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NAME | SYNOPSIS | DESCRIPTION | NOTES | ARCHITECTURE-SPECIFIC NOTES | SEE ALSO | COLOPHON |
VDSO(7) Linux Programmer's Manual VDSO(7)
vdso - overview of the virtual ELF dynamic shared object
#include <sys/auxv.h>
void *vdso = (uintptr_t) getauxval(AT_SYSINFO_EHDR);
The "vDSO" (virtual dynamic shared object) is a small shared library
that the kernel automatically maps into the address space of all
user-space applications. Applications usually do not need to concern
themselves with these details as the vDSO is most commonly called by
the C library. This way you can code in the normal way using
standard functions and the C library will take care of using any
functionality that is available via the vDSO.
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.
Example background
Making system calls can be slow. In x86 32-bit systems, you can
trigger a software interrupt (int $0x80) to tell the kernel you wish
to make a system call. However, this instruction is expensive: it
goes through the full interrupt-handling paths in the processor's
microcode as well as in the kernel. Newer processors have faster
(but backward incompatible) instructions to initiate system calls.
Rather than require the C library to figure out if this functionality
is available at run time, the C library can use functions provided by
the kernel in the vDSO.
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.
Finding the vDSO
The base address of the vDSO (if one exists) is passed by the kernel
to each program in the initial auxiliary vector (see getauxval(3)),
via the AT_SYSINFO_EHDR tag.
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.
File format
Since the vDSO is a fully formed ELF image, you can do symbol lookups
on it. This allows new symbols to be added with newer kernel
releases, and allows the C library to detect available functionality
at run time when running under different kernel versions. Oftentimes
the C library will do detection with the first call and then cache
the result for subsequent calls.
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".
You use the standard C calling conventions when calling any of these
functions. No need to worry about weird register or stack behavior.
Source
When you compile the kernel, it will automatically compile and link
the vDSO code for you. You will frequently find it under the
architecture-specific directory:
find arch/$ARCH/ -name '*vdso*.so*' -o -name '*gate*.so*'
vDSO names
The name of the vDSO varies across architectures. It will often show
up in things like glibc's ldd(1) output. The exact name should not
matter to any code, so do not hardcode it.
user ABI vDSO name
─────────────────────────────
aarch64 linux-vdso.so.1
arm linux-vdso.so.1
ia64 linux-gate.so.1
mips linux-vdso.so.1
ppc/32 linux-vdso32.so.1
ppc/64 linux-vdso64.so.1
s390 linux-vdso32.so.1
s390x linux-vdso64.so.1
sh linux-gate.so.1
i386 linux-gate.so.1
x86_64 linux-vdso.so.1
x86/x32 linux-vdso.so.1
strace(1) and the vDSO
When tracing systems calls with strace(1), symbols (system calls)
that are exported by the vDSO will not appear in the trace output.
The subsections below provide architecture-specific notes on the
vDSO.
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.
ARM functions
The table below lists the symbols exported by the vDSO.
symbol version
────────────────────────────────────────────────────────────
__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 kernel
documentation as it's extremely detailed and covers everything you
need to know: Documentation/arm/kernel_user_helpers.txt.
aarch64 functions
The table below lists the symbols exported by the vDSO.
symbol version
──────────────────────────────────────
__kernel_rt_sigreturn LINUX_2.6.39
__kernel_gettimeofday LINUX_2.6.39
__kernel_clock_gettime LINUX_2.6.39
__kernel_clock_getres LINUX_2.6.39
bfin (Blackfin) functions
As this CPU lacks a memory management unit (MMU), it doesn't set up a
vDSO in the normal sense. Instead, it maps at boot time a few raw
functions into a fixed location in memory. User-space applications
then call directly into that region. There is no provision for
backward compatibility beyond sniffing raw opcodes, but as this is an
embedded CPU, it can get away with things—some of the object formats
it runs aren't even ELF based (they're bFLT/FLAT).
For information on this code page, it's best to refer to the public
documentation:
http://docs.blackfin.uclinux.org/doku.php?id=linux-kernel:fixed-code
mips functions
The table below lists the symbols exported by the vDSO.
symbol version
──────────────────────────────────────────────────────────────
__kernel_gettimeofday LINUX_2.6 (exported since Linux 4.4)
__kernel_clock_gettime LINUX_2.6 (exported since Linux 4.4)
ia64 (Itanium) functions
The table below lists the symbols exported by the vDSO.
symbol version
───────────────────────────────────────
__kernel_sigtramp LINUX_2.5
__kernel_syscall_via_break LINUX_2.5
__kernel_syscall_via_epc LINUX_2.5
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.
function
────────────────
clock_gettime
getcpu
getpid
getppid
gettimeofday
set_tid_address
parisc (hppa) functions
The parisc port has a code page full of utility functions called a
gateway page. Rather than use the normal ELF auxiliary vector
approach, it passes the address of the page to the process via the
SR2 register. The permissions on the page are such that merely
executing those addresses automatically executes with kernel
privileges and not in user space. This is done to match the way HP-
UX works.
