perf symbols: Check access permission when reading symbol files

There 2 problems when reading symbols files:

*  It doesn't report any errors even if when users specify symbol
   files which don't exist with --kallsyms or --vmlinux. The result
   just shows the address without symbols, which is not what is expected.
   So it's better to report errors and exit the program.

*  When using command perf report --kallsyms=/proc/kallsyms with a
   non-root user, symbols are resolved. Then select one symbol and
   annotate it, it reports the error as the following:
   Can't annotate __clear_user: No vmlinux file with build id xxx was
   found.

   The problem is caused by reading /proc/kcore without access permission.
   /proc/kcore requires CAP_SYS_RAWIO capability to access, so it needs to
   change access permission to allow a specific user to read /proc/kcore or
   use root to execute the perf command.

This patch is to report errors when symbol files specified by users
don't exist. And check access permission of /proc/kcore when reading it.

Signed-off-by: Li Zhang <zhlcindy@linux.vnet.ibm.com>
Cc: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Link: http://lkml.kernel.org/r/1434704253-2632-1-git-send-email-zhlcindy@linux.vnet.ibm.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-report.c

@@ -742,6 +742,17 @@ int cmd_report(int argc, const char **argv, const char *prefix __maybe_unused)
 
 	argc = parse_options(argc, argv, options, report_usage, 0);
 
+	if (symbol_conf.vmlinux_name &&
+	    access(symbol_conf.vmlinux_name, R_OK)) {
+		pr_err("Invalid file: %s\n", symbol_conf.vmlinux_name);
+		return -EINVAL;
+	}
+	if (symbol_conf.kallsyms_name &&
+	    access(symbol_conf.kallsyms_name, R_OK)) {
+		pr_err("Invalid file: %s\n", symbol_conf.kallsyms_name);
+		return -EINVAL;
+	}
+
 	if (report.use_stdio)
 		use_browser = 0;
 	else if (report.use_tui)

tools/perf/util/symbol.c

@@ -1132,8 +1132,11 @@ static int dso__load_kcore(struct dso *dso, struct map *map,
 	INIT_LIST_HEAD(&md.maps);
 
 	fd = open(kcore_filename, O_RDONLY);
-	if (fd < 0)
+	if (fd < 0) {
+		pr_err("%s requires CAP_SYS_RAWIO capability to access.\n",
+			kcore_filename);
 		return -EINVAL;
+	}
 
 	/* Read new maps into temporary lists */
 	err = file__read_maps(fd, md.type == MAP__FUNCTION, kcore_mapfn, &md,