1VDSO(7)                    Linux Programmer's Manual                   VDSO(7)
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3
4

NAME

6       vdso - overview of the virtual ELF dynamic shared object
7

SYNOPSIS

9       #include <sys/auxv.h>
10
11       void *vdso = (uintptr_t) getauxval(AT_SYSINFO_EHDR);
12

DESCRIPTION

14       The  "vDSO"  (virtual  dynamic shared object) is a small shared library
15       that the kernel automatically maps into the address space of all  user-
16       space  applications.  Applications usually do not need to concern them‐
17       selves with these details as the vDSO is most commonly called by the  C
18       library.   This way you can code in the normal way using standard func‐
19       tions and the C library will take care of using any functionality  that
20       is available via the vDSO.
21
22       Why does the vDSO exist at all?  There are some system calls the kernel
23       provides that user-space code ends up using frequently,  to  the  point
24       that  such calls can dominate overall performance.  This is due both to
25       the frequency of the call as well as the context-switch  overhead  that
26       results from exiting user space and entering the kernel.
27
28       The  rest  of  this documentation is geared toward the curious and/or C
29       library writers rather than general developers.  If  you're  trying  to
30       call  the vDSO in your own application rather than using the C library,
31       you're most likely doing it wrong.
32
33   Example background
34       Making system calls can be slow.  In x86 32-bit systems, you can  trig‐
35       ger  a  software  interrupt  (int $0x80) to tell the kernel you wish to
36       make a system call.  However, this instruction is  expensive:  it  goes
37       through  the full interrupt-handling paths in the processor's microcode
38       as well as in the kernel.  Newer processors have faster  (but  backward
39       incompatible)  instructions  to initiate system calls.  Rather than re‐
40       quire the C library to figure out if this functionality is available at
41       run time, the C library can use functions provided by the kernel in the
42       vDSO.
43
44       Note that the terminology can be confusing.  On x86 systems,  the  vDSO
45       function used to determine the preferred method of making a system call
46       is named "__kernel_vsyscall", but on x86-64, the term  "vsyscall"  also
47       refers to an obsolete way to ask the kernel what time it is or what CPU
48       the caller is on.
49
50       One frequently used system call is gettimeofday(2).  This  system  call
51       is  called  both  directly  by user-space applications as well as indi‐
52       rectly by the C library.  Think timestamps or timing loops or  polling—
53       all  of  these frequently need to know what time it is right now.  This
54       information is also not secret—any application in  any  privilege  mode
55       (root  or  any  unprivileged  user) will get the same answer.  Thus the
56       kernel arranges for the information required to answer this question to
57       be  placed  in memory the process can access.  Now a call to gettimeof‐
58       day(2) changes from a system call to a normal function call and  a  few
59       memory accesses.
60
61   Finding the vDSO
62       The base address of the vDSO (if one exists) is passed by the kernel to
63       each program in the initial auxiliary vector  (see  getauxval(3)),  via
64       the AT_SYSINFO_EHDR tag.
65
66       You  must  not  assume the vDSO is mapped at any particular location in
67       the user's memory map.  The base address will usually be randomized  at
68       run time every time a new process image is created (at execve(2) time).
69       This is done for security reasons, to prevent "return-to-libc" attacks.
70
71       For some architectures, there is also an AT_SYSINFO tag.  This is  used
72       only for locating the vsyscall entry point and is frequently omitted or
73       set to 0 (meaning it's not available).  This tag is a throwback to  the
74       initial vDSO work (see History below) and its use should be avoided.
75
76   File format
77       Since  the  vDSO is a fully formed ELF image, you can do symbol lookups
78       on it.  This allows new symbols to be added with newer kernel releases,
79       and  allows the C library to detect available functionality at run time
80       when running under different kernel versions.  Oftentimes the C library
81       will  do  detection  with  the first call and then cache the result for
82       subsequent calls.
83
84       All symbols are also versioned (using the GNU  version  format).   This
85       allows  the  kernel  to  update the function signature without breaking
86       backward compatibility.  This means changing  the  arguments  that  the
87       function  accepts as well as the return value.  Thus, when looking up a
88       symbol in the vDSO, you must always include the version  to  match  the
89       ABI you expect.
90
91       Typically  the vDSO follows the naming convention of prefixing all sym‐
92       bols with "__vdso_" or "__kernel_" so as to distinguish them from other
93       standard  symbols.   For  example, the "gettimeofday" function is named
94       "__vdso_gettimeofday".
95
96       You use the standard C calling conventions when calling  any  of  these
97       functions.  No need to worry about weird register or stack behavior.
98

