PR_SET_SYSCALL_USER_DISPATCH(2const) PR_SET_SYSCALL_USER_DISPATCH(2const)
NAME
PR_SET_SYSCALL_USER_DISPATCH - set the system-call user dispatch mecha-
nism for the calling thread
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#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
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. Inter-
ception takes the form of a thread-directed SIGSYS signal that is deliv-
ered 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 inter-
cepted, 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 sys-
tem 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_FIL-
TER_ALLOW to temporarily allow them to be executed. This value
is checked by the kernel on every system call entry, and any un-
expected value will raise an uncatchable SIGSYS at that time,
killing the application.
When a system call is intercepted, the kernel sends a thread-di-
rected SIGSYS signal to the triggering thread. Various fields
will be set in the siginfo_t structure (see sigaction(2)) associ-
ated with the signal:
• si_signo will contain SIGSYS.
• si_call_addr will show the address of the system call instruc-
tion.
• si_syscall and si_arch will indicate which system call was at-
tempted.
• 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 in-
struction).
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 regis-
ter context stored in the ucontext argument of the signal han-
dler. See sigaction(2), sigreturn(2), and getcontext(3) for more
information.
PR_SYS_DISPATCH_OFF
Syscall User Dispatch is disabled for that thread.
The setting is not preserved across fork(2), clone(2), or execve(2).
RETURN VALUE
On success, 0 is returned. On error, -1 is returned, and errno is set
to indicate the error.
ERRORS
EFAULT switch is an invalid address.
EINVAL op is PR_SYS_DISPATCH_ON and the memory range specified is out-
side the address space of the process.
EINVAL op is invalid.
STANDARDS
Linux. x86 only.
HISTORY
Linux 5.11 (x86).
SEE ALSO
prctl(2)
For more information, see the kernel source file Documentation/ad-
min-guide/syscall-user-dispatch.rst
Linux man-pages 6.9.1 2024-06-PR_SET_SYSCALL_USER_DISPATCH(2const)
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