vDSO(7) Miscellaneous Information Manual vDSO(7)
NAME
vdso - overview of the virtual ELF dynamic shared object
SYNOPSIS
#include <sys/auxv.h>
void *vdso = (uintptr_t) getauxval(AT_SYSINFO_EHDR);
DESCRIPTION
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 them-
selves 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 func-
tions 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 li-
brary 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 incompati-
ble) 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 un-
privileged 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 subse-
quent calls.
All symbols are also versioned (using the GNU version format). This al-
lows the kernel to update the function signature without breaking back-
ward 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 ex-
pect.
Typically the vDSO follows the naming convention of prefixing all sym-
bols 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.
NOTES
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
riscv linux-vdso.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), seccomp(2), and the vDSO
When tracing system calls with strace(1), symbols (system calls) that
are exported by the vDSO will not appear in the trace output. Those
system calls will likewise not be visible to seccomp(2) filters.
ARCHITECTURE-SPECIFIC NOTES
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 do-
ing 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 doc-
umentation as it's extremely detailed and covers everything you need to
know: Documentation/arm/kernel_user_helpers.rst.
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 (port removed in Linux 4.17)
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 func-
tions into a fixed location in memory. User-space applications then
call directly into that region. There is no provision for backward com-
patibility 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 doc-
umentation:
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 documenta-
tion 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 with 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 do-
ing symbol lookups or versioning. Simply call into the appropriate off-
set via the branch instruction, for example:
ble <offset>(%sr2, %r0)
offset function
────────────────────────────────────────────
00b0 lws_entry (CAS operations)
00e0 set_thread_pointer (used by glibc)
0100 linux_gateway_entry (syscall)
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_clock_gettime64 LINUX_5.11
__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
Before Linux 5.6, the CLOCK_REALTIME_COARSE and CLOCK_MONOTONIC_COARSE
clocks are not supported by the __kernel_clock_getres and __ker-
nel_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
Before Linux 4.16, the CLOCK_REALTIME_COARSE and CLOCK_MONOTONIC_COARSE
clocks are not supported by the __kernel_clock_getres and __ker-
nel_clock_gettime interfaces; the kernel falls back to the real system
call.
riscv functions
The table below lists the symbols exported by the vDSO.
symbol version
──────────────────────────────────
__vdso_rt_sigreturn LINUX_4.15
__vdso_gettimeofday LINUX_4.15
__vdso_clock_gettime LINUX_4.15
__vdso_clock_getres LINUX_4.15
__vdso_getcpu LINUX_4.15
__vdso_flush_icache LINUX_4.15
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.
SEE ALSO
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.rst
Documentation/vDSO/* (includes examples of using the vDSO)
find arch/ -iname '*vdso*' -o -iname '*gate*'
Linux man-pages 6.9.1 2024-05-02 vDSO(7)
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