Some resilience to malformed ELF header (#628)

* Som resilience to malformed ELF header

* Better enum fallback

* String table lazy load

Author: sevaa

elftools/elf/descriptions.py

@@ -27,7 +27,7 @@ def describe_ei_data(x):
 
 
 def describe_ei_version(x):
-    s = '%d' % ENUM_E_VERSION[x]
+    s = str(ENUM_E_VERSION.get(x, f"{x} <unknown>"))
     if x == 'EV_CURRENT':
         s += ' (current)'
     return s
@@ -41,9 +41,10 @@ def describe_e_type(x, elffile=None):
     if elffile is not None and x == 'ET_DYN':
         # Detect whether this is a normal SO or a PIE executable
         dynamic = elffile.get_section_by_name('.dynamic')
-        for t in dynamic.iter_tags('DT_FLAGS_1'):
-            if t.entry.d_val & ENUM_DT_FLAGS_1['DF_1_PIE']:
-                return 'DYN (Position-Independent Executable file)'
+        if dynamic:
+            for t in dynamic.iter_tags('DT_FLAGS_1'):
+                if t.entry.d_val & ENUM_DT_FLAGS_1['DF_1_PIE']:
+                    return 'DYN (Position-Independent Executable file)'
     return _DESCR_E_TYPE.get(x, _unknown)
 
 
@@ -52,7 +53,7 @@ def describe_e_machine(x):
 
 
 def describe_e_version_numeric(x):
-    return '0x%x' % ENUM_E_VERSION[x]
+    return f"{ENUM_E_VERSION.get(x, x):#x}"
 
 
 def describe_p_type(x):

elftools/elf/elffile.py

@@ -84,8 +84,7 @@ def __init__(self, stream, stream_loader=None):
         self.stream.seek(0)
         self.e_ident_raw = self.stream.read(16)
 
-        self._section_header_stringtable = \
-            self._get_section_header_stringtable()
+        self._section_header_stringtable = None # Lazy load
         self._section_name_map = None
         self.stream_loader = stream_loader
 
@@ -693,8 +692,10 @@ def _get_section_name(self, section_header):
         """ Given a section header, find this section's name in the file's
             string table
         """
-        if self._section_header_stringtable is None:
-            raise ELFParseError("String Table not found")
+        if self._section_header_stringtable is None: # Try to lazy load
+            self._section_header_stringtable = self._get_section_header_stringtable()
+            if self._section_header_stringtable is None: # If absent (or badly malformed)
+                raise ELFParseError("String Table not found")
 
         name_offset = section_header['sh_name']
         return self._section_header_stringtable.get_string(name_offset)