NOTES

100   Source
101       When you compile the kernel, it will automatically compile and link the
102       vDSO code for you.  You will frequently find it under the architecture-
103       specific directory:
104
105           find arch/$ARCH/ -name '*vdso*.so*' -o -name '*gate*.so*'
106
107   vDSO names
108       The  name  of the vDSO varies across architectures.  It will often show
109       up in things like glibc's ldd(1) output.  The  exact  name  should  not
110       matter to any code, so do not hardcode it.
111
112       user ABI   vDSO name
113       ─────────────────────────────
114       aarch64    linux-vdso.so.1
115       arm        linux-vdso.so.1
116       ia64       linux-gate.so.1
117       mips       linux-vdso.so.1
118       ppc/32     linux-vdso32.so.1
119       ppc/64     linux-vdso64.so.1
120       riscv      linux-vdso.so.1
121       s390       linux-vdso32.so.1
122       s390x      linux-vdso64.so.1
123       sh         linux-gate.so.1
124       i386       linux-gate.so.1
125       x86-64     linux-vdso.so.1
126       x86/x32    linux-vdso.so.1
127
128   strace(1), seccomp(2), and the vDSO
129       When  tracing systems calls with strace(1), symbols (system calls) that
130       are exported by the vDSO will not appear in the  trace  output.   Those
131       system calls will likewise not be visible to seccomp(2) filters.
132

