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

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Date:   Tue, 20 Apr 2021 14:02:29 +0200
From:   Florent Revest <revest@...omium.org>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
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 9:34 PM Andrii Nakryiko
<andrii.nakryiko@...il.com> wrote:
>
> 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>

Thank you for the all the fast and in-depth reviews Andrii! :)