bpf: implement variadic printk helper

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. A libbpf convienience macro, bpf_vprintk, is added to support
true vararg calling style.

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.

Hypothetically libbpf's bpf_printk convenience macro could be modified
to use bpf_trace_vprintk under the hood. This patchset does not attempt
to do this, though, nor am I confident that it's desired.

This feature was proposed by Andrii in libbpf mirror's issue tracker
[1].

[1] https://github.com/libbpf/libbpf/issues/315

Dave Marchevsky (5):
  bpf: merge printk and seq_printf VARARG max macros
  bpf: add bpf_trace_vprintk helper
  libbpf: Add bpf_vprintk convenience macro
  bpftool: only probe trace_vprintk feature in 'full' mode
  selftests/bpf: add trace_vprintk test prog

 include/linux/bpf.h                           |  3 +
 include/uapi/linux/bpf.h                      | 23 ++++++
 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                | 23 ++++++
 tools/lib/bpf/bpf_helpers.h                   | 18 +++++
 tools/testing/selftests/bpf/Makefile          |  3 +-
 .../selftests/bpf/prog_tests/trace_vprintk.c  | 75 +++++++++++++++++++
 .../selftests/bpf/progs/trace_vprintk.c       | 25 +++++++
 tools/testing/selftests/bpf/test_bpftool.py   | 22 +++---
 12 files changed, 238 insertions(+), 20 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/trace_vprintk.c
 create mode 100644 tools/testing/selftests/bpf/progs/trace_vprintk.c

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2.30.2