.. SPDX-License-Identifier: GPL-2.0

=====================
Syscall User Dispatch
=====================

Background
----------

Compatibility layers like Wine need a way to efficiently emulate system
calls of only a part of their process - the part that has the
incompatible code - while being able to execute native syscalls without
a high performance penalty on the native part of the process.  Seccomp
falls short on this task, since it has limited support to efficiently
filter syscalls based on memory regions, and it doesn't support removing
filters.  Therefore a new mechanism is necessary.

Syscall User Dispatch brings the filtering of the syscall dispatcher
address back to userspace.  The application is in control of a flip
switch, indicating the current personality of the process.  A
multiple-personality application can then flip the switch without
invoking the kernel, when crossing the compatibility layer API
boundaries, to enable/disable the syscall redirection and execute
syscalls directly (disabled) or send them to be emulated in userspace
through a SIGSYS.

The goal of this design is to provide very quick compatibility layer
boundary crosses, which is achieved by not executing a syscall to change
personality every time the compatibility layer executes.  Instead, a
userspace memory region exposed to the kernel indicates the current
personality, and the application simply modifies that variable to
configure the mechanism.

There is a relatively high cost associated with handling signals on most
architectures, like x86, but at least for Wine, syscalls issued by
native Windows code are currently not known to be a performance problem,
since they are quite rare, at least for modern gaming applications.

Since this mechanism is designed to capture syscalls issued by
non-native applications, it must function on syscalls whose invocation
ABI is completely unexpected to Linux.  Syscall User Dispatch, therefore
doesn't rely on any of the syscall ABI to make the filtering.  It uses
only the syscall dispatcher address and the userspace key.

As the ABI of these intercepted syscalls is unknown to Linux, these
syscalls are not instrumentable via ptrace or the syscall tracepoints.

Interface
---------

A thread can setup this mechanism on supported kernels by executing the
following prctl:

  prctl(PR_SET_SYSCALL_USER_DISPATCH, <op>, <offset>, <length>, [selector])

<op> is either PR_SYS_DISPATCH_ON or PR_SYS_DISPATCH_OFF, to enable and
disable the mechanism globally for that thread.  When
PR_SYS_DISPATCH_OFF is used, the other fields must be zero.

[<offset>, <offset>+<length>) delimit a memory region interval
from which syscalls are always executed directly, regardless of the
userspace selector.  This provides a fast path for the C library, which
includes the most common syscall dispatchers in the native code
applications, and also provides a way for the signal handler to return
without triggering a nested SIGSYS on (rt\_)sigreturn.  Users of this
interface should make sure that at least the signal trampoline code is
included in this region. In addition, for syscalls that implement the
trampoline code on the vDSO, that trampoline is never intercepted.

[selector] is a pointer to a char-sized region in the process memory
region, that provides a quick way to enable disable syscall redirection
thread-wide, without the need to invoke the kernel directly.  selector
can be set to PR_SYS_DISPATCH_ON or PR_SYS_DISPATCH_OFF.  Any other
value should terminate the program with a SIGSYS.

Security Notes
--------------

Syscall User Dispatch provides functionality for compatibility layers to
quickly capture system calls issued by a non-native part of the
application, while not impacting the Linux native regions of the
process.  It is not a mechanism for sandboxing system calls, and it
should not be seen as a security mechanism, since it is trivial for a
malicious application to subvert the mechanism by jumping to an allowed
dispatcher region prior to executing the syscall, or to discover the
address and modify the selector value.  If the use case requires any
kind of security sandboxing, Seccomp should be used instead.

Any fork or exec of the existing process resets the mechanism to
PR_SYS_DISPATCH_OFF.
