From 86995ee24a8601b15a503c2af66e96a2c9ad6284 Mon Sep 17 00:00:00 2001
From: Andrea Righi <andrea.righi@canonical.com>
Date: Fri, 2 Feb 2024 09:52:22 +0100
Subject: [PATCH] virtme-ng: print an explicit error when a valid pts is not
 found

When running in script-mode we need to channel the stdin/stdout/stderr
of the guest command into the host's stdin/stdout/stderr.

However, if we don't have read/write permission to operate on the
current pts we may get the following (quite obscure) error:

 qemu-system-x86_64: -chardev file,id=stdout,path=/proc/self/fd/1: Could not open '/proc/self/fd/1': Permission denied

For example, it is really easy to reproduce this problem switching to a
different user and then running vng:

  arighi@gpd$ sudo su - sbuild
  sbuild@gpd:~$ vng -r -- uname -r
  qemu-system-x86_64: -chardev file,id=stdout,path=/proc/self/fd/1: Could not open '/proc/self/fd/1': Permission denied

Instead of printing the qemu error, let's check directly the current
stdin/stdout/stderr and if they don't exist or if we don't have the
proper permissions to access them, report a more clear error (with a
short hint on how to solve this issue):

  sbuild@gpd:~$ vng -r -- uname -r
  ERROR: not a valid pts, try to run vng inside tmux or screen

This can help to save some time for those that may run virtme-ng after
switching to a different user or any other similar scenarios, as
discussed in #60.

Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
---
 virtme/commands/run.py | 24 +++++++++++++++++++-----
 1 file changed, 19 insertions(+), 5 deletions(-)

diff --git a/virtme/commands/run.py b/virtme/commands/run.py
index 762c6d8..87dadcc 100644
--- a/virtme/commands/run.py
+++ b/virtme/commands/run.py
@@ -725,13 +725,11 @@ def sanitize_disk_args(func: str, arg: str) -> Tuple[str, str]:
     return name, fn
 
 
-def can_use_kvm(args):
-    if args.disable_kvm:
-        return False
-    if not os.path.exists("/dev/kvm"):
+def can_access_file(path):
+    if not os.path.exists(path):
         return False
     try:
-        fd = os.open('/dev/kvm', os.O_RDWR | os.O_CLOEXEC)
+        fd = os.open(path, os.O_RDWR | os.O_CLOEXEC)
         os.close(fd)
         return True
 
@@ -739,6 +737,12 @@ def can_use_kvm(args):
         return False
 
 
+def can_use_kvm(args):
+    if args.disable_kvm:
+        return False
+    return can_access_file("/dev/kvm")
+
+
 def can_use_microvm(args):
     return not args.disable_microvm and not args.numa and args.arch == "x86_64" and can_use_kvm(args)
 
@@ -1084,6 +1088,16 @@ def do_script(shellcmd: str, ret_path=None, show_boot_console=False) -> None:
             # Turn off default I/O
             qemuargs.extend(arch.qemu_nodisplay_args())
 
+        # Check if we can redirect stdin/stdout/stderr.
+        if not can_access_file("/proc/self/fd/0") or \
+           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",
+                file=sys.stderr,
+            )
+            sys.exit(1)
+
         # Configure kernel console output
         if show_boot_console:
             output = "/proc/self/fd/2"