perf trace: Do not hardcode the size of the tracepoint common_ fields

perf trace: Do not hardcode the size of the tracepoint common_ fields

We shouldn't hardcode the size of the tracepoint common_ fields, use the
offset of the 'id'/'__syscallnr' field in the sys_enter event instead.

This caused the augmented syscalls code to fail on a particular build of a
PREEMPT_RT_FULL kernel where these extra 'common_migrate_disable' and
'common_padding' fields were before the syscall id one:

  # cat /sys/kernel/debug/tracing/events/raw_syscalls/sys_enter/format
  name: sys_enter
  ID: 22
  format:
	field:unsigned short common_type;	offset:0;	size:2;	signed:0;
	field:unsigned char common_flags;	offset:2;	size:1;	signed:0;
	field:unsigned char common_preempt_count;	offset:3;	size:1;	signed:0;
	field:int common_pid;	offset:4;	size:4;	signed:1;
	field:unsigned short common_migrate_disable;	offset:8;	size:2;	signed:0;
	field:unsigned short common_padding;	offset:10;	size:2;	signed:0;

	field:long id;	offset:16;	size:8;	signed:1;
	field:unsigned long args[6];	offset:24;	size:48;	signed:0;

  print fmt: "NR %ld (%lx, %lx, %lx, %lx, %lx, %lx)", REC->id, REC->args[0], REC->args[1], REC->args[2], REC->args[3], REC->args[4], REC->args[5]
  #

All those 'common_' prefixed fields are zeroed when they hit a BPF tracepoint
hook, we better just discard those, i.e. somehow pass an offset to the
BPF program from the start of the ctx and make adjustments in the 'perf trace'
handlers to adjust the offset of the syscall arg offsets obtained from tracefs.

Till then, fix it the quick way and add this to the augmented_raw_syscalls.c to
bet it to work in such kernels:

  diff --git a/tools/perf/examples/bpf/augmented_raw_syscalls.c b/tools/perf/examples/bpf/augmented_raw_syscalls.c
  index 53c233370fae..1f746f931e13 100644
  --- a/tools/perf/examples/bpf/augmented_raw_syscalls.c
  +++ b/tools/perf/examples/bpf/augmented_raw_syscalls.c
  @@ -38,12 +38,14 @@ struct bpf_map SEC("maps") syscalls = {

   struct syscall_enter_args {
          unsigned long long common_tp_fields;
  +       long               rt_common_tp_fields;
          long               syscall_nr;
          unsigned long      args[6];
   };

   struct syscall_exit_args {
          unsigned long long common_tp_fields;
  +       long               rt_common_tp_fields;
          long               syscall_nr;
          long               ret;
   };

Just to check that this was the case. Fix it properly later, for now remove the
hardcoding of the offset in the 'perf trace' side and document the situation
with this patch.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Luis Cláudio Gonçalves <lclaudio@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: https://lkml.kernel.org/n/tip-2pqavrktqkliu5b9nzouio21@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>