vDSO (virtual dynamic shared object) is a kernel mechanism for exporting a carefully selected set of kernel space routines to

user space A modern computer operating system usually segregates virtual memory into user space and kernel space. Primarily, this separation serves to provide memory protection and hardware protection from malicious or errant software behaviour. Kernel ...

applications so that applications can call these kernel space routines in-process, without incurring the performance penalty of a mode switch from

user mode A modern computer operating system usually segregates virtual memory into user space and kernel space. Primarily, this separation serves to provide memory protection and hardware protection from malicious or errant software behaviour. Kernel ...

kernel mode In computer science, hierarchical protection domains, often called protection rings, are mechanisms to protect data and functionality from faults (by improving fault tolerance) and malicious behavior (by providing computer security). Computer ...

that is inherent when calling these same kernel space routines by means of the

system call In computing, a system call (commonly abbreviated to syscall) is the programmatic way in which a computer program requests a service from the operating system on which it is executed. This may include hardware-related services (for example, acc ...

interface. vDSO uses standard mechanisms for linking and loading i.e. standard

Executable and Linkable Format In computing, the Executable and Linkable FormatTool Interface Standard (TIS) Portable Formats SpecificationVersion 1.1'' (October 1993) (ELF, formerly named Extensible Linking Format), is a common standard file format for executable files, ...

(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 Address space layout randomization (ASLR) is a computer security technique involved in preventing exploitation of memory corruption vulnerabilities. In order to prevent an attacker from reliably jumping to, for example, a particular exploited fu ...

, and supports more than four system calls. Some

C standard libraries The C standard library or libc is the standard library for the C programming language, as specified in the ISO C standard. ISO/IEC (2018). '' ISO/IEC 9899:2018(E): Programming Languages - C §7'' Starting from the original ANSI C standard, it was ...

, like

glibc The GNU C Library, commonly known as glibc, is the GNU Project's implementation of the C standard library. Despite its name, it now also directly supports C++ (and, indirectly, other programming languages). It was started in the 1980s by ...

, may provide vDSO links so that if the kernel does not have vDSO support, a traditional

syscall In computing, a system call (commonly abbreviated to syscall) is the programmatic way in which a computer program requests a service from the operating system on which it is executed. This may include hardware-related services (for example, acc ...

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 In computer engineering, computer architecture is a description of the structure of a computer system made from component parts. It can sometimes be a high-level description that ignores details of the implementation. At a more detailed level, the ...

such as

IA-32 IA-32 (short for "Intel Architecture, 32-bit", commonly called i386) is the 32-bit version of the x86 instruction set architecture, designed by Intel and first implemented in the 80386 microprocessor in 1985. IA-32 is the first incarnation o ...

. An advantage over other methods is that such exported routines can provide proper

DWARF Dwarf or dwarves may refer to: Common uses *Dwarf (folklore), a being from Germanic mythology and folklore * Dwarf, a person or animal with dwarfism Arts, entertainment, and media Fictional entities * Dwarf (''Dungeons & Dragons''), a humanoid ...

(Debug With Attributed Record Format) debugging information. Implementation generally implies hooks in 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 In computer software, an application binary interface (ABI) is an interface between two binary program modules. Often, one of these modules is a library or operating system facility, and the other is a program that is being run by a user. An ' ...

(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.

References

{{Reflist Linux kernel features