Add a snprintf eBPF helper

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-reve...

- 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-reve...

Florent Revest (5):
  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                           |   5 +
 include/uapi/linux/bpf.h                      |  28 +++
 kernel/bpf/verifier.c                         | 178 ++++++++++++++++++
 kernel/trace/bpf_trace.c                      | 110 +++++++++++
 tools/include/uapi/linux/bpf.h                |  28 +++
 tools/lib/bpf/bpf_tracing.h                   |  45 ++++-
 .../selftests/bpf/prog_tests/snprintf.c       |  71 +++++++
 .../selftests/bpf/progs/test_snprintf.c       |  71 +++++++
 8 files changed, 535 insertions(+), 1 deletion(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/snprintf.c
 create mode 100644 tools/testing/selftests/bpf/progs/test_snprintf.c

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2.30.1.766.gb4fecdf3b7-goog