perf header: Assign proper ff->ph in perf_event__synthesize_features()

bpf/btf write_* functions need ff->ph->env.

With this missing, pipe-mode (perf record -o -)  would crash like:

Program terminated with signal SIGSEGV, Segmentation fault.

This patch assign proper ph value to ff.

Committer testing:

  (gdb) run record -o -
  Starting program: /root/bin/perf record -o -
  PERFILE2
  <SNIP start of perf.data headers>
  Thread 1 "perf" received signal SIGSEGV, Segmentation fault.
  __do_write_buf (size=4, buf=0x160, ff=0x7fffffff8f80) at util/header.c:126
  126		memcpy(ff->buf + ff->offset, buf, size);
  (gdb) bt
  #0  __do_write_buf (size=4, buf=0x160, ff=0x7fffffff8f80) at util/header.c:126
  #1  do_write (ff=ff@entry=0x7fffffff8f80, buf=buf@entry=0x160, size=4) at util/header.c:137
  #2  0x00000000004eddba in write_bpf_prog_info (ff=0x7fffffff8f80, evlist=<optimized out>) at util/header.c:912
  #3  0x00000000004f69d7 in perf_event__synthesize_features (tool=tool@entry=0x97cc00 <record>, session=session@entry=0x7fffe9c6d010,
      evlist=0x7fffe9cae010, process=process@entry=0x4435d0 <process_synthesized_event>) at util/header.c:3695
  #4  0x0000000000443c79 in record__synthesize (tail=tail@entry=false, rec=0x97cc00 <record>) at builtin-record.c:1214
  #5  0x0000000000444ec9 in __cmd_record (rec=0x97cc00 <record>, argv=<optimized out>, argc=0) at builtin-record.c:1435
  #6  cmd_record (argc=0, argv=<optimized out>) at builtin-record.c:2450
  #7  0x00000000004ae3e9 in run_builtin (p=p@entry=0x98e058 <commands+216>, argc=argc@entry=3, argv=0x7fffffffd670) at perf.c:304
  #8  0x000000000042eded in handle_internal_command (argv=<optimized out>, argc=<optimized out>) at perf.c:356
  #9  run_argv (argcp=<optimized out>, argv=<optimized out>) at perf.c:400
  #10 main (argc=3, argv=<optimized out>) at perf.c:522
  (gdb)

After the patch the SEGSEGV is gone.

Reported-by: David Carrillo Cisneros <davidca@fb.com>
Signed-off-by: Song Liu <songliubraving@fb.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: kernel-team@fb.com
Cc: stable@vger.kernel.org # v5.1+
Fixes: 606f972 ("perf bpf: Save bpf_prog_info information as headers to perf.data")
Link: http://lkml.kernel.org/r/20190620010453.4118689-1-songliubraving@fb.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/header.c

@@ -3602,6 +3602,7 @@ int perf_event__synthesize_features(struct perf_tool *tool,
 		return -ENOMEM;
 
 	ff.size = sz - sz_hdr;
+	ff.ph = &session->header;
 
 	for_each_set_bit(feat, header->adds_features, HEADER_FEAT_BITS) {
 		if (!feat_ops[feat].synthesize) {