[bpf-next,v3,0/6] Add a snprintf eBPF helper

Message

Florent Revest April 12, 2021, 3:37 p.m. UTC

We have a usecase where we want to audit symbol names (if available) in
callback registration hooks. (ex: fentry/nf_register_net_hook)

A few months back, I proposed a bpf_kallsyms_lookup series but it was
decided in the reviews that a more generic helper, bpf_snprintf, would
be more useful.

This series implements the helper according to the feedback received in
https://lore.kernel.org/bpf/[e-mail]/T/#u

- A new arg type guarantees the NULL-termination of string arguments and
  lets us pass format strings in only one arg
- A new helper is implemented using that guarantee. Because the format
  string is known at verification time, the format string validation is
  done by the verifier
- To implement a series of tests for bpf_snprintf, the logic for
  marshalling variadic args in a fixed-size array is reworked as per:
https://lore.kernel.org/bpf/[e-mail]/T/#u

---
Changes in v3:
- Simplified temporary buffer acquisition with try_get_fmt_tmp_buf()
- Made zero-termination check more consistent
- Allowed NULL output_buffer
- Simplified the BPF_CAST_FMT_ARG macro
- Three new test cases: number padding, simple string with no arg and
  string length extraction only with a NULL output buffer
- Clarified helper's description for edge cases (eg: str_size == 0)
- Lots of cosmetic changes

---
Changes in v2:
- Extracted the format validation/argument sanitization in a generic way
  for all printf-like helpers.
- bpf_snprintf's str_size can now be 0
- bpf_snprintf is now exposed to all BPF program types
- We now preempt_disable when using a per-cpu temporary buffer
- Addressed a few cosmetic changes

Florent Revest (6):
  bpf: Factorize bpf_trace_printk and bpf_seq_printf
  bpf: Add a ARG_PTR_TO_CONST_STR argument type
  bpf: Add a bpf_snprintf helper
  libbpf: Initialize the bpf_seq_printf parameters array field by field
  libbpf: Introduce a BPF_SNPRINTF helper macro
  selftests/bpf: Add a series of tests for bpf_snprintf

 include/linux/bpf.h                           |   7 +
 include/uapi/linux/bpf.h                      |  28 +
 kernel/bpf/helpers.c                          |   2 +
 kernel/bpf/verifier.c                         |  82 +++
 kernel/trace/bpf_trace.c                      | 579 +++++++++---------
 tools/include/uapi/linux/bpf.h                |  28 +
 tools/lib/bpf/bpf_tracing.h                   |  58 +-
 .../selftests/bpf/prog_tests/snprintf.c       |  81 +++
 .../selftests/bpf/progs/test_snprintf.c       |  74 +++
 9 files changed, 647 insertions(+), 292 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/snprintf.c
 create mode 100644 tools/testing/selftests/bpf/progs/test_snprintf.c