perf report: fix crash showing warning related to kernel maps

While testing https://lkml.org/lkml/2012/4/10/123 I hit this crash:

(gdb) bt

  0  0x000000000042000f in __cmd_report (rep=0x7fff80cec580) at builtin-report.c:380
  1  cmd_report (argc=0, argv=<optimized out>, prefix=<optimized out>) at builtin-report.c:759
  2  0x0000000000414513 in run_builtin (p=0x7724a8, argc=3, argv=0x7fff80ceca70) at perf.c:273
  3  0x0000000000413d41 in handle_internal_command (argv=0x7fff80ceca70, argc=3) at perf.c:345
  4  run_argv (argv=0x7fff80cec880, argcp=0x7fff80cec88c) at perf.c:389
  5  main (argc=3, argv=0x7fff80ceca70) at perf.c:487

kernel_map can be NULL, so need to handle it while dumping a warning to user.

Signed-off-by David Ahern<[email protected]>

---
  tools/perf/builtin-report.c |   17 ++++++++++++-----
  1 files changed, 12 insertions(+), 5 deletions(-)

tools/perf/builtin-report.c +12 -5

index 2e31743..002c644 100644
@@ -374,16 +374,23 @@ static int __cmd_report(struct perf_report *rep)
  	    (kernel_map->dso->hit&&
  	(kernel_kmap->ref_reloc_sym == NULL ||
  	      kernel_kmap->ref_reloc_sym->addr == 0))) {
-		const struct dso *kdso = kernel_map->dso;
+		const char *desc =
+		    "As no suitable kallsyms nor vmlinux was found, kernel samples\n"
+		    "can't be resolved.";
+
+		if (kernel_map) {
+			const struct dso *kdso = kernel_map->dso;
+			if (RB_EMPTY_ROOT(&kdso->symbols[MAP__FUNCTION])) {
+				desc = "If some relocation was applied (e.g. "
+				       "kexec) symbols may be misresolved.";
+			}
+		}
 
  		ui__warning(
  "Kernel address maps (/proc/{kallsyms,modules}) were restricted.\n\n"
  "Check /proc/sys/kernel/kptr_restrict before running 'perf record'.\n\n%s\n\n"
  "Samples in kernel modules can't be resolved as well.\n\n",
-			    RB_EMPTY_ROOT(&kdso->symbols[MAP__FUNCTION]) ?
-"As no suitable kallsyms nor vmlinux was found, kernel samples\n"
-"can't be resolved." :
-"If some relocation was applied (e.g. kexec) symbols may be misresolved.");
+		desc);
  	}
 
  	if (dump_trace) {