ARCHITECTURE-SPECIFIC NOTES

134       The subsections below provide architecture-specific notes on the vDSO.
135
136       Note  that the vDSO that is used is based on the ABI of your user-space
137       code and not the ABI of the kernel.  Thus, for example, when you run an
138       i386  32-bit ELF binary, you'll get the same vDSO regardless of whether
139       you run it under an i386 32-bit kernel or under an x86-64  64-bit  ker‐
140       nel.   Therefore,  the name of the user-space ABI should be used to de‐
141       termine which of the sections below is relevant.
142
143   ARM functions
144       The table below lists the symbols exported by the vDSO.
145
146       symbol                 version
147       ────────────────────────────────────────────────────────────
148       __vdso_gettimeofday    LINUX_2.6 (exported since Linux 4.1)
149       __vdso_clock_gettime   LINUX_2.6 (exported since Linux 4.1)
150
151       Additionally, the ARM port has a code page full of  utility  functions.
152       Since it's just a raw page of code, there is no ELF information for do‐
153       ing symbol lookups or versioning.  It does provide support for  differ‐
154       ent versions though.
155
156       For  information  on  this  code page, it's best to refer to the kernel
157       documentation as it's extremely detailed and covers everything you need
158       to know: Documentation/arm/kernel_user_helpers.txt.
159
160   aarch64 functions
161       The table below lists the symbols exported by the vDSO.
162
163       symbol                   version
164       ──────────────────────────────────────
165       __kernel_rt_sigreturn    LINUX_2.6.39
166       __kernel_gettimeofday    LINUX_2.6.39
167       __kernel_clock_gettime   LINUX_2.6.39
168       __kernel_clock_getres    LINUX_2.6.39
169
170   bfin (Blackfin) functions (port removed in Linux 4.17)
171       As  this  CPU lacks a memory management unit (MMU), it doesn't set up a
172       vDSO in the normal sense.  Instead, it maps at  boot  time  a  few  raw
173       functions  into  a  fixed  location in memory.  User-space applications
174       then call directly into that region.  There is no provision  for  back‐
175       ward  compatibility  beyond sniffing raw opcodes, but as this is an em‐
176       bedded CPU, it can get away with things—some of the object  formats  it
177       runs aren't even ELF based (they're bFLT/FLAT).
178
179       For  information  on  this  code page, it's best to refer to the public
180       documentation:
181       http://docs.blackfin.uclinux.org/doku.php?id=linux-kernel:fixed-code
182
183   mips functions
184       The table below lists the symbols exported by the vDSO.
185
186       symbol                   version
187       ──────────────────────────────────────────────────────────────
188       __kernel_gettimeofday    LINUX_2.6 (exported since Linux 4.4)
189       __kernel_clock_gettime   LINUX_2.6 (exported since Linux 4.4)
190
191   ia64 (Itanium) functions
192       The table below lists the symbols exported by the vDSO.
193
194       symbol                       version
195       ───────────────────────────────────────
196       __kernel_sigtramp            LINUX_2.5
197       __kernel_syscall_via_break   LINUX_2.5
198
199       __kernel_syscall_via_epc     LINUX_2.5
200
201       The Itanium port is somewhat tricky.  In addition to the vDSO above, it
202       also  has "light-weight system calls" (also known as "fast syscalls" or
203       "fsys").  You can invoke these via  the  __kernel_syscall_via_epc  vDSO
204       helper.  The system calls listed here have the same semantics as if you
205       called them directly via syscall(2), so refer to the relevant  documen‐
206       tation  for  each.   The  table below lists the functions available via
207       this mechanism.
208
209       function
210       ────────────────
211       clock_gettime
212       getcpu
213       getpid
214       getppid
215       gettimeofday
216       set_tid_address
217
218   parisc (hppa) functions
219       The parisc port has a code page with utility functions called a gateway
220       page.   Rather  than  use  the normal ELF auxiliary vector approach, it
221       passes the address of the page to the process  via  the  SR2  register.
222       The  permissions  on  the page are such that merely executing those ad‐
223       dresses automatically executes with kernel privileges and not  in  user
224       space.  This is done to match the way HP-UX works.
225
226       Since it's just a raw page of code, there is no ELF information for do‐
227       ing symbol lookups or versioning.  Simply  call  into  the  appropriate
228       offset via the branch instruction, for example:
229
230           ble <offset>(%sr2, %r0)
231
232       offset   function
233       ────────────────────────────────────────────
234       00b0     lws_entry (CAS operations)
235       00e0     set_thread_pointer (used by glibc)
236       0100     linux_gateway_entry (syscall)
237
238   ppc/32 functions
239       The  table below lists the symbols exported by the vDSO.  The functions
240       marked with a * are available only  when  the  kernel  is  a  PowerPC64
241       (64-bit) kernel.
242
243       symbol                     version
244       ────────────────────────────────────────
245       __kernel_clock_getres      LINUX_2.6.15
246       __kernel_clock_gettime     LINUX_2.6.15
247       __kernel_clock_gettime64   LINUX_5.11
248       __kernel_datapage_offset   LINUX_2.6.15
249       __kernel_get_syscall_map   LINUX_2.6.15
250       __kernel_get_tbfreq        LINUX_2.6.15
