vdso

       #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
       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  trig-
       ger  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 indi-
       rectly 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 gettimeof-
       day(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

   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 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
       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 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 dif-
       ferent 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 back-
       ward 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

   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 documen-
       tation for each.  The table below lists  the  functions  available  via

   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

   ppc/64 functions
       The table below lists the symbols exported by the vDSO.

       __kernel_sync_dicache_p5   LINUX_2.6.15

   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
       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.txt
           Documentation/vDSO/* (includes examples of using the vDSO)

           find arch/ -iname '*vdso*' -o -iname '*gate*'

COLOPHON
       This page is part of release 4.04 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
       http://www.kernel.org/doc/man-pages/.

Linux                             2015-12-28                           VDSO(7)
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