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hhŒ	_document”hŒsource”NŒline”NubhŒcomment”“”)”}”(hŒ SPDX-License-Identifier: GPL-2.0”h]”hŒ SPDX-License-Identifier: GPL-2.0”…””}”hh£sbah}”(h]”h ]”h"]”h$]”h&]”Œ	xml:space”Œpreserve”uh1h¡hhhžhhŸŒO/var/lib/git/docbuild/linux/Documentation/admin-guide/syscall-user-dispatch.rst”h KubhŒsection”“”)”}”(hhh]”(hŒtitle”“”)”}”(hŒSyscall User Dispatch”h]”hŒSyscall User Dispatch”…””}”(hh»hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h¹hh¶hžhhŸh³h Kubhµ)”}”(hhh]”(hº)”}”(hŒ
Background”h]”hŒ
Background”…””}”(hhÌhžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h¹hhÉhžhhŸh³h KubhŒ	paragraph”“”)”}”(hXÕ  Compatibility layers like Wine need a way to efficiently emulate system
calls of only a part of their process - the part that has the
incompatible code - while being able to execute native syscalls without
a high performance penalty on the native part of the process.  Seccomp
falls short on this task, since it has limited support to efficiently
filter syscalls based on memory regions, and it doesn't support removing
filters.  Therefore a new mechanism is necessary.”h]”hX×  Compatibility layers like Wine need a way to efficiently emulate system
calls of only a part of their process - the part that has the
incompatible code - while being able to execute native syscalls without
a high performance penalty on the native part of the process.  Seccomp
falls short on this task, since it has limited support to efficiently
filter syscalls based on memory regions, and it doesnâ€™t support removing
filters.  Therefore a new mechanism is necessary.”…””}”(hhÜhžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÚhŸh³h K
hhÉhžhubhÛ)”}”(hXá  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.”h]”hXá  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.”…””}”(hhêhžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÚhŸh³h KhhÉhžhubhÛ)”}”(hXq  The goal of this design is to provide very quick compatibility layer
boundary crosses, which is achieved by not executing a syscall to change
personality every time the compatibility layer executes.  Instead, a
userspace memory region exposed to the kernel indicates the current
personality, and the application simply modifies that variable to
configure the mechanism.”h]”hXq  The goal of this design is to provide very quick compatibility layer
boundary crosses, which is achieved by not executing a syscall to change
personality every time the compatibility layer executes.  Instead, a
userspace memory region exposed to the kernel indicates the current
personality, and the application simply modifies that variable to
configure the mechanism.”…””}”(hhøhžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÚhŸh³h KhhÉhžhubhÛ)”}”(hX  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.”h]”hX  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.”…””}”(hj  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÚhŸh³h K"hhÉhžhubhÛ)”}”(hXP  Since this mechanism is designed to capture syscalls issued by
non-native applications, it must function on syscalls whose invocation
ABI is completely unexpected to Linux.  Syscall User Dispatch, therefore
doesn't rely on any of the syscall ABI to make the filtering.  It uses
only the syscall dispatcher address and the userspace key.”h]”hXR  Since this mechanism is designed to capture syscalls issued by
non-native applications, it must function on syscalls whose invocation
ABI is completely unexpected to Linux.  Syscall User Dispatch, therefore
doesnâ€™t rely on any of the syscall ABI to make the filtering.  It uses
only the syscall dispatcher address and the userspace key.”…””}”(hj  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÚhŸh³h K'hhÉhžhubhÛ)”}”(hŒŠAs the ABI of these intercepted syscalls is unknown to Linux, these
syscalls are not instrumentable via ptrace or the syscall tracepoints.”h]”hŒŠAs the ABI of these intercepted syscalls is unknown to Linux, these
syscalls are not instrumentable via ptrace or the syscall tracepoints.”…””}”(hj"  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÚhŸh³h K-hhÉhžhubeh}”(h]”Œ
background”ah ]”h"]”Œ
background”ah$]”h&]”uh1h´hh¶hžhhŸh³h Kubhµ)”}”(hhh]”(hº)”}”(hŒ	Interface”h]”hŒ	Interface”…””}”(hj;  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h¹hj8  hžhhŸh³h K1ubhÛ)”}”(hŒXA thread can setup this mechanism on supported kernels by executing the
following prctl:”h]”hŒXA thread can setup this mechanism on supported kernels by executing the
following prctl:”…””}”(hjI  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÚhŸh³h K3hj8  hžhubhŒblock_quote”“”)”}”(hŒJprctl(PR_SET_SYSCALL_USER_DISPATCH, <op>, <offset>, <length>, [selector])
”h]”hÛ)”}”(hŒIprctl(PR_SET_SYSCALL_USER_DISPATCH, <op>, <offset>, <length>, [selector])”h]”hŒIprctl(PR_SET_SYSCALL_USER_DISPATCH, <op>, <offset>, <length>, [selector])”…””}”(hj]  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÚhŸh³h K6hjY  ubah}”(h]”h ]”h"]”h$]”h&]”uh1jW  hŸh³h K6hj8  hžhubhÛ)”}”(hŒà<op> is either PR_SYS_DISPATCH_EXCLUSIVE_ON/PR_SYS_DISPATCH_INCLUSIVE_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.”h]”hŒà<op> is either PR_SYS_DISPATCH_EXCLUSIVE_ON/PR_SYS_DISPATCH_INCLUSIVE_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.”…””}”(hjq  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÚhŸh³h K8hj8  hžhubhÛ)”}”(hXˆ  For PR_SYS_DISPATCH_EXCLUSIVE_ON [<offset>, <offset>+<length>) delimit
