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VDSO(7)                             Linux Programmer's Manual                             VDSO(7)

NAME
       vDSO - overview of the virtual ELF dynamic shared object

SYNOPSIS
       #include 

       void *vdso = (uintptr_t) getauxval(AT_SYSINFO_EHDR);

DESCRIPTION
       The "vDSO" (virtual dynamic shared object) is a small shared library that the kernel auto‐
       matically maps into the address space of all user-space applications.   Applications  usu‐
       ally  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 over‐
       all 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 inter‐
       rupt (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 proces‐
       sor'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 time‐
       stamps 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  avail‐
       able).   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.   Often‐
       times  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 chang‐
       ing the arguments that the function accepts as well as the return value.  Thus, when look‐
       ing  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.

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.

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  deter‐
       mine 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/ker‐
       nel_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
       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

   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 this mechanism.

       function
       ────────────────
       clock_gettime
       getcpu
       getpid
       getppid
       gettimeofday
       set_tid_address

   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  exe‐
       cuting  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 (%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.

       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

   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.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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