Add a --debug flag to the CLI to help retrieve more logs.

When the flag is set:

- The `RUNSC_DEBUG=1` environment variable is added to the outer container ;
- the stderr from the outer container is attached to the exception, and
  displayed to the user on failures.

dangerzone/cli.py

@@ -42,6 +42,11 @@ def print_header(s: str) -> None:
     type=click.UNPROCESSED,
     callback=args.validate_input_filenames,
 )
+@click.option(
+    "--debug",
+    "debug",
+    flag_value=True,
+    help="Run Dangerzone in debug mode, and get more logs from the conversion process.")
 @click.version_option(version=get_version(), message="%(version)s")
 @errors.handle_document_errors
 def cli_main(
@@ -50,6 +55,7 @@ def cli_main(
     filenames: List[str],
     archive: bool,
     dummy_conversion: bool,
+    debug: bool,
 ) -> None:
     setup_logging()
 
@@ -58,7 +64,7 @@ def cli_main(
     elif is_qubes_native_conversion():
         dangerzone = DangerzoneCore(Qubes())
     else:
-        dangerzone = DangerzoneCore(Container())
+        dangerzone = DangerzoneCore(Container(debug=debug))
 
     display_banner()
     if len(filenames) == 1 and output_filename:

dangerzone/conversion/errors.py

@@ -18,11 +18,20 @@ class ConversionException(Exception):
     error_message = "Unspecified error"
     error_code = ERROR_SHIFT
 
-    def __init__(self, error_message: Optional[str] = None) -> None:
+    def __init__(
+        self, error_message: Optional[str] = None, logs: Optional[str] = None
+    ) -> None:
         if error_message:
             self.error_message = error_message
+        self.logs = logs
         super().__init__(self.error_message)
 
+    def __str__(self):
+        msg = f"{self.error_message}"
+        if self.logs:
+            msg += f" {self.logs}"
+        return msg
+
     @classmethod
     def get_subclasses(cls) -> List[Type["ConversionException"]]:
         subclasses = [cls]
@@ -100,9 +109,10 @@ class UnexpectedConversionError(ConversionException):
 
 def exception_from_error_code(
     error_code: int,
+    logs: Optional[str] = None,
 ) -> Union[ConversionException, ValueError]:
     """returns the conversion exception corresponding to the error code"""
     for cls in ConversionException.get_subclasses():
         if cls.error_code == error_code:
-            return cls()
-    return UnexpectedConversionError(f"Unknown error code '{error_code}'")
+            return cls(logs=logs)
+    return UnexpectedConversionError(f"Unknown error code '{error_code}', logs= {logs}")

dangerzone/isolation_provider/base.py

@@ -5,6 +5,7 @@ import platform
 import signal
 import subprocess
 import sys
+import tempfile
 from abc import ABC, abstractmethod
 from pathlib import Path
 from typing import IO, Callable, Iterator, Optional
@@ -87,12 +88,19 @@ class IsolationProvider(ABC):
     Abstracts an isolation provider
     """
 
-    def __init__(self) -> None:
-        if getattr(sys, "dangerzone_dev", False) is True:
+    def __init__(self, debug) -> None:
+        self.debug = debug
+        if self.should_display_errors():
             self.proc_stderr = subprocess.PIPE
         else:
+            # We do not trust STDERR from the inner container,
+            # which could be exploited to ask users to open the document with
+            # a default PDF reader for instance.
             self.proc_stderr = subprocess.DEVNULL
 
+    def should_display_errors(self) -> bool:
+        return self.debug or getattr(sys, "dangerzone_dev", False)
+
     @abstractmethod
     def install(self) -> bool:
         pass
@@ -220,6 +228,17 @@ class IsolationProvider(ABC):
         text = "Successfully converted document"
         self.print_progress(document, False, text, 100)
 
+        if self.should_display_errors():
+            assert p.stderr
+            debug_log = read_debug_text(p.stderr, MAX_CONVERSION_LOG_CHARS)
+            p.stderr.close()
+            log.debug(
+                "Conversion output (doc to pixels)\n"
+                f"{DOC_TO_PIXELS_LOG_START}\n"
+                f"{debug_log}"  # no need for an extra newline here
+                f"{DOC_TO_PIXELS_LOG_END}"
+            )
+
     def print_progress(
         self, document: Document, error: bool, text: str, percentage: float
     ) -> None:
@@ -252,7 +271,10 @@ class IsolationProvider(ABC):
                 "Encountered an I/O error during document to pixels conversion,"
                 f" but the status of the conversion process is unknown (PID: {p.pid})"
             )
-        return errors.exception_from_error_code(error_code)
+        logs = None
+        if self.debug:
+            logs = "".join([line.decode() for line in p.stderr.readlines()])
+        return errors.exception_from_error_code(error_code, logs=logs)
 
     @abstractmethod
     def get_max_parallel_conversions(self) -> int:

dangerzone/isolation_provider/container.py

@@ -93,7 +93,7 @@ class Container(IsolationProvider):
         return runtime
 
     @staticmethod
-    def get_runtime_security_args() -> List[str]:
+    def get_runtime_security_args(debug: bool) -> List[str]:
         """Security options applicable to the outer Dangerzone container.
 
         Our security precautions for the outer Dangerzone container are the following:
@@ -137,6 +137,9 @@ class Container(IsolationProvider):
         security_args += ["--network=none"]
         security_args += ["-u", "dangerzone"]
 
+        if debug:
+            security_args += ["-e", "RUNSC_DEBUG=1"]
+
         return security_args
 
     @staticmethod
@@ -250,7 +253,7 @@ class Container(IsolationProvider):
         extra_args: List[str] = [],
     ) -> subprocess.Popen:
         container_runtime = self.get_runtime()
-        security_args = self.get_runtime_security_args()
+        security_args = self.get_runtime_security_args(self.debug)
         enable_stdin = ["-i"]
         set_name = ["--name", name]
         prevent_leakage_args = ["--rm"]