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NAME

       vdso - overview of the virtual ELF dynamic shared object

SYNOPSIS

       #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 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.

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
       mips       linux-vdso.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), 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.

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
       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.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

   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 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

       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

       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.

   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 4.16 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 https://www.kernel.org/doc/man-pages/.