linux-kernel - Re: [PATCH bpf-next v5 0/6] Add a snprintf eBPF helper

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Date:   Mon, 19 Apr 2021 12:33:54 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Florent Revest <revest@...omium.org>
Cc:     bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Yonghong Song <yhs@...com>, KP Singh <kpsingh@...nel.org>,
        Brendan Jackman <jackmanb@...omium.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next v5 0/6] Add a snprintf eBPF helper

On Mon, Apr 19, 2021 at 8:52 AM Florent Revest <revest@...omium.org> wrote:
>
> 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/20201126165748.1748417-1-revest@google.com/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/20210310015455.1095207-1-revest@chromium.org/T/#u
>
> ---
> Changes in v5:
> - Fixed the bpf_printf_buf_used counter logic in try_get_fmt_tmp_buf
> - Added a couple of extra incorrect specifiers tests
> - Call test_snprintf_single__destroy unconditionally
> - Fixed a C++-style comment
>
> ---
> Changes in v4:
> - Moved bpf_snprintf, bpf_printf_prepare and bpf_printf_cleanup to
>   kernel/bpf/helpers.c so that they get built without CONFIG_BPF_EVENTS
> - Added negative test cases (various invalid format strings)
> - Renamed put_fmt_tmp_buf() as bpf_printf_cleanup()
> - Fixed a mistake that caused temporary buffers to be unconditionally
>   freed in bpf_printf_prepare
> - Fixed a mistake that caused missing 0 character to be ignored
> - Fixed a warning about integer to pointer conversion
> - Misc cleanups
>
> ---
> 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                           |  22 ++
>  include/uapi/linux/bpf.h                      |  28 ++
>  kernel/bpf/helpers.c                          | 306 ++++++++++++++
>  kernel/bpf/verifier.c                         |  82 ++++
>  kernel/trace/bpf_trace.c                      | 373 ++----------------
>  tools/include/uapi/linux/bpf.h                |  28 ++
>  tools/lib/bpf/bpf_tracing.h                   |  58 ++-
>  .../selftests/bpf/prog_tests/snprintf.c       | 125 ++++++
>  .../selftests/bpf/progs/test_snprintf.c       |  73 ++++
>  .../bpf/progs/test_snprintf_single.c          |  20 +
>  10 files changed, 770 insertions(+), 345 deletions(-)
>  create mode 100644 tools/testing/selftests/bpf/prog_tests/snprintf.c
>  create mode 100644 tools/testing/selftests/bpf/progs/test_snprintf.c
>  create mode 100644 tools/testing/selftests/bpf/progs/test_snprintf_single.c
>
> --
> 2.31.1.368.gbe11c130af-goog
>

Looks great, thank you!

For the series:

Acked-by: Andrii Nakryiko <andrii@...nel.org>