251       __kernel_getcpu *          LINUX_2.6.15
252       __kernel_gettimeofday      LINUX_2.6.15
253       __kernel_sigtramp_rt32     LINUX_2.6.15
254       __kernel_sigtramp32        LINUX_2.6.15
255       __kernel_sync_dicache      LINUX_2.6.15
256       __kernel_sync_dicache_p5   LINUX_2.6.15
257
258       In  kernel  versions  before  Linux  5.6, the CLOCK_REALTIME_COARSE and
259       CLOCK_MONOTONIC_COARSE  clocks  are  not  supported   by   the   __ker‐
260       nel_clock_getres  and  __kernel_clock_gettime  interfaces;  the  kernel
261       falls back to the real system call.
262
263   ppc/64 functions
264       The table below lists the symbols exported by the vDSO.
265
266       symbol                     version
267       ────────────────────────────────────────
268       __kernel_clock_getres      LINUX_2.6.15
269       __kernel_clock_gettime     LINUX_2.6.15
270       __kernel_datapage_offset   LINUX_2.6.15
271       __kernel_get_syscall_map   LINUX_2.6.15
272       __kernel_get_tbfreq        LINUX_2.6.15
273       __kernel_getcpu            LINUX_2.6.15
274       __kernel_gettimeofday      LINUX_2.6.15
275       __kernel_sigtramp_rt64     LINUX_2.6.15
276       __kernel_sync_dicache      LINUX_2.6.15
277       __kernel_sync_dicache_p5   LINUX_2.6.15
278
279       In kernel versions before Linux  4.16,  the  CLOCK_REALTIME_COARSE  and
280       CLOCK_MONOTONIC_COARSE   clocks   are   not  supported  by  the  __ker‐
281       nel_clock_getres  and  __kernel_clock_gettime  interfaces;  the  kernel
282       falls back to the real system call.
283
284   riscv functions
285       The table below lists the symbols exported by the vDSO.
286
287       symbol                   version
288       ────────────────────────────────────
289       __kernel_rt_sigreturn    LINUX_4.15
290       __kernel_gettimeofday    LINUX_4.15
291       __kernel_clock_gettime   LINUX_4.15
292       __kernel_clock_getres    LINUX_4.15
293       __kernel_getcpu          LINUX_4.15
294       __kernel_flush_icache    LINUX_4.15
295
296   s390 functions
297       The table below lists the symbols exported by the vDSO.
298
299       symbol                   version
300       ──────────────────────────────────────
301       __kernel_clock_getres    LINUX_2.6.29
302       __kernel_clock_gettime   LINUX_2.6.29
303       __kernel_gettimeofday    LINUX_2.6.29
304
305   s390x functions
306       The table below lists the symbols exported by the vDSO.
307
308       symbol                   version
309       ──────────────────────────────────────
310       __kernel_clock_getres    LINUX_2.6.29
311       __kernel_clock_gettime   LINUX_2.6.29
312       __kernel_gettimeofday    LINUX_2.6.29
313
314   sh (SuperH) functions
315       The table below lists the symbols exported by the vDSO.
316
317       symbol                  version
318       ──────────────────────────────────
319       __kernel_rt_sigreturn   LINUX_2.6
320       __kernel_sigreturn      LINUX_2.6
321       __kernel_vsyscall       LINUX_2.6
322
323   i386 functions
324       The table below lists the symbols exported by the vDSO.
325
326       symbol                  version
327       ──────────────────────────────────────────────────────────────
328       __kernel_sigreturn      LINUX_2.5
329       __kernel_rt_sigreturn   LINUX_2.5
330       __kernel_vsyscall       LINUX_2.5
331       __vdso_clock_gettime    LINUX_2.6 (exported since Linux 3.15)
332
333       __vdso_gettimeofday     LINUX_2.6 (exported since Linux 3.15)
334       __vdso_time             LINUX_2.6 (exported since Linux 3.15)
335
336   x86-64 functions
337       The  table  below lists the symbols exported by the vDSO.  All of these
338       symbols are also available without the "__vdso_" prefix, but you should
339       ignore those and stick to the names below.
340
341       symbol                 version
342       ─────────────────────────────────
343       __vdso_clock_gettime   LINUX_2.6
344       __vdso_getcpu          LINUX_2.6
345       __vdso_gettimeofday    LINUX_2.6
346       __vdso_time            LINUX_2.6
347
348   x86/x32 functions
349       The table below lists the symbols exported by the vDSO.
350
351       symbol                 version
352       ─────────────────────────────────
353       __vdso_clock_gettime   LINUX_2.6
354       __vdso_getcpu          LINUX_2.6
355       __vdso_gettimeofday    LINUX_2.6
356       __vdso_time            LINUX_2.6
357
358   History
359       The  vDSO was originally just a single function—the vsyscall.  In older
360       kernels, you might see that name in a process's memory map rather  than
361       "vdso".  Over time, people realized that this mechanism was a great way
362       to pass more functionality to user space, so it was  reconceived  as  a
363       vDSO in the current format.
364

SEE ALSO

366       syscalls(2), getauxval(3), proc(5)
367
368       The documents, examples, and source code in the Linux source code tree:
369
370           Documentation/ABI/stable/vdso
371           Documentation/ia64/fsys.txt
372           Documentation/vDSO/* (includes examples of using the vDSO)
373
374           find arch/ -iname '*vdso*' -o -iname '*gate*'
375

COLOPHON

377       This  page  is  part of release 5.13 of the Linux man-pages project.  A
378       description of the project, information about reporting bugs,  and  the
379       latest     version     of     this    page,    can    be    found    at
380       https://www.kernel.org/doc/man-pages/.
381
382
383
384Linux                             2021-08-27                           VDSO(7)
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