[PATCH bpf-next v3 0/4] bpf: Improve error reporting for freplace attachment failure

[Leon Hwang <leon.hwang@linux.dev>]

This patch series improves error reporting for BPF_LINK_CREATE when
attaching freplace programs. Inspired by the discussion in
"[PATCH bpf-next v2] bpf: Add bpf_check_attach_target_with_klog method to
output failure logs to kernel"[0], this series enhances that freplace
attachment failure returns meaningful logs to userspace, aiding debugging.

For example, when attempting to attach a freplace program to a static
function:

libbpf: prog 'new_test_pkt_access': failed to attach to freplace: -EINVAL
libbpf: prog 'new_test_pkt_access': attach log: subprog_tail() is not a global function

Patch breakdown:
1. bpf, verifier: Add missing newline of bpf_log in bpf_check_attach_target
    * Add the missing newline in
      bpf_log(log, "Target program bound device mismatch").
2. bpf: Report log of freplace attach failure
    * Extends BPF_LINK_CREATE to report detailed error logs.
3. bpf, libbpf: Capture log of freplace attach failure
    * Modifies libbpf to capture and print attachment logs.
4. selftests/bpf: Add a test case for freplace attachment failure logging
    * Introduces a selftest to validate error reporting.

Links:
[0] https://lore.kernel.org/bpf/CAEf4BzbbyojuFSS7xQ3+jZb=dHzOaZfMbtT+WnypW2LPwOUwRw@mail.gmail.com/

Changes:
v2: https://lore.kernel.org/bpf/20240725051511.57112-1-me@manjusaka.me/
v2 -> v3:
  * Address comment from Andrii:
    * Report back the reason for declining freplace attachment instead of
      logging in dmesg.

Leon Hwang (4):
  bpf, verifier: Add missing newline of bpf_log in
    bpf_check_attach_target
  bpf: Improve error reporting for freplace attachment failure
  bpf, libbpf: Capture and log freplace attachment failure
  selftests/bpf: Add test case for freplace attachment failure logging

 include/uapi/linux/bpf.h                      |  2 +
 kernel/bpf/syscall.c                          | 51 ++++++++++++++++---
 kernel/bpf/verifier.c                         |  2 +-
 tools/include/uapi/linux/bpf.h                |  2 +
 tools/lib/bpf/bpf.c                           |  6 ++-
 tools/lib/bpf/bpf.h                           |  2 +
 tools/lib/bpf/libbpf.c                        | 14 ++++-
 .../bpf/prog_tests/tracing_link_attach_log.c  | 42 +++++++++++++++
 8 files changed, 109 insertions(+), 12 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/tracing_link_attach_log.c

-- 
2.47.1