# vdso

## 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
           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.1
    
       **strace(1), seccomp(2), and the vDSO**
           When tracing system 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.rst_.
    
       **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 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 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 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 (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_clock_gettime64   LINUX_5.11
           __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
    
           Before Linux 5.6, 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
    
           Before Linux 4.16, 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.
    
       **riscv functions**
           The table below lists the symbols exported by the vDSO.
           symbol                 version
           ──────────────────────────────────
           __vdso_rt_sigreturn    LINUX_4.15
           __vdso_gettimeofday    LINUX_4.15
           __vdso_clock_gettime   LINUX_4.15
           __vdso_clock_getres    LINUX_4.15
           __vdso_getcpu          LINUX_4.15
           __vdso_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.
    


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