a memory region interval from which syscalls are always executed directly,
regardless of the userspace selector.  This provides a fast path for the
C library, which includes the most common syscall dispatchers in the native
code applications, and also provides a way for the signal handler to return
without triggering a nested SIGSYS on (rt\_)sigreturn.  Users of this
interface should make sure that at least the signal trampoline code is
included in this region. In addition, for syscalls that implement the
trampoline code on the vDSO, that trampoline is never intercepted.”h]”hXˆ  For PR_SYS_DISPATCH_EXCLUSIVE_ON [<offset>, <offset>+<length>) delimit
a memory region interval from which syscalls are always executed directly,
regardless of the userspace selector.  This provides a fast path for the
C library, which includes the most common syscall dispatchers in the native
code applications, and also provides a way for the signal handler to return
without triggering a nested SIGSYS on (rt _)sigreturn.  Users of this
interface should make sure that at least the signal trampoline code is
included in this region. In addition, for syscalls that implement the
trampoline code on the vDSO, that trampoline is never intercepted.”…””}”(hj  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÚhŸh³h K<hj8  hžhubhÛ)”}”(hŒäFor PR_SYS_DISPATCH_INCLUSIVE_ON [<offset>, <offset>+<length>) delimit
a memory region interval from which syscalls are dispatched based on
the userspace selector. Syscalls from outside of the range are always
executed directly.”h]”hŒäFor PR_SYS_DISPATCH_INCLUSIVE_ON [<offset>, <offset>+<length>) delimit
a memory region interval from which syscalls are dispatched based on
the userspace selector. Syscalls from outside of the range are always
executed directly.”…””}”(hj  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÚhŸh³h KFhj8  hžhubhÛ)”}”(hX]  [selector] is a pointer to a char-sized region in the process memory
region, that provides a quick way to enable disable syscall redirection
thread-wide, without the need to invoke the kernel directly.  selector
can be set to SYSCALL_DISPATCH_FILTER_ALLOW or SYSCALL_DISPATCH_FILTER_BLOCK.
Any other value should terminate the program with a SIGSYS.”h]”hX]  [selector] is a pointer to a char-sized region in the process memory
region, that provides a quick way to enable disable syscall redirection
thread-wide, without the need to invoke the kernel directly.  selector
can be set to SYSCALL_DISPATCH_FILTER_ALLOW or SYSCALL_DISPATCH_FILTER_BLOCK.
Any other value should terminate the program with a SIGSYS.”…””}”(hj›  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÚhŸh³h KKhj8  hžhubhÛ)”}”(hŒÈAdditionally, a tasks syscall user dispatch configuration can be peeked
and poked via the PTRACE_(GET|SET)_SYSCALL_USER_DISPATCH_CONFIG ptrace
requests. This is useful for checkpoint/restart software.”h]”hŒÈAdditionally, a tasks syscall user dispatch configuration can be peeked
and poked via the PTRACE_(GET|SET)_SYSCALL_USER_DISPATCH_CONFIG ptrace
requests. This is useful for checkpoint/restart software.”…””}”(hj©  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÚhŸh³h KQhj8  hžhubeh}”(h]”Œ	interface”ah ]”h"]”Œ	interface”ah$]”h&]”uh1h´hh¶hžhhŸh³h K1ubhµ)”}”(hhh]”(hº)”}”(hŒSecurity Notes”h]”hŒSecurity Notes”…””}”(hjÂ  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h¹hj¿  hžhhŸh³h KVubhÛ)”}”(hXb  Syscall User Dispatch provides functionality for compatibility layers to
quickly capture system calls issued by a non-native part of the
application, while not impacting the Linux native regions of the
process.  It is not a mechanism for sandboxing system calls, and it
should not be seen as a security mechanism, since it is trivial for a
malicious application to subvert the mechanism by jumping to an allowed
dispatcher region prior to executing the syscall, or to discover the
address and modify the selector value.  If the use case requires any
kind of security sandboxing, Seccomp should be used instead.”h]”hXb  Syscall User Dispatch provides functionality for compatibility layers to
quickly capture system calls issued by a non-native part of the
application, while not impacting the Linux native regions of the
process.  It is not a mechanism for sandboxing system calls, and it
should not be seen as a security mechanism, since it is trivial for a
malicious application to subvert the mechanism by jumping to an allowed
dispatcher region prior to executing the syscall, or to discover the
address and modify the selector value.  If the use case requires any
kind of security sandboxing, Seccomp should be used instead.”…””}”(hjÐ  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÚhŸh³h KXhj¿  hžhubhÛ)”}”(hŒUAny fork or exec of the existing process resets the mechanism to
PR_SYS_DISPATCH_OFF.”h]”hŒUAny fork or exec of the existing process resets the mechanism to
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