simpletrace: improved error handling on struct unpack

A failed call to `read_header` wouldn't be handled the same for the two
different code paths (one path would try to use `None` as a list).
Changed to raise exception to be handled centrally. This also allows for
easier unpacking, as errors has been filtered out.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Mads Ynddal <m.ynddal@samsung.com>
Message-id: 20230926103436.25700-7-mads@ynddal.dk
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

scripts/simpletrace.py

@@ -37,26 +37,24 @@ def read_header(fobj, hfmt):
     hlen = struct.calcsize(hfmt)
     hdr = fobj.read(hlen)
     if len(hdr) != hlen:
-        return None
+        raise ValueError('Error reading header. Wrong filetype provided?')
     return struct.unpack(hfmt, hdr)
 
 def get_record(event_mapping, event_id_to_name, rechdr, fobj):
     """Deserialize a trace record from a file into a tuple
        (name, timestamp, pid, arg1, ..., arg6)."""
-    if rechdr is None:
-        return None
-    if rechdr[0] != dropped_event_id:
-        event_id = rechdr[0]
+    event_id, timestamp_ns, length, pid = rechdr
+    if event_id != dropped_event_id:
         name = event_id_to_name[event_id]
-        rec = (name, rechdr[1], rechdr[3])
         try:
             event = event_mapping[name]
         except KeyError as e:
-            sys.stderr.write('%s event is logged but is not declared ' \
+            sys.stderr.write(f'{e} event is logged but is not declared ' \
                              'in the trace events file, try using ' \
-                             'trace-events-all instead.\n' % str(e))
+                             'trace-events-all instead.\n')
             sys.exit(1)
 
+        rec = (name, timestamp_ns, pid)
         for type, name in event.args:
             if is_string(type):
                 l = fobj.read(4)
@@ -67,9 +65,8 @@ def get_record(event_mapping, event_id_to_name, rechdr, fobj):
                 (value,) = struct.unpack('=Q', fobj.read(8))
                 rec = rec + (value,)
     else:
-        rec = ("dropped", rechdr[1], rechdr[3])
-        (value,) = struct.unpack('=Q', fobj.read(8))
-        rec = rec + (value,)
+        (dropped_count,) = struct.unpack('=Q', fobj.read(8))
+        rec = ("dropped", timestamp_ns, pid, dropped_count)
     return rec
 
 def get_mapping(fobj):
@@ -86,22 +83,16 @@ def read_record(event_mapping, event_id_to_name, fobj):
 
 def read_trace_header(fobj):
     """Read and verify trace file header"""
-    header = read_header(fobj, log_header_fmt)
-    if header is None:
-        raise ValueError('Not a valid trace file!')
-    if header[0] != header_event_id:
-        raise ValueError('Not a valid trace file, header id %d != %d' %
-                         (header[0], header_event_id))
-    if header[1] != header_magic:
-        raise ValueError('Not a valid trace file, header magic %d != %d' %
-                         (header[1], header_magic))
-
-    log_version = header[2]
+    _header_event_id, _header_magic, log_version = read_header(fobj, log_header_fmt)
+    if _header_event_id != header_event_id:
+        raise ValueError(f'Not a valid trace file, header id {_header_event_id} != {header_event_id}')
+    if _header_magic != header_magic:
+        raise ValueError(f'Not a valid trace file, header magic {_header_magic} != {header_magic}')
+
     if log_version not in [0, 2, 3, 4]:
-        raise ValueError('Unknown version of tracelog format!')
+        raise ValueError(f'Unknown version {log_version} of tracelog format!')
     if log_version != 4:
-        raise ValueError('Log format %d not supported with this QEMU release!'
-                         % log_version)
+        raise ValueError(f'Log format {log_version} not supported with this QEMU release!')
 
 def read_trace_records(event_mapping, event_id_to_name, fobj):
     """Deserialize trace records from a file, yielding record tuples (event_num, timestamp, pid, arg1, ..., arg6).