PR_SET_SYSCALL_USER_DISPATCH(2const) - Linux manual page (original) (raw)

PRSETSYSCALLUSERDISPATCH(2const) PRSETSYSCALLUSERDISPATCH(2const)

NAME top

   PR_SET_SYSCALL_USER_DISPATCH - set the system-call user dispatch
   mechanism for the calling thread

LIBRARY top

   Standard C library (_libc_, _-lc_)

SYNOPSIS top

   **#include <linux/prctl.h>** /* Definition of **PR_*** constants */
   **#include <sys/prctl.h>**

   **int prctl(PR_SET_SYSCALL_USER_DISPATCH, long** _op_**, ...);**

   **int prctl(PR_SET_SYSCALL_USER_DISPATCH, PR_SYS_DISPATCH_ON,**
             **unsigned long** _off_**, unsigned long** _size_**, int8_t ***_switch_**);**
   **int prctl(PR_SET_SYSCALL_USER_DISPATCH, PR_SYS_DISPATCH_OFF, 0L, 0L, 0L);**

DESCRIPTION top

   Configure the Syscall User Dispatch mechanism for the calling
   thread.  This mechanism allows an application to selectively
   intercept system calls so that they can be handled within the
   application itself.  Interception takes the form of a thread-
   directed **SIGSYS** signal that is delivered to the thread when it
   makes a system call.  If intercepted, the system call is not
   executed by the kernel.

   **PR_SYS_DISPATCH_ON**
          Enable this mechanism.

          Once enabled, further system calls will be selectively
          intercepted, depending on a control variable provided by
          user space.  In this case, _off_ and _size_ respectively
          identify the offset and size of a single contiguous memory
          region in the process address space from where system calls
          are always allowed to be executed, regardless of the
          control variable.  (Typically, this area would include the
          area of memory containing the C library.)

          _switch_ points to a variable that is a fast switch to
          allow/block system call execution without the overhead of
          doing another system call to re-configure Syscall User
          Dispatch.  This control variable can either be set to
          **SYSCALL_DISPATCH_FILTER_BLOCK** to block system calls from
          executing or to **SYSCALL_DISPATCH_FILTER_ALLOW** to
          temporarily allow them to be executed.  This value is
          checked by the kernel on every system call entry, and any
          unexpected value will raise an uncatchable **SIGSYS** at that
          time, killing the application.

          When a system call is intercepted, the kernel sends a
          thread-directed **SIGSYS** signal to the triggering thread.
          Various fields will be set in the _siginfot_ structure (see
          [sigaction(2)](../man2/sigaction.2.html)) associated with the signal:

          •  _sisigno_ will contain **SIGSYS**.

          •  _sicalladdr_ will show the address of the system call
             instruction.

          •  _sisyscall_ and _siarch_ will indicate which system call
             was attempted.

          •  _sicode_ will contain **SYS_USER_DISPATCH**.

          •  _sierrno_ will be set to 0.

          The program counter will be as though the system call
          happened (i.e., the program counter will not point to the
          system call instruction).

          When the signal handler returns to the kernel, the system
          call completes immediately and returns to the calling
          thread, without actually being executed.  If necessary
          (i.e., when emulating the system call on user space.), the
          signal handler should set the system call return value to a
          sane value, by modifying the register context stored in the
          _ucontext_ argument of the signal handler.  See [sigaction(2)](../man2/sigaction.2.html),
          [sigreturn(2)](../man2/sigreturn.2.html), and [getcontext(3)](../man3/getcontext.3.html) for more information.

   **PR_SYS_DISPATCH_OFF**
          Syscall User Dispatch is disabled for that thread.

   The setting is not preserved across [fork(2)](../man2/fork.2.html), [clone(2)](../man2/clone.2.html), or
   [execve(2)](../man2/execve.2.html).

RETURN VALUE top

   On success, 0 is returned.  On error, -1 is returned, and _[errno](../man3/errno.3.html)_ is
   set to indicate the error.

ERRORS top

   **EFAULT** _switch_ is an invalid address.

   **EINVAL** _op_ is **PR_SYS_DISPATCH_ON** and the memory range specified is
          outside the address space of the process.

   **EINVAL** _op_ is invalid.

STANDARDS top

   Linux.  x86 only.

HISTORY top

   Linux 5.11 (x86).

SEE ALSO top

   [prctl(2)](../man2/prctl.2.html)

   For more information, see the kernel source file _Documentation/_
   _admin-guide/syscall-user-dispatch.rst_

COLOPHON top

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Linux man-pages 6.10 2024-0P_7_R-__2_S_3_ETSYSCALLUSERDISPATCH(2const)

Pages that refer to this page:prctl(2)