"perf ftrace" segfault because ->cpus!=NULL but ->all_cpus==NULL

[Jann Horn]

I was clumsily trying to use "perf ftrace" from git master (I might
very well be using it wrong), and it's falling over with a NULL deref.
I don't really understand the perf code, but it looks to me like it
might be related to Andi Kleen's refactoring that introduced
evlist->all_cpus?

I think the problem is that evlist_close() assumes that ->cpus!=NULL
implies ->all_cpus!=NULL, but perf_evlist__propagate_maps() doesn't
set ->all_cpus if the evlist is empty.

Here's the crash I encountered:

root@vm:~# /linux-llvm/tools/perf/perf ftrace -a -T kmem_cache_alloc
failed to reset ftrace
Segmentation fault (core dumped)
root@vm:~# gdb /linux-llvm/tools/perf/perf core
[...]
Core was generated by `/linux-llvm/tools/perf/perf ftrace -a -T
kmem_cache_alloc'.
Program terminated with signal SIGSEGV, Segmentation fault.
#0  perf_cpu_map__cpu (cpus=0x0, idx=idx@entry=0) at cpumap.c:250
[...]
(gdb) list
245 return cpus;
246 }
247
248 int perf_cpu_map__cpu(const struct perf_cpu_map *cpus, int idx)
249 {
250 if (idx < cpus->nr)
251 return cpus->map[idx];
252
253 return -1;
254 }
(gdb) print cpus
$1 = (const struct perf_cpu_map *) 0x0
(gdb) bt
#0  perf_cpu_map__cpu (cpus=0x0, idx=idx@entry=0) at cpumap.c:250
#1  0x00005579cbf38731 in evlist__close
(evlist=evlist@entry=0x5579cd1c22c0) at util/evlist.c:1222
#2  0x00005579cbf388f6 in evlist__delete (evlist=0x5579cd1c22c0) at
util/evlist.c:152
#3  0x00005579cbf389c1 in evlist__delete (evlist=<optimized out>) at
util/evlist.c:148
#4  0x00005579cbe9e3b5 in cmd_ftrace (argc=0, argv=0x7ffe818a16e0) at
builtin-ftrace.c:520
#5  0x00005579cbf2287d in run_builtin (p=0x5579cc2a9740
<commands+672>, argc=4, argv=0x7ffe818a16e0) at perf.c:312
#6  0x00005579cbe96baa in handle_internal_command
(argv=0x7ffe818a16e0, argc=4) at perf.c:364
#7  run_argv (argcp=<synthetic pointer>, argv=<synthetic pointer>) at perf.c:408
#8  main (argc=4, argv=0x7ffe818a16e0) at perf.c:538
(gdb) frame 1
#1  0x00005579cbf38731 in evlist__close
(evlist=evlist@entry=0x5579cd1c22c0) at util/evlist.c:1222
1222 evlist__for_each_cpu(evlist, i, cpu) {
(gdb) print evlist
$2 = (struct evlist *) 0x5579cd1c22c0
(gdb) print *evlist
$3 = {core = {entries = {next = 0x5579cd1c22c0, prev =
0x5579cd1c22c0}, nr_entries = 0, has_user_cpus = false, cpus =
0x5579cd1b8fe0, all_cpus = 0x0, threads = 0x5579cd1c2d60,
    nr_mmaps = 0, mmap_len = 0, pollfd = {nr = 0, nr_alloc = 0,
nr_autogrow = 64, entries = 0x0, priv = 0x0}, heads = {{first = 0x0}
<repeats 256 times>}, mmap = 0x0, mmap_ovw = 0x0,
    mmap_first = 0x0, mmap_ovw_first = 0x0}, nr_groups = 0, enabled =
false, id_pos = 0, is_pos = 0, combined_sample_type = 0,
bkw_mmap_state = BKW_MMAP_NOTREADY, workload = {cork_fd = 0,
    pid = -1}, mmap = 0x0, overwrite_mmap = 0x0, selected = 0x0, stats
= {total_period = 0, total_non_filtered_period = 0, total_lost = 0,
total_lost_samples = 0, total_aux_lost = 0,
    total_aux_partial = 0, total_invalid_chains = 0, nr_events = {0
<repeats 82 times>}, nr_non_filtered_samples = 0, nr_lost_warned = 0,
nr_unknown_events = 0, nr_invalid_chains = 0,
    nr_unknown_id = 0, nr_unprocessable_samples = 0,
nr_auxtrace_errors = {0, 0}, nr_proc_map_timeout = 0}, env = 0x0,
trace_event_sample_raw = 0x0, first_sample_time = 0,
  last_sample_time = 0, thread = {th = 0, done = 0}}
(gdb)