GNU.WIKI: The GNU/Linux Knowledge Base

  [HOME] [PHP Manual] [HowTo] [ABS] [MAN1] [MAN2] [MAN3] [MAN4] [MAN5] [MAN6] [MAN7] [MAN8] [MAN9]

  [0-9] [Aa] [Bb] [Cc] [Dd] [Ee] [Ff] [Gg] [Hh] [Ii] [Jj] [Kk] [Ll] [Mm] [Nn] [Oo] [Pp] [Qq] [Rr] [Ss] [Tt] [Uu] [Vv] [Ww] [Xx] [Yy] [Zz]


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
       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

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 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

   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

   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

   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 3.65 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.



  All copyrights belong to their respective owners. Other content (c) 2014-2018, GNU.WIKI. Please report site errors to webmaster@gnu.wiki.
Page load time: 0.119 seconds. Last modified: November 04 2018 12:49:43.