Why does the vDSO exist at all? There are some system calls the kernel provides that user-space code ends up using frequently, to the point that such calls can dominate overall performance. This is due both to the frequency of the call as well as the context-switch overhead that results from exiting user space and entering the kernel.
The rest of this documentation is geared toward the curious and/or C library writers rather than general developers. If you're trying to call the vDSO in your own application rather than using the C library, you're most likely doing it wrong.
Note that the terminology can be confusing. On x86 systems, the vDSO function used to determine the preferred method of making a system call is named "__kernel_vsyscall", but on x86-64, the term "vsyscall" also refers to an obsolete way to ask the kernel what time it is or what CPU the caller is on.
One frequently used system call is gettimeofday(2). This system call is called both directly by user-space applications as well as indirectly by the C library. Think timestamps or timing loops or polling---all of these frequently need to know what time it is right now. This information is also not secret---any application in any privilege mode (root or any unprivileged user) will get the same answer. Thus the kernel arranges for the information required to answer this question to be placed in memory the process can access. Now a call to gettimeofday(2) changes from a system call to a normal function call and a few memory accesses.
You must not assume the vDSO is mapped at any particular location in the user's memory map. The base address will usually be randomized at run time every time a new process image is created (at execve(2) time). This is done for security reasons, to prevent "return-to-libc" attacks.
For some architectures, there is also an AT_SYSINFO tag. This is used only for locating the vsyscall entry point and is frequently omitted or set to 0 (meaning it's not available). This tag is a throwback to the initial vDSO work (see History below) and its use should be avoided.
All symbols are also versioned (using the GNU version format). This allows the kernel to update the function signature without breaking backward compatibility. This means changing the arguments that the function accepts as well as the return value. Thus, when looking up a symbol in the vDSO, you must always include the version to match the ABI you expect.
Typically the vDSO follows the naming convention of prefixing all symbols with "__vdso_" or "__kernel_" so as to distinguish them from other standard symbols. For example, the "gettimeofday" function is named "__vdso_gettimeofday".
|user ABI||vDSO name|
Note that the vDSO that is used is based on the ABI of your user-space code and not the ABI of the kernel. Thus, for example, when you run an i386 32-bit ELF binary, you'll get the same vDSO regardless of whether you run it under an i386 32-bit kernel or under an x86-64 64-bit kernel. Therefore, the name of the user-space ABI should be used to determine which of the sections below is relevant.
|__vdso_gettimeofday||LINUX_2.6 (exported since Linux 4.1)|
|__vdso_clock_gettime||LINUX_2.6 (exported since Linux 4.1)|
Additionally, the ARM port has a code page full of utility functions. Since it's just a raw page of code, there is no ELF information for doing symbol lookups or versioning. It does provide support for different versions though.
For information on this code page,
it's best to refer to the public documentation:
The table below lists the symbols exported by the vDSO.
|__kernel_gettimeofday||LINUX_2.6 (exported since Linux 4.4)|
|__kernel_clock_gettime||LINUX_2.6 (exported since Linux 4.4)|
The Itanium port is somewhat tricky. In addition to the vDSO above, it also has "light-weight system calls" (also known as "fast syscalls" or "fsys"). You can invoke these via the __kernel_syscall_via_epc vDSO helper. The system calls listed here have the same semantics as if you called them directly via syscall(2), so refer to the relevant documentation for each. The table below lists the functions available via this mechanism.
Since it's just a raw page of code, there is no ELF information for doing symbol lookups or versioning. Simply call into the appropriate offset via the branch instruction, for example:
ble <offset>(%sr2, %r0)
|00b0||lws_entry (CAS operations)|
|00e0||set_thread_pointer (used by glibc)|
|__vdso_clock_gettime||LINUX_2.6 (exported since Linux 3.15)|
|__vdso_gettimeofday||LINUX_2.6 (exported since Linux 3.15)|
|__vdso_time||LINUX_2.6 (exported since Linux 3.15)|
The documents, examples, and source code in the Linux source code tree:
Documentation/ABI/stable/vdso Documentation/ia64/fsys.txt Documentation/vDSO/* (includes examples of using the vDSO)