'\" t
.\" Written by Mike Frysinger <vapier@gentoo.org>
.\"
.\" %%%LICENSE_START(PUBLIC_DOMAIN)
.\" This page is in the public domain.
.\" %%%LICENSE_END
.\"
.\" Useful background:
.\"   http://articles.manugarg.com/systemcallinlinux2_6.html
.\"   https://lwn.net/Articles/446528/
.\"   http://www.linuxjournal.com/content/creating-vdso-colonels-other-chicken
.\"   http://www.trilithium.com/johan/2005/08/linux-gate/
.\"
.TH vDSO 7 2024-05-02 "Linux man-pages (unreleased)"
.SH NAME
vdso \- overview of the virtual ELF dynamic shared object
.SH SYNOPSIS
.nf
.B #include <sys/auxv.h>
.P
.B void *vdso = (uintptr_t) getauxval(AT_SYSINFO_EHDR);
.fi
.SH 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 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.
.P
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.
.P
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.
.SS Example background
Making system calls can be slow.
In x86 32-bit systems, you can trigger a software interrupt
.RI ( "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.
.P
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.
.P
One frequently used system call is
.BR 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\[em]all of these
frequently need to know what time it is right now.
This information is also not secret\[em]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
.BR gettimeofday (2)
changes from a system call to a normal function
call and a few memory accesses.
.SS 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
.BR getauxval (3)),
via the
.B AT_SYSINFO_EHDR
tag.
.P
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
.BR execve (2)
time).
This is done for security reasons,
to prevent "return-to-libc" attacks.
.P
For some architectures, there is also an
.B 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
.I History
below) and its use should be avoided.
.SS 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.
.P
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.
.P
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".
.P
You use the standard C calling conventions when calling
any of these functions.
No need to worry about weird register or stack behavior.
.SH NOTES
.SS 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:
.P
.in +4n
.EX
find arch/$ARCH/ \-name \[aq]*vdso*.so*\[aq] \-o \-name \[aq]*gate*.so*\[aq]
.EE
.in
.\"
.SS vDSO names
The name of the vDSO varies across architectures.
It will often show up in things like glibc's
.BR ldd (1)
output.
The exact name should not matter to any code, so do not hardcode it.
.if t \{\
.ft CW
\}
.TS
l l.
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
.TE
.if t \{\
.in
.ft P
\}
.SS strace(1), seccomp(2), and the vDSO
When tracing system calls with
.BR strace (1),
symbols (system calls) that are exported by the vDSO will
.I not
appear in the trace output.
Those system calls will likewise not be visible to
.BR seccomp (2)
filters.
.SH ARCHITECTURE-SPECIFIC NOTES
The subsections below provide architecture-specific notes
on the vDSO.
.P
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.
.SS ARM functions
.\" See linux/arch/arm/vdso/vdso.lds.S
.\" Commit: 8512287a8165592466cb9cb347ba94892e9c56a5
The table below lists the symbols exported by the vDSO.
.if t \{\
.ft CW
\}
.TS
l l.
symbol	version
_
__vdso_gettimeofday	LINUX_2.6 (exported since Linux 4.1)
__vdso_clock_gettime	LINUX_2.6 (exported since Linux 4.1)
.TE
.if t \{\
.in
.ft P
\}
.P
.\" See linux/arch/arm/kernel/entry-armv.S
.\" See linux/Documentation/arm/kernel_user_helpers.rst
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.
.P
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:
.IR Documentation/arm/kernel_user_helpers.rst .
.SS aarch64 functions
.\" See linux/arch/arm64/kernel/vdso/vdso.lds.S
The table below lists the symbols exported by the vDSO.
.if t \{\
.ft CW
\}
.TS
l l.
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
.TE
.if t \{\
.in
.ft P
\}
.SS bfin (Blackfin) functions (port removed in Linux 4.17)
.\" See linux/arch/blackfin/kernel/fixed_code.S
.\" See http://docs.blackfin.uclinux.org/doku.php?id=linux-kernel:fixed-code
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\[em]some of the
object formats it runs aren't even ELF based (they're bFLT/FLAT).
.P
For information on this code page,
it's best to refer to the public documentation:
.br
http://docs.blackfin.uclinux.org/doku.php?id=linux\-kernel:fixed\-code
.SS mips functions
.\" See linux/arch/mips/vdso/vdso.ld.S
The table below lists the symbols exported by the vDSO.
.if t \{\
.ft CW
\}
.TS
l l.
symbol	version
_
__kernel_gettimeofday	LINUX_2.6 (exported since Linux 4.4)
__kernel_clock_gettime	LINUX_2.6 (exported since Linux 4.4)
.TE
.if t \{\
.in
.ft P
\}
.SS ia64 (Itanium) functions
.\" See linux/arch/ia64/kernel/gate.lds.S
.\" Also linux/arch/ia64/kernel/fsys.S and linux/Documentation/ia64/fsys.rst
The table below lists the symbols exported by the vDSO.
.if t \{\
.ft CW
\}
.TS
l l.
symbol	version
_
__kernel_sigtramp	LINUX_2.5
__kernel_syscall_via_break	LINUX_2.5
__kernel_syscall_via_epc	LINUX_2.5
.TE
.if t \{\
.in
.ft P
\}
.P
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
.I __kernel_syscall_via_epc
vDSO helper.
The system calls listed here have the same semantics as if you called them
directly via
.BR syscall (2),
so refer to the relevant
documentation for each.
The table below lists the functions available via this mechanism.
.if t \{\
.ft CW
\}
.TS
l.
function
_
clock_gettime
getcpu
getpid
getppid
gettimeofday
set_tid_address
.TE
.if t \{\
.in
.ft P
\}
.SS parisc (hppa) functions
.\" See linux/arch/parisc/kernel/syscall.S
.\" See linux/Documentation/parisc/registers.rst
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.
