fix: where mode named pipe permissions

We change the permissions on the named pipe to allow processes not
necessarily spawned by the superuse to write to it, thus allowing the
where mode to work in greater generality.

echion/__main__.py

@@ -31,19 +31,24 @@ def detach(pid: int) -> None:
 def attach(args: argparse.Namespace) -> None:
     from hypno import inject_py
 
-    script = dedent(
-        f"""
-        from echion.bootstrap.attach import attach
-        attach({args.__dict__!r})
-        """
-    ).strip()
-
     pid = args.pid or args.where
 
     try:
+        pipe_name = None
         if args.where:
             pipe_name = Path(tempfile.gettempdir()) / f"echion-{pid}"
             os.mkfifo(pipe_name)
+            # This named pipe is likely created by the superuser, so we need to
+            # make it writable by everyone to allow the target process to write
+            # to it.
+            os.chmod(pipe_name, 0o666)
+
+        script = dedent(
+            f"""
+            from echion.bootstrap.attach import attach
+            attach({args.__dict__!r}, {repr(str(pipe_name)) if pipe_name is not None else str(None)})
+            """
+        ).strip()
 
         inject_py(pid, script)
 
@@ -53,6 +58,7 @@ def attach(args: argparse.Namespace) -> None:
                 from time import monotonic as time
 
                 end = time() + args.exposure
+
             while not args.where:
                 try:
                     os.kill(pid, 0)
@@ -61,11 +67,12 @@ def attach(args: argparse.Namespace) -> None:
                 if end is not None and time() > end:
                     break
                 os.sched_yield()
+
         except (KeyboardInterrupt, ProcessLookupError):
             pass
 
         # Read the output
-        if args.where and pipe_name.exists():
+        if args.where and pipe_name is not None and pipe_name.exists():
             with pipe_name.open("r") as f:
                 while True:
                     line = f.readline()
@@ -76,7 +83,7 @@ def attach(args: argparse.Namespace) -> None:
         detach(pid)
 
     finally:
-        if args.where and pipe_name.exists():
+        if args.where and pipe_name is not None and pipe_name.exists():
             pipe_name.unlink()
 
 

echion/bootstrap/attach.py

@@ -6,7 +6,7 @@
 import typing as t
 
 
-def attach(config: t.Dict[str, str]) -> None:
+def attach(config: t.Dict[str, str], pipe_name: t.Optional[str] = None) -> None:
     os.environ["ECHION_CPU"] = str(int(config["cpu"]))
     os.environ["ECHION_NATIVE"] = str(int(config["native"]))
     os.environ["ECHION_OUTPUT"] = config["output"]
@@ -15,6 +15,12 @@ def attach(config: t.Dict[str, str]) -> None:
 
     from echion.bootstrap import start
 
+    # This is used in where mode for IPC.
+    if pipe_name is not None:
+        from echion.core import set_pipe_name
+
+        set_pipe_name(pipe_name)
+
     start()
 
 

echion/config.h

@@ -26,6 +26,9 @@ static int native = 0;
 // Where mode
 static int where = 0;
 
+// Pipe name (where mode IPC)
+static std::string pipe_name;
+
 // ----------------------------------------------------------------------------
 static PyObject *
 set_interval(PyObject *Py_UNUSED(m), PyObject *args)
@@ -77,3 +80,16 @@ set_where(PyObject *Py_UNUSED(m), PyObject *args)
 
     Py_RETURN_NONE;
 }
+
+// ----------------------------------------------------------------------------
+static PyObject *
+set_pipe_name(PyObject *Py_UNUSED(m), PyObject *args)
+{
+    const char *name;
+    if (!PyArg_ParseTuple(args, "s", &name))
+        return NULL;
+
+    pipe_name = name;
+
+    Py_RETURN_NONE;
+}

echion/core.pyi

@@ -23,3 +23,4 @@ def set_interval(interval: int) -> None: ...
 def set_cpu(cpu: bool) -> None: ...
 def set_native(native: bool) -> None: ...
 def set_where(where: bool) -> None: ...
+def set_pipe_name(name: str) -> None: ...

echion/coremodule.cc

@@ -138,15 +138,13 @@ _sampler()
 
     if (where)
     {
-        auto pipe_name = std::filesystem::temp_directory_path() / ("echion-" + std::to_string(getpid()));
         std::ofstream pipe(pipe_name, std::ios::out);
-        if (!pipe)
-        {
-            std::cerr << "Failed to open pipe " << pipe_name << std::endl;
-            return;
-        }
 
-        do_where(pipe);
+        if (pipe)
+            do_where(pipe);
+
+        else
+            std::cerr << "Failed to open pipe " << pipe_name << std::endl;
 
         running = 0;
 
@@ -398,6 +396,7 @@ static PyMethodDef echion_core_methods[] = {
     {"set_cpu", set_cpu, METH_VARARGS, "Set whether to use CPU time instead of wall time"},
     {"set_native", set_native, METH_VARARGS, "Set whether to sample the native stacks"},
     {"set_where", set_where, METH_VARARGS, "Set whether to use where mode"},
+    {"set_pipe_name", set_pipe_name, METH_VARARGS, "Set the pipe name"},
     {NULL, NULL, 0, NULL} /* Sentinel */
 };