elf: detect Android OS via note and dependencies

CHANGELOG.md

@@ -22,6 +22,7 @@
 
 - Fix binja backend stack string detection. #1473 @xusheng6
 - linter: skip native API check for NtProtectVirtualMemory #1675 @williballenthin 
+- OS: detect Android ELF files #1705 @williballenthin
 
 ### capa explorer IDA Pro plugin
 - fix unhandled exception when resolving rule path #1693 @mike-hunhoff

capa/features/extractors/elf.py

@@ -54,6 +54,7 @@ class OS(str, Enum):
     CLOUD = "cloud"
     SYLLABLE = "syllable"
     NACL = "nacl"
+    ANDROID = "android"
 
 
 # via readelf: https://github.com/bminor/binutils-gdb/blob/c0e94211e1ac05049a4ce7c192c9d14d1764eb3e/binutils/readelf.c#L19635-L19658
@@ -764,6 +765,11 @@ def guess_os_from_ph_notes(elf: ELF) -> Optional[OS]:
         elif note.name == "FreeBSD":
             logger.debug("note owner: %s", "FREEBSD")
             return OS.FREEBSD
+        elif note.name == "Android":
+            logger.debug("note owner: %s", "Android")
+            # see the following for parsing the structure:
+            # https://android.googlesource.com/platform/ndk/+/master/parse_elfnote.py
+            return OS.ANDROID
         elif note.name == "GNU":
             abi_tag = note.abi_tag
             if abi_tag:
@@ -855,6 +861,8 @@ def guess_os_from_needed_dependencies(elf: ELF) -> Optional[OS]:
             return OS.HURD
         if needed.startswith("libhurduser.so"):
             return OS.HURD
+        if needed.startswith("libandroid.so"):
+            return OS.ANDROID
 
     return None
 

tests/fixtures.py

@@ -308,6 +308,8 @@ def get_data_path_by_name(name) -> Path:
         return CD / "data" / "2bf18d0403677378adad9001b1243211.elf_"
     elif name.startswith("ea2876"):
         return CD / "data" / "ea2876e9175410b6f6719f80ee44b9553960758c7d0f7bed73c0fe9a78d8e669.dll_"
+    elif name.startswith("1038a2"):
+        return CD / "data" / "1038a23daad86042c66bfe6c9d052d27048de9653bde5750dc0f240c792d9ac8.elf_"
     else:
         raise ValueError(f"unexpected sample fixture: {name}")
 

tests/test_os_detection.py

@@ -80,6 +80,18 @@ def test_elf_symbol_table():
         assert capa.features.extractors.elf.detect_elf_os(f) == "linux"
 
 
+def test_elf_android_notes():
+    # DEBUG:capa.features.extractors.elf:guess: osabi: None
+    # DEBUG:capa.features.extractors.elf:guess: ph notes: OS.ANDROID
+    # DEBUG:capa.features.extractors.elf:guess: sh notes: None
+    # DEBUG:capa.features.extractors.elf:guess: linker: None
+    # DEBUG:capa.features.extractors.elf:guess: ABI versions needed: None
+    # DEBUG:capa.features.extractors.elf:guess: needed dependencies: OS.ANDROID
+    path = get_data_path_by_name("1038a2")
+    with Path(path).open("rb") as f:
+        assert capa.features.extractors.elf.detect_elf_os(f) == "android"
+
+
 def test_elf_parse_capa_pyinstaller_header():
     # error after misidentified large pydata section with address 0; fixed in #1454
     # compressed ELF header of capa-v5.1.0-linux