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

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 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
       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 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
       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  ker‐
       nel.   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	 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
       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 3.63 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/.

Linux				  2014-02-01			       VDSO(7)
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