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__cmd_diff() sets result of perf_session__new() to d->session.

In case of failure, it's errno and perf-diff may crash with:

  failed to open perf.data: Permission denied
  Failed to open perf.data
  Segmentation fault (core dumped)

From the coredump:

0  0x00005569a62b5955 in auxtrace__free (session=0xffffffffffffffff)
    at util/auxtrace.c:2681
1  0x00005569a626b37d in perf_session__delete (session=0xffffffffffffffff)
    at util/session.c:295
2  perf_session__delete (session=0xffffffffffffffff) at util/session.c:291
3  0x00005569a618008a in __cmd_diff () at builtin-diff.c:1239
4  cmd_diff (argc=<optimized out>, argv=<optimized out>) at builtin-diff.c:2011
[..]

Funny enough, it won't always crash. For me it crashes only if failed
file is second in cmd-line: the reason is that cmd_diff() check files for
branch-stacks [in check_file_brstack()] and if the first file doesn't
have brstacks, it doesn't proceed to try open other files from c...