.P
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:
.P
.in +4n
.EX
ble <offset>(%sr2, %r0)
.EE
.in
.if t \{\
.ft CW
\}
.TS
l l.
offset	function
_
00b0	lws_entry (CAS operations)
00e0	set_thread_pointer (used by glibc)
0100	linux_gateway_entry (syscall)
.TE
.if t \{\
.in
.ft P
\}
.SS ppc/32 functions
.\" See linux/arch/powerpc/kernel/vdso32/vdso32.lds.S
The table below lists the symbols exported by the vDSO.
The functions marked with a
.I *
are available only when the kernel is
a PowerPC64 (64-bit) kernel.
.if t \{\
.ft CW
\}
.TS
l l.
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 \fI*\fR	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
.TE
.if t \{\
.in
.ft P
\}
.P
Before Linux 5.6,
.\" commit 654abc69ef2e69712e6d4e8a6cb9292b97a4aa39
the
.B CLOCK_REALTIME_COARSE
and
.B CLOCK_MONOTONIC_COARSE
clocks are
.I not
supported by the
.I __kernel_clock_getres
and
.I __kernel_clock_gettime
interfaces;
the kernel falls back to the real system call.
.SS ppc/64 functions
.\" See linux/arch/powerpc/kernel/vdso64/vdso64.lds.S
The table below lists the symbols exported by the vDSO.
.if t \{\
.ft CW
\}
.TS
l l.
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
.TE
.if t \{\
.in
.ft P
\}
.P
Before Linux 4.16,
.\" commit 5c929885f1bb4b77f85b1769c49405a0e0f154a1
the
.B CLOCK_REALTIME_COARSE
and
.B CLOCK_MONOTONIC_COARSE
clocks are
.I not
supported by the
.I __kernel_clock_getres
and
.I __kernel_clock_gettime
interfaces;
the kernel falls back to the real system call.
.SS riscv functions
.\" See linux/arch/riscv/kernel/vdso/vdso.lds.S
The table below lists the symbols exported by the vDSO.
.if t \{\
.ft CW
\}
.TS
l l.
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
.TE
.if t \{\
.in
.ft P
\}
.SS s390 functions
.\" See linux/arch/s390/kernel/vdso32/vdso32.lds.S
The table below lists the symbols exported by the vDSO.
.if t \{\
.ft CW
\}
.TS
l l.
symbol	version
_
__kernel_clock_getres	LINUX_2.6.29
__kernel_clock_gettime	LINUX_2.6.29
__kernel_gettimeofday	LINUX_2.6.29
.TE
.if t \{\
.in
.ft P
\}
.SS s390x functions
.\" See linux/arch/s390/kernel/vdso64/vdso64.lds.S
The table below lists the symbols exported by the vDSO.
.if t \{\
.ft CW
\}
.TS
l l.
symbol	version
_
__kernel_clock_getres	LINUX_2.6.29
__kernel_clock_gettime	LINUX_2.6.29
__kernel_gettimeofday	LINUX_2.6.29
.TE
.if t \{\
.in
.ft P
\}
.SS sh (SuperH) functions
.\" See linux/arch/sh/kernel/vsyscall/vsyscall.lds.S
The table below lists the symbols exported by the vDSO.
.if t \{\
.ft CW
\}
.TS
l l.
symbol	version
_
__kernel_rt_sigreturn	LINUX_2.6
__kernel_sigreturn	LINUX_2.6
__kernel_vsyscall	LINUX_2.6
.TE
.if t \{\
.in
.ft P
\}
.SS i386 functions
.\" See linux/arch/x86/vdso/vdso32/vdso32.lds.S
The table below lists the symbols exported by the vDSO.
.if t \{\
.ft CW
\}
.TS
l l.
symbol	version
_
__kernel_sigreturn	LINUX_2.5
__kernel_rt_sigreturn	LINUX_2.5
__kernel_vsyscall	LINUX_2.5
.\" Added in 7a59ed415f5b57469e22e41fc4188d5399e0b194 and updated
.\" in 37c975545ec63320789962bf307f000f08fabd48.
__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)
.TE
.if t \{\
.in
.ft P
\}
.SS x86-64 functions
.\" See linux/arch/x86/vdso/vdso.lds.S
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.
.if t \{\
.ft CW
\}
.TS
l l.
symbol	version
_
__vdso_clock_gettime	LINUX_2.6
__vdso_getcpu	LINUX_2.6
__vdso_gettimeofday	LINUX_2.6
__vdso_time	LINUX_2.6
.TE
.if t \{\
.in
.ft P
\}
.SS x86/x32 functions
.\" See linux/arch/x86/vdso/vdso32.lds.S
The table below lists the symbols exported by the vDSO.
.if t \{\
.ft CW
\}
.TS
l l.
symbol	version
_
__vdso_clock_gettime	LINUX_2.6
__vdso_getcpu	LINUX_2.6
__vdso_gettimeofday	LINUX_2.6
__vdso_time	LINUX_2.6
.TE
.if t \{\
.in
.ft P
\}
.SS History
The vDSO was originally just a single function\[em]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.
.SH SEE ALSO
.BR syscalls (2),
.BR getauxval (3),
.BR proc (5)
.P
The documents, examples, and source code in the Linux source code tree:
.P
.in +4n
.EX
Documentation/ABI/stable/vdso
Documentation/ia64/fsys.rst
Documentation/vDSO/* (includes examples of using the vDSO)
.P
find arch/ \-iname \[aq]*vdso*\[aq] \-o \-iname \[aq]*gate*\[aq]
.EE
.in