kunit: tool: stop using a shell to run kernel under QEMU

Note: this potentially breaks custom qemu_configs if people are using
them! But the fix for them is simple, don't specify multiple arguments
in one string and don't add on a redundant ''.

It feels a bit iffy to be using a shell in the first place.

There's the usual shenanigans where people could pass in arbitrary shell
commands via --kernel_arg (since we're just adding '' around the
kernel_cmdline) or via a custom qemu_config.
This isn't too much of a concern given the nature of this script (and
the qemu_config file is in python, you can do w/e you want already).

But it does have some other drawbacks.

One example of a kunit-specific pain point:
If the relevant qemu binary is missing, we get output like this:
> /bin/sh: line 1: qemu-system-aarch64: command not found
This in turn results in our KTAP parser complaining about
missing/invalid KTAP, but we don't directly show the error!
It's even more annoying to debug when you consider --raw_output only
shows KUnit output by default, i.e. you need --raw_output=all to see it.

Whereas directly invoking the binary, Python will raise a
FileNotFoundError for us, which is a noisier but more clear.

Making this change requires
* splitting parameters like ['-m 256'] into ['-m', '256'] in
  kunit/qemu_configs/*.py
* change [''] to [] in kunit/qemu_configs/*.py since otherwise
  QEMU fails w/ 'Device needs media, but drive is empty'
* dropping explicit quoting of the kernel cmdline
* using shlex.quote() when we print what command we're running
  so the user can copy-paste and run it

Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/kunit/kunit_kernel.py

@@ -11,6 +11,7 @@ import importlib.util
 import logging
 import subprocess
 import os
+import shlex
 import shutil
 import signal
 import threading
@@ -118,16 +119,17 @@ class LinuxSourceTreeOperationsQemu(LinuxSourceTreeOperations):
 				'-nodefaults',
 				'-m', '1024',
 				'-kernel', kernel_path,
-				'-append', '\'' + ' '.join(params + [self._kernel_command_line]) + '\'',
+				'-append', ' '.join(params + [self._kernel_command_line]),
 				'-no-reboot',
 				'-nographic',
-				'-serial stdio'] + self._extra_qemu_params
-		print('Running tests with:\n$', ' '.join(qemu_command))
-		return subprocess.Popen(' '.join(qemu_command),
-					   stdin=subprocess.PIPE,
-					   stdout=subprocess.PIPE,
-					   stderr=subprocess.STDOUT,
-					   text=True, shell=True, errors='backslashreplace')
+				'-serial', 'stdio'] + self._extra_qemu_params
+		# Note: shlex.join() does what we want, but requires python 3.8+.
+		print('Running tests with:\n$', ' '.join(shlex.quote(arg) for arg in qemu_command))
+		return subprocess.Popen(qemu_command,
+					stdin=subprocess.PIPE,
+					stdout=subprocess.PIPE,
+					stderr=subprocess.STDOUT,
+					text=True, errors='backslashreplace')
 
 class LinuxSourceTreeOperationsUml(LinuxSourceTreeOperations):
 	"""An abstraction over command line operations performed on a source tree."""