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Subject[tip: core/entry] docs: Document Syscall User Dispatch
The following commit has been merged into the core/entry branch of tip:

Commit-ID: a4452e671c6770e1bb80764f39995934067f70a0
Author: Gabriel Krisman Bertazi <>
AuthorDate: Fri, 27 Nov 2020 14:32:38 -05:00
Committer: Thomas Gleixner <>
CommitterDate: Wed, 02 Dec 2020 15:07:57 +01:00

docs: Document Syscall User Dispatch

Explain the interface, provide some background and security notes.

[ tglx: Add note about non-visibility, add it to the index and fix the
kerneldoc warning ]

Signed-off-by: Gabriel Krisman Bertazi <>
Signed-off-by: Thomas Gleixner <>
Reviewed-by: Kees Cook <>
Reviewed-by: Andy Lutomirski <>
Acked-by: Peter Zijlstra (Intel) <>
Documentation/admin-guide/index.rst | 1 +-
Documentation/admin-guide/syscall-user-dispatch.rst | 90 ++++++++++++-
2 files changed, 91 insertions(+)
create mode 100644 Documentation/admin-guide/syscall-user-dispatch.rst

diff --git a/Documentation/admin-guide/index.rst b/Documentation/admin-guide/index.rst
index 4e0c4ae..b29d3c1 100644
--- a/Documentation/admin-guide/index.rst
+++ b/Documentation/admin-guide/index.rst
@@ -111,6 +111,7 @@ configure specific aspects of kernel behavior to your liking.
+ syscall-user-dispatch
diff --git a/Documentation/admin-guide/syscall-user-dispatch.rst b/Documentation/admin-guide/syscall-user-dispatch.rst
new file mode 100644
index 0000000..a380d65
--- /dev/null
+++ b/Documentation/admin-guide/syscall-user-dispatch.rst
@@ -0,0 +1,90 @@
+.. SPDX-License-Identifier: GPL-2.0
+Syscall User Dispatch
+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.
+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
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