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VDSO(7)                    Linux Programmer's ManualVDSO(7)NAME       vdso - overview of the virtual ELF dynamic shared objectSYNOPSIS       #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 re-       quire 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 isgettimeofday(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  (seegetauxval(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 (atexecve(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'sldd(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       riscv      linux-vdso.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.1strace(1),seccomp(2), and the vDSO       When  tracing systems calls withstrace(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 toseccomp(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  ker-       nel.   Therefore,  the name of the user-space ABI should be used to de-       termine 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 do-       ing symbol lookups or versioning.  It does provide support for  differ-       ent 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 (port removed in Linux 4.17)       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 em-       bedded 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 viasyscall(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 with 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 ad-       dresses 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 do-       ing 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 (CAS operations)       00e0     set_thread_pointer (used by glibc)       0100     linux_gateway_entry (syscall)   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  in-       terfaces; 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  in-       terfaces; the kernel falls back to the real system call.   riscv functions       The table below lists the symbols exported by the vDSO.       symbol                   version       ------------------------------------       __kernel_rt_sigreturn    LINUX_4.15       __kernel_gettimeofday    LINUX_4.15       __kernel_clock_gettime   LINUX_4.15       __kernel_clock_getres    LINUX_4.15       __kernel_getcpu          LINUX_4.15       __kernel_flush_icache    LINUX_4.15   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 ALSOsyscalls(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 5.05 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                             2019-08-02VDSO(7)