perf inject: Enter namespace when reading build-id

It should be in a proper mnt namespace when accessing the file.

I think this had no problem since the build-id was actually read from
map__load() -> dso__load() already.  But I'd like to change it in the
following commit.
Acked-by: Jiri Olsa <jolsa@redhat.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20201012070214.2074921-4-namhyung@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-inject.c

@@ -23,6 +23,7 @@
 #include "util/symbol.h"
 #include "util/synthetic-events.h"
 #include "util/thread.h"
+#include "util/namespaces.h"
 #include <linux/err.h>
 
 #include <subcmd/parse-options.h>
@@ -419,16 +420,19 @@ static int perf_event__repipe_tracing_data(struct perf_session *session,
 
 static int dso__read_build_id(struct dso *dso)
 {
+	struct nscookie nsc;
+
 	if (dso->has_build_id)
 		return 0;
 
+	nsinfo__mountns_enter(dso->nsinfo, &nsc);
 	if (filename__read_build_id(dso->long_name, dso->build_id,
 				    sizeof(dso->build_id)) > 0) {
 		dso->has_build_id = true;
-		return 0;
 	}
+	nsinfo__mountns_exit(&nsc);
 
-	return -1;
+	return dso->has_build_id ? 0 : -1;
 }
 
 static int dso__inject_build_id(struct dso *dso, struct perf_tool *tool,