PR_SET_SYSCALL_USER_DISPATCH(2const) — Linux manual page



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, ...);

                 unsigned long off, unsigned long size, int8_t *switch);

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.

              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 siginfo_t structure (see
              sigaction(2)) associated with the signal:

              •  si_signo will contain SIGSYS.

              •  si_call_addr will show the address of the system call

              •  si_syscall and si_arch will indicate which system call
                 was attempted.

              •  si_code will contain SYS_USER_DISPATCH.

              •  si_errno 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), sigreturn(2), and getcontext(3) for more

              Syscall User Dispatch is disabled for that thread.

       The setting is not preserved across fork(2), clone(2), or

RETURN VALUE         top

       On success, 0 is returned.  On error, -1 is returned, and errno
       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


       For more information, see the kernel source file Documentation/

COLOPHON         top

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Linux man-pages 6.9.1          2024-06P-R0_1SET_SYSCA...SER_DISPATCH(2const)

Pages that refer to this page: prctl(2)