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)
offset function
───────────────────────────────────────
00b0 lws_entry
00e0 set_thread_pointer
0100 linux_gateway_entry (syscall)
0268 syscall_nosys
0274 tracesys
0324 tracesys_next
0368 tracesys_exit
03a0 tracesys_sigexit
03b8 lws_start
03dc lws_exit_nosys
03e0 lws_exit
03e4 lws_compare_and_swap64
03e8 lws_compare_and_swap
0404 cas_wouldblock
0410 cas_action
ppc/32 functions
The table below lists the symbols exported by the vDSO. The
functions marked with a * are available only when the kernel is a
PowerPC64 (64-bit) kernel.
symbol version
────────────────────────────────────────
__kernel_clock_getres LINUX_2.6.15
__kernel_clock_gettime LINUX_2.6.15
__kernel_datapage_offset LINUX_2.6.15
__kernel_get_syscall_map LINUX_2.6.15
__kernel_get_tbfreq LINUX_2.6.15
__kernel_getcpu * LINUX_2.6.15
__kernel_gettimeofday LINUX_2.6.15
__kernel_sigtramp_rt32 LINUX_2.6.15
__kernel_sigtramp32 LINUX_2.6.15
__kernel_sync_dicache LINUX_2.6.15
__kernel_sync_dicache_p5 LINUX_2.6.15
The CLOCK_REALTIME_COARSE and CLOCK_MONOTONIC_COARSE clocks are not
supported by the __kernel_clock_getres and __kernel_clock_gettime
interfaces; the kernel falls back to the real system call.
ppc/64 functions
The table below lists the symbols exported by the vDSO.
symbol version
────────────────────────────────────────
__kernel_clock_getres LINUX_2.6.15
__kernel_clock_gettime LINUX_2.6.15
__kernel_datapage_offset LINUX_2.6.15
__kernel_get_syscall_map LINUX_2.6.15
__kernel_get_tbfreq LINUX_2.6.15
__kernel_getcpu LINUX_2.6.15
__kernel_gettimeofday LINUX_2.6.15
__kernel_sigtramp_rt64 LINUX_2.6.15
__kernel_sync_dicache LINUX_2.6.15
__kernel_sync_dicache_p5 LINUX_2.6.15
The CLOCK_REALTIME_COARSE and CLOCK_MONOTONIC_COARSE clocks are not
supported by the __kernel_clock_getres and __kernel_clock_gettime
interfaces; the kernel falls back to the real system call.
s390 functions
The table below lists the symbols exported by the vDSO.
symbol version
──────────────────────────────────────
__kernel_clock_getres LINUX_2.6.29
__kernel_clock_gettime LINUX_2.6.29
__kernel_gettimeofday LINUX_2.6.29
s390x functions
The table below lists the symbols exported by the vDSO.
symbol version
──────────────────────────────────────
__kernel_clock_getres LINUX_2.6.29
__kernel_clock_gettime LINUX_2.6.29
__kernel_gettimeofday LINUX_2.6.29
sh (SuperH) functions
The table below lists the symbols exported by the vDSO.
symbol version
──────────────────────────────────
__kernel_rt_sigreturn LINUX_2.6
__kernel_sigreturn LINUX_2.6
__kernel_vsyscall LINUX_2.6
i386 functions
The table below lists the symbols exported by the vDSO.
symbol version
──────────────────────────────────────────────────────────────
__kernel_sigreturn LINUX_2.5
__kernel_rt_sigreturn LINUX_2.5
__kernel_vsyscall LINUX_2.5
__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)
x86_64 functions
The table below lists the symbols exported by the vDSO. All of these
symbols are also available without the "__vdso_" prefix, but you
should ignore those and stick to the names below.
symbol version
─────────────────────────────────
__vdso_clock_gettime LINUX_2.6
__vdso_getcpu LINUX_2.6
__vdso_gettimeofday LINUX_2.6
__vdso_time LINUX_2.6
x86/x32 functions
The table below lists the symbols exported by the vDSO.
symbol version
─────────────────────────────────
__vdso_clock_gettime LINUX_2.6
__vdso_getcpu LINUX_2.6
__vdso_gettimeofday LINUX_2.6
__vdso_time LINUX_2.6
History
The vDSO was originally just a single function—the vsyscall. In
older kernels, you might see that name in a process's memory map
rather than "vdso". Over time, people realized that this mechanism
was a great way to pass more functionality to user space, so it was
reconceived as a vDSO in the current format.
syscalls(2), getauxval(3), proc(5)
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)
find arch/ -iname '*vdso*' -o -iname '*gate*'
This page is part of release 4.12 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2016-05-09 VDSO(7)
Pages that refer to this page: ldd(1), clock_getres(2), getcpu(2), gettimeofday(2), getunwind(2), sigreturn(2), syscall(2), syscalls(2), time(2), getauxval(3), proc(5), libc(7)