Re: [RFC PATCH bpf-next v2 7/9] tools: bpftool: add C-style "#define" output for probes

On 12/20/2018 01:24 PM, Quentin Monnet wrote:

Make bpftool able to dump a subset of the parameters collected by
probing the system as a listing of C-style #define macros, so that
external projects can reuse the result of this probing and build
BPF-based project in accordance with the features available on the
system.

The new "macros" keyword is used to select this output. An additional
"prefix" keyword is added so that users can select a custom prefix for
macro names, in order to avoid any namespace conflict.

Sample output:

    # bpftool feature probe kernel macros prefix FOO_
    /*** System call availability ***/
    #define FOO_HAVE_BPF_SYSCALL

    /*** eBPF program types ***/
    #define FOO_HAVE_SOCKET_FILTER_PROG_TYPE
    #define FOO_HAVE_KPROBE_PROG_TYPE
    #define FOO_HAVE_SCHED_CLS_PROG_TYPE
    ...

    /*** eBPF map types ***/
    #define FOO_HAVE_HASH_MAP_TYPE
    #define FOO_HAVE_ARRAY_MAP_TYPE
    #define FOO_HAVE_PROG_ARRAY_MAP_TYPE
    ...

    /*** eBPF helper functions ***/
    ...
    #define FOO_BPF_SKB_CHANGE_HEAD_HELPER_COMPAT_LIST      ""      \
            "lwt_xmit "     \
            "sk_skb "
    #define FOO_BPF_XDP_ADJUST_HEAD_HELPER_COMPAT_LIST      ""      \
            "xdp "
    #define FOO_BPF_PROBE_READ_STR_HELPER_COMPAT_LIST       ""      \
            "kprobe "       \
            "tracepoint "   \
            "perf_event "   \
            "raw_tracepoint "
    ...

Rest looks good to me, just a comment here. How would programs use the
compat list? Wouldn't it be nicer to provide a PROG(HELPER) availability
query along the lines of ...

#if FOO_HAVE_SOCKET_FILTER(skb_load_bytes) == 1
// ...
#endif

... where the helper provides these macro functions and results in the
header file? So they can be used easily in the code from BPF prog or
normal C applications?

v2:
- #define-based output added as a distinct patch.
- "HAVE_" prefix appended to all macro names.
- Output limited to bpf() syscall availability, BPF prog and map types,
  helper functions. In this version kernel config options, procfs
  parameter or kernel version are intentionally left aside.
- Following the change on helper probes, format for helper probes in
  this output style has changed (now a list of compatible program
  types).

Signed-off-by: Quentin Monnet <redacted>