cli: Open /proc/kcore via sudo when not root

Non-root users can now run Drgn against the running kernel. Drgn will
attempt to use sudo to open /proc/kcore and transmit the opened file
descriptor back to the user process. The file descriptor is then passed
to Program.set_fd(). The user must still have sudo privileges.

Signed-off-by: Stephen Brennan <stephen@brennan.io>

drgn/cli.py

@@ -20,6 +20,7 @@
 
 import drgn
 from drgn.internal.rlcompleter import Completer
+from drgn.internal.sudohelper import open_via_sudo
 
 __all__ = ("run_interactive", "version_header")
 
@@ -263,8 +264,10 @@ def _main() -> None:
             prog.set_core_dump(args.core)
         elif args.pid is not None:
             prog.set_pid(args.pid or os.getpid())
-        else:
+        elif os.getuid() == 0:
             prog.set_kernel()
+        else:
+            prog.set_fd(open_via_sudo("/proc/kcore"))
     except PermissionError as e:
         print(e, file=sys.stderr)
         if args.pid is not None:

drgn/internal/sudohelper.py

@@ -0,0 +1,125 @@
+# Copyright (c) Stephen Brennan <stephen@brennan.io>
+# SPDX-License-Identifier: LGPL-2.1-or-later
+
+"""Helper for opening a file as root and transmitting it via unix socket"""
+import argparse
+import array
+import errno
+import os
+from pathlib import Path
+import socket
+import subprocess
+import sys
+import tempfile
+from typing import Union
+
+
+def recv_fd(sock: socket.socket) -> int:
+    """Receive a file descripter on the socket"""
+    # Borrows heavily from the Python docs for sock.recvmsg()
+    fds = array.array("i")
+    msg, ancdata, flags, addr = sock.recvmsg(4096, socket.CMSG_SPACE(fds.itemsize))
+    if msg == b"success":
+        level, typ, data = ancdata[0]
+        assert level == socket.SOL_SOCKET
+        assert typ == socket.SCM_RIGHTS
+        data = data[: fds.itemsize]
+        fds.frombytes(data)
+        return fds[0]
+    raise Exception(msg.decode())
+
+
+def recv_error(sock: socket.socket) -> None:
+    """Try to receive an error message from the sudo helper"""
+    sock.setblocking(False)
+    try:
+        msg, ancdata, flags, addr = sock.recvmsg(4096)
+    except OSError as e:
+        if e.errno in (errno.EAGAIN, errno.EWOULDBLOCK):
+            raise Exception("sudo helper never connected to socket") from None
+        raise
+    raise Exception(msg.decode())
+
+
+def send_fd(sock: socket.socket, fd: int) -> None:
+    """Send a file descripter on the socket"""
+    # Borrows heavily from the Python docs for sock.sendmsg()
+    fds = array.array("i", [fd])
+    sock.sendmsg(
+        [b"success"],
+        [(socket.SOL_SOCKET, socket.SCM_RIGHTS, fds)],
+    )
+
+
+def open_via_sudo(
+    path: Union[Path, str],
+    flags: int = os.O_RDONLY,
+    mode: int = 0o777,
+) -> int:
+    """Implements os.open() using sudo to get permissions"""
+    # Currently does not support dir_fd argument
+    path = str(path)
+    with tempfile.TemporaryDirectory() as td:
+        sockpath = Path(td) / "sock"
+        sock = socket.socket(socket.AF_UNIX, socket.SOCK_DGRAM)
+        sock.bind(str(sockpath))
+
+        try:
+            proc = subprocess.run(
+                [
+                    "sudo",
+                    sys.executable,
+                    "-B",
+                    __file__,
+                    str(sockpath),
+                    path,
+                    str(flags),
+                    str(mode),
+                ],
+                check=False,
+            )
+            if proc.returncode == 0:
+                return recv_fd(sock)
+            else:
+                recv_error(sock)
+        finally:
+            # sudo tends to mess up the terminal, get it into a sane state
+            subprocess.run(["stty", "sane"])
+
+
+def main() -> None:
+    parser = argparse.ArgumentParser()
+    parser.add_argument(
+        "socket",
+        type=Path,
+        help="path to unix domain socket",
+    )
+    parser.add_argument(
+        "file",
+        type=Path,
+        help="filename to open",
+    )
+    parser.add_argument(
+        "flags",
+        type=int,
+        help="numeric flags",
+    )
+    parser.add_argument(
+        "mode",
+        type=int,
+        help="numeric mode",
+    )
+    args = parser.parse_args()
+
+    sock = socket.socket(socket.AF_UNIX, socket.SOCK_DGRAM)
+    sock.connect(str(args.socket))
+    try:
+        fd = os.open(args.file, args.flags, args.mode)
+        send_fd(sock, fd)
+    except Exception as e:
+        sock.sendmsg([str(e).encode()])
+        sys.exit(1)
+
+
+if __name__ == "__main__":
+    main()