81
-- 
2.52.0