simpleperf: fix is_elf_file.

When running by python3, is_elf_file() decodes first 4 bytes of
a file, which may cause UnicodeDecodeError.

Bug: none
Test: run test.py TestTools.test_is_elf_file.
Change-Id: Ie19e4869f876bb92155d849c814c8f39989c443b

2 files changed, 12 insertions, 4 deletions

diff --git a/simpleperf/scripts/test.py b/simpleperf/scripts/test.py
index 24a04356..e24f58f1 100755
--- a/simpleperf/scripts/test.py
+++ b/simpleperf/scripts/test.py
@@ -56,7 +56,7 @@ from binary_cache_builder import BinaryCacheBuilder
 from simpleperf_report_lib import ReportLib
 from utils import log_exit, log_info, log_fatal
 from utils import AdbHelper, Addr2Nearestline, bytes_to_str, find_tool_path, get_script_dir
-from utils import is_python3, is_windows, Objdump, ReadElf, remove, SourceFileSearcher
+from utils import is_elf_file, is_python3, is_windows, Objdump, ReadElf, remove, SourceFileSearcher
 from utils import str_to_bytes
 
 try:
@@ -1188,6 +1188,16 @@ class TestTools(unittest.TestCase):
                         'simpleperf/simpleperfexampleofkotlin/MainActivity.kt'),
             searcher.get_real_path('MainActivity.kt'))
 
+    def test_is_elf_file(self):
+        self.assertTrue(is_elf_file(os.path.join(
+            'testdata', 'simpleperf_runtest_two_functions_arm')))
+        with open('not_elf', 'wb') as fh:
+            fh.write(b'\x90123')
+        try:
+            self.assertFalse(is_elf_file('not_elf'))
+        finally:
+            remove('not_elf')
+
 
 class TestNativeLibDownloader(unittest.TestCase):
     def setUp(self):
diff --git a/simpleperf/scripts/utils.py b/simpleperf/scripts/utils.py
index 4b942d8c..9891e606 100644
--- a/simpleperf/scripts/utils.py
+++ b/simpleperf/scripts/utils.py
@@ -374,9 +374,7 @@ def open_report_in_browser(report_path):
 def is_elf_file(path):
     if os.path.isfile(path):
         with open(path, 'rb') as fh:
-            data = fh.read(4)
-            if len(data) == 4 and bytes_to_str(data) == '\x7fELF':
-                return True
+            return fh.read(4) == b'\x7fELF'
     return False
 
 def find_real_dso_path(dso_path_in_record_file, binary_cache_path):