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17-396: VDSO ( virtual dynamic shared object ) is a kernel mechanism for exporting a carefully selected set of kernel space routines to user space applications so that applications can call these kernel space routines in-process, without incurring the performance penalty of a mode switch from user mode to kernel mode that is inherent when calling these same kernel space routines by means of

34-450: A hierarchy of privileges. These architectures are often said to have ring-based security, wherein the hierarchy of privileges resembles a set of concentric rings, with the kernel mode in the center. Multics hardware was the first significant implementation of ring security, but many other hardware platforms have been designed along similar lines, including the Intel 80286 protected mode , and

51-547: A mode that prevents direct access to input/output devices and some other hardware facilities to code running in that mode. The first mode is referred to by names such as supervisor state (System/360), executive mode (PDP-6/PDP-10), master mode (GE-600 series), control mode (B5000 series), and control state (B6500 series). The second mode is referred to by names such as problem state (System/360), user mode (PDP-6/PDP-10), slave mode (GE-600 series), and normal state (B6500 series); there are multiple non-control modes in

68-523: A single address space for all user-mode code. (The kernel-mode code may be in the same address space, or it may be in a second address space). Other operating systems have a per-process address space, with a separate address space for each user-mode process. Another approach taken in experimental operating systems is to have a single address space for all software, and rely on a programming language's semantics to make sure that arbitrary memory cannot be accessed – applications cannot acquire any references to

85-569: A status word in the CPU itself, but every access to memory may additionally be validated against a separate ring number for the virtual-memory segment targeted by the access, and/or against a ring number for the physical page (if any) being targeted. This has been demonstrated with the PSP handheld system . Hardware that meets the Popek and Goldberg virtualization requirements makes writing software to efficiently support

102-602: A virtual machine much simpler. Such a system can run software that "believes" it is running in supervisor mode, but is actually running in user mode. Several computer systems introduced in the 1960s, such as the IBM System/360 , DEC PDP-6 / PDP-10 , the GE-600 / Honeywell 6000 series, and the Burroughs B5000 series and B6500 series, support two CPU modes; a mode that grants full privileges to code running in that mode, and

119-437: Is strictly reserved for running a privileged operating system kernel , kernel extensions, and most device drivers . In contrast, user space is the memory area where application software and some drivers execute, typically one address space per process. The term user space (or userland ) refers to all code that runs outside the operating system's kernel. User space usually refers to the various programs and libraries that

136-424: Is the basic design of many virtual machine systems available today. The unrestricted mode is often called kernel mode, but many other designations exist ( master mode , supervisor mode , privileged mode , etc.). Restricted modes are usually referred to as user modes, but are also known by many other names ( slave mode, problem state, etc.). Some CPU architectures support more modes than those, often with

153-503: The IA-64 as well, though it is referred to by a different name in these cases. Mode protection may extend to resources beyond the CPU hardware itself. Hardware registers track the current operating mode of the CPU, but additional virtual-memory registers, page-table entries, and other data may track mode identifiers for other resources. For example, a CPU may be operating in Ring 0 as indicated by

170-459: The system call interface. vDSO uses standard mechanisms for linking and loading i.e. standard Executable and Linkable Format (ELF) format. vDSO is a memory area allocated in user space which exposes some kernel functionalities. vDSO is dynamically allocated , offers improved safety through address space layout randomization , and supports more than four system calls. Some C standard libraries , like glibc , may provide vDSO links so that if

187-443: The CPU. For example, this design allows an operating system to run with more privileges than application software by running the operating systems and applications in different modes. Ideally, only highly trusted kernel code is allowed to execute in the unrestricted mode; everything else (including non-supervisory portions of the operating system) runs in a restricted mode and must use a system call (via interrupt ) to request

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204-533: The dynamic linker to find the vDSOs. vDSO was developed to offer the vsyscall features while overcoming its limitations: a small amount of statically allocated memory, which allows only four system calls, and the same addresses application binary interface (ABI) in each process, which compromises security. This security issue has been mitigated by emulating a virtual system call , but the emulation introduces additional latency. glibc have support for getrandom () vDSO. Kernel space Kernel space

221-451: The kernel does not have vDSO support, a traditional syscall is made. vDSO helps to reduce the calling overhead on simple kernel routines, and it also can work as a way to select the best system-call method on some computer architectures such as IA-32 . An advantage over other methods is that such exported routines can provide proper DWARF (Debug With Attributed Record Format) debugging information. Implementation generally implies hooks in

238-431: The kernel perform on its behalf any operation that could damage or compromise the system, making it impossible for untrusted programs to alter or damage other programs (or the computing system itself). Device drivers are designed to be part of the kernel due to the need for frequent I/O access. Multiple modes can be implemented, e.g. allowing a hypervisor to run multiple operating system supervisors beneath it, which

255-663: The kernel to map part of another process's memory space to its own, as is the case for debuggers . Programs can also request shared memory regions with other processes, although other techniques are also available to allow inter-process communication . The most common way of implementing a user mode separate from kernel mode involves operating system protection rings . Protection rings, in turn, are implemented using CPU modes . Typically, kernel space programs run in kernel mode , also called supervisor mode ; normal applications in user space run in user mode. Some operating systems are single address space operating systems —they have

272-529: The objects that they are not allowed to access. This approach has been implemented in JXOS , Unununium as well as Microsoft's Singularity research project. CPU modes CPU modes (also called processor modes, CPU states, CPU privilege levels and other names) are operating modes for the central processing unit of most computer architectures that place restrictions on the type and scope of operations that can be performed by instructions being executed by

289-619: The operating system uses to interact with the kernel: software that performs input/output , manipulates file system objects, application software , etc. Each user space process normally runs in its own virtual memory space, and, unless explicitly allowed, cannot access the memory of other processes. This is the basis for memory protection in today's mainstream operating systems, and a building block for privilege separation . A separate user mode can also be used to build efficient virtual machines – see Popek and Goldberg virtualization requirements . With enough privileges, processes can request

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