NAME | SYNOPSIS | DESCRIPTION | NOTES | ARCHITECTURE-SPECIFIC NOTES | SEE ALSO | COLOPHON

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

NAME         top

       vdso - overview of the virtual ELF dynamic shared object

SYNOPSIS         top

       #include <sys/auxv.h>

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

DESCRIPTION         top

       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         top

   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) 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         top

       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         top

       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         top

       This page is part of release 4.06 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/.

Linux                            2016-05-09                          VDSO(7)