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NAME | SYNOPSIS | DESCRIPTION | NOTES | ARCHITECTURE-SPECIFIC NOTES | SEE ALSO |
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vDSO(7) Miscellaneous Information 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
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 systems 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.
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.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 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 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 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 (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 __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
Before Linux 4.16, 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.
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.
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 (unreleased) (date) vDSO(7)
Pages that refer to this page: ldd(1), clock_getres(2), getcpu(2), gettimeofday(2), getunwind(2), seccomp(2), sigreturn(2), syscall(2), syscalls(2), time(2), getauxval(3), core(5), proc(5), libc(7)
Copyright and license for this manual page
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HTML rendering created 2023-12-22 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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