virtme: always print kernel panic / oops in interactive mode

As mentioned in #217 it would be nice to always print the output of
critical kernel errors (oops / panic), instead of suppressing all the
kernel logs completely by default.

Therefore, keep suppressing the boot kernel log, but always dump
panic/oops to stderr by default when running in interactive mode.

Example with this change applied:

arighi@virtme-ng~/s/linux (master)> vng
          _      _
   __   _(_)_ __| |_ _ __ ___   ___       _ __   __ _
   \ \ / / |  __| __|  _   _ \ / _ \_____|  _ \ / _  |
    \ V /| | |  | |_| | | | | |  __/_____| | | | (_| |
     \_/ |_|_|   \__|_| |_| |_|\___|     |_| |_|\__  |
                                                |___/
   kernel version: 6.13.0-virtme x86_64
   (CTRL+d to exit)

arighi@virtme-ng~/s/linux (master)> echo c | sudo tee /proc/sysrq-trigger
[    8.923672] sysrq: Trigger a crash
[    8.923980] Kernel panic - not syncing: sysrq triggered crash
[    8.924183] CPU: 0 UID: 0 PID: 198 Comm: tee Not tainted 6.13.0-virtme #2
[    8.924632] Call Trace:
[    8.924704]  <TASK>
[    8.924783]  panic+0x349/0x3b0
[    8.925055]  sysrq_handle_crash+0x36/0x80
[    8.925181]  __handle_sysrq+0xed/0x270
[    8.925274]  write_sysrq_trigger+0x6a/0x90
[    8.925380]  proc_reg_write+0x56/0xa0
[    8.925489]  vfs_write+0x105/0x590
[    8.925600]  ksys_write+0x74/0xf0
[    8.925682]  do_syscall_64+0xbb/0x1d0
[    8.925767]  entry_SYSCALL_64_after_hwframe+0x77/0x7f
[    8.925891] RIP: 0033:0x7fdfed54ba84

Signed-off-by: Andrea Righi <[email protected]>

virtme/commands/run.py

@@ -1282,25 +1282,27 @@ def do_it() -> int:
     if args.graphics is None and not args.script_sh and not args.script_exec:
         qemuargs.extend(["-echr", "1"])
 
-        if args.verbose:
-            # Check if we have permission to access the current stderr.
-            if not can_access_file("/proc/self/fd/2"):
-                print(
-                    "ERROR: not a valid pts, try to run vng inside tmux or screen",
-                    file=sys.stderr,
-                )
-                sys.exit(1)
-
-            # Redirect kernel messages to stderr, creating a separate console
+        # Redirect kernel errors to stderr, creating a separate console.
+        #
+        # If we don't have access to stderr via procfs (for example when
+        # running inside a container), print a warning and implicitly
+        # suppress the kernel errors redirection.
+        if can_access_file("/proc/self/fd/2"):
             qemuargs.extend(["-chardev", "file,path=/proc/self/fd/2,id=dmesg"])
             qemuargs.extend(["-device", arch.virtio_dev_type("serial")])
             qemuargs.extend(["-device", "virtconsole,chardev=dmesg"])
             kernelargs.extend(["console=hvc0"])
+        else:
+            print(
+                "WARNING: unable to write kernel messages, try to run vng with a valid PTS (e.g., inside tmux or screen)",
+                file=sys.stderr,
+            )
 
-            # Unfortunately we can't use hvc0 to redirect early console
-            # messages to stderr, so just send them to the main console, in
-            # this way we don't lose early printk's in verbose mode and we can
-            # catch potential boot issues.
+        # Unfortunately we can't use hvc0 to redirect early console
+        # messages to stderr, so just send them to the main console, in
+        # this way we don't lose early printk's in verbose mode and we can
+        # catch potential boot issues.
+        if args.verbose:
             kernelargs.extend(arch.earlyconsole_args())
 
         qemuargs.extend(["-chardev", "stdio,id=console,signal=off,mux=on"])
@@ -1407,7 +1409,7 @@ def do_script(shellcmd: str, ret_path=None, show_boot_console=False) -> None:
            not can_access_file("/proc/self/fd/1") or \
            not can_access_file("/proc/self/fd/2"):
             print(
-                "ERROR: not a valid pts, try to run vng inside tmux or screen",
+                "ERROR: not a valid pts, try to run vng with a valid PTS (e.g., inside tmux or screen)",
                 file=sys.stderr,
             )
             sys.exit(1)
@@ -1671,11 +1673,11 @@ def get_net_mac(index):
         )
         initrdpath = None
 
-    if not args.verbose:
-        kernelargs.append("quiet")
-        kernelargs.append("loglevel=0")
-    else:
+    if args.verbose:
         kernelargs.append("debug")
+    else:
+        kernelargs.append("quiet")
+        kernelargs.append("loglevel=1")
 
     # Now that we're done setting up kernelargs, append user-specified args
     # and then initargs