Date:   Mon, 13 Sep 2021 22:52:14 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Dave Marchevsky <davemarchevsky@...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>, Networking <netdev@...r.kernel.org>
Subject: Re: [PATCH v5 bpf-next 0/9] bpf: implement variadic printk helper

On Sun, Sep 12, 2021 at 8:56 PM Dave Marchevsky <davemarchevsky@...com> wrote:
>
> This series introduces a new helper, bpf_trace_vprintk, which functions
> like bpf_trace_printk but supports > 3 arguments via a pseudo-vararg u64
> array. The bpf_printk libbpf convenience macro is modified to use
> bpf_trace_vprintk when > 3 varargs are passed, otherwise the previous
> behavior - using bpf_trace_printk - is retained.
>
> Helper functions and macros added during the implementation of
> bpf_seq_printf and bpf_snprintf do most of the heavy lifting for
> bpf_trace_vprintk. There's no novel format string wrangling here.
>
> Usecase here is straightforward: Giving BPF program writers a more
> powerful printk will ease development of BPF programs, particularly
> during debugging and testing, where printk tends to be used.
>
> This feature was proposed by Andrii in libbpf mirror's issue tracker
> [1].
>
> [1] https://github.com/libbpf/libbpf/issues/315
>
> v4 -> v5:
>
> * patch 8: added test for "%pS" format string w/ NULL fmt arg [Daniel]
> * patch 8: dmesg -> /sys/kernel/debug/tracing/trace_pipe in commit message [Andrii]
> * patch 9: squash into patch 8, remove the added test in favor of just bpf_printk'ing in patch 8's test [Andrii]
>     * migrate comment to /* */
> * header comments improved$
>     * uapi/linux/bpf.h: u64 -> long return type [Daniel]
>     * uapi/linux/bpf.h: function description explains benefit of bpf_trace_vprintk over bpf_trace_printk [Daniel]
>     * uapi/linux/bpf.h: added patch explaining that data_len should be a multiple of 8 in bpf_seq_printf, bpf_snprintf descriptions [Daniel]
>     * tools/lib/bpf/bpf_helpers.h: move comment to new bpf_printk [Andrii]
> * rebase
>
> v3 -> v4:
> * Add patch 2, which migrates reference_tracking prog_test away from
>   bpf_program__load. Could be placed a bit later in the series, but
>   wanted to keep the actual vprintk-related patches contiguous
> * Add patch 9, which adds a program w/ 0 fmt arg bpf_printk to vprintk
>   test
> * bpf_printk convenience macro isn't multiline anymore, so simplify [Andrii]
> * Add some comments to ___bpf_pick_printk to make it more obvious when
>   implementation switches from printk to vprintk [Andrii]
> * BPF_PRINTK_FMT_TYPE -> BPF_PRINTK_FMT_MOD for 'static const' fmt string
>   in printk wrapper macro [Andrii]
>     * checkpatch.pl doesn't like this, says "Macros with complex values
>       should be enclosed in parentheses". Strange that it didn't have similar
>       complaints about v3's BPF_PRINTK_FMT_TYPE. Regardless, IMO the complaint
>       is not highlighting a real issue in the case of this macro.
> * Fix alignment of __bpf_vprintk and __bpf_pick_printk [Andrii]
> * rebase
>
> v2 -> v3:
> * Clean up patch 3's commit message [Alexei]
> * Add patch 4, which modifies __bpf_printk to use 'static const char' to
>   store fmt string with fallback for older kernels [Andrii]
> * rebase
>
> v1 -> v2:
>
> * Naming conversation seems to have gone in favor of keeping
>   bpf_trace_vprintk, names are unchanged
>
> * Patch 3 now modifies bpf_printk convenience macro to choose between
>   __bpf_printk and __bpf_vprintk 'implementation' macros based on arg
>   count. __bpf_vprintk is a renaming of bpf_vprintk convenience macro
>   from v1, __bpf_printk is the existing bpf_printk implementation.
>
>   This patch could use some scrutiny as I think current implementation
>   may regress developer experience in a specific case, turning a
>   compile-time error into a load-time error. Unclear to me how
>   common the case is, or whether the macro magic I chose is ideal.
>
> * char ___fmt[] to static const char ___fmt[] change was not done,
>   wanted to leave __bpf_printk 'implementation' macro unchanged for v2
>   to ease discussion of above point
>
> * Removed __always_inline from __set_printk_clr_event [Andrii]
> * Simplified bpf_trace_printk docstring to refer to other functions
>   instead of copy/pasting and avoid specifying 12 vararg limit [Andrii]
> * Migrated trace_printk selftest to use ASSERT_ instead of CHECK
>   * Adds new patch 5, previous patch 5 is now 6
> * Migrated trace_vprintk selftest to use ASSERT_ instead of CHECK,
>   open_and_load instead of separate open, load [Andrii]
> * Patch 2's commit message now correctly mentions trace_pipe instead of
>   dmesg [Andrii]
>
> Dave Marchevsky (9):
>   bpf: merge printk and seq_printf VARARG max macros
>   selftests/bpf: stop using bpf_program__load
>   bpf: add bpf_trace_vprintk helper
>   libbpf: Modify bpf_printk to choose helper based on arg count
>   libbpf: use static const fmt string in __bpf_printk
>   bpftool: only probe trace_vprintk feature in 'full' mode
>   selftests/bpf: Migrate prog_tests/trace_printk CHECKs to ASSERTs
>   selftests/bpf: add trace_vprintk test prog
>   bpf: clarify data_len param in bpf_snprintf and bpf_seq_printf
>     comments
>
>  include/linux/bpf.h                           |  3 +
>  include/uapi/linux/bpf.h                      | 16 ++++-
>  kernel/bpf/core.c                             |  5 ++
>  kernel/bpf/helpers.c                          |  6 +-
>  kernel/trace/bpf_trace.c                      | 54 ++++++++++++++-
>  tools/bpf/bpftool/feature.c                   |  1 +
>  tools/include/uapi/linux/bpf.h                | 16 ++++-
>  tools/lib/bpf/bpf_helpers.h                   | 51 +++++++++++---
>  tools/testing/selftests/bpf/Makefile          |  3 +-
>  .../bpf/prog_tests/reference_tracking.c       | 39 ++++++++---
>  .../selftests/bpf/prog_tests/trace_printk.c   | 24 +++----
>  .../selftests/bpf/prog_tests/trace_vprintk.c  | 68 +++++++++++++++++++
>  .../selftests/bpf/progs/trace_vprintk.c       | 33 +++++++++
>  tools/testing/selftests/bpf/test_bpftool.py   | 22 +++---
>  14 files changed, 286 insertions(+), 55 deletions(-)
>  create mode 100644 tools/testing/selftests/bpf/prog_tests/trace_vprintk.c
>  create mode 100644 tools/testing/selftests/bpf/progs/trace_vprintk.c
>
> --
> 2.30.2
>

This is a great feature and nice and clean implementation, thanks.
Unfortunately it's conflicting with recently landed
bpf_get_branch_snapshot() series, so you'll have to rebase and
resubmit.

For the series:

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

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