VDSO (Virutal Dynamic Shared Object)

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About VDSO (Virutal Dynamic Shared Object)

* http://mail.nl.linux.org/kernelnewbies/2008-02/msg00257.html
* http://lwn.net/Articles/190139/
* http://www.trilithium.com/johan/2005/08/linux-gate/
* http://linux.derkeiler.com/Mailing-Lists/Kernel/2007-01/msg03739.html
 (Roland MCGrath's VDSO Linux Patch)

Virtual Dynamically-linked Shared Object, a kernel-provided shared library that helps userspace
perform a few kernel actions without the overhead of a system call, as well as automatically
choosing the most efficient syscall mechanism. Also called the "vsyscall page".

vDSO is virtual Dynamic Shared Object and the vdso32 is designed to prevent stack overflow.
A shared object exposed by the kernel at a fixed address in every process' memory.
We can verify  vDSO map information via "#>cat /proc//maps".
So that instead doing int80 or SYSCALL, the app can simply jump to vDSO address and
passing required parameters. the vDSO will decide which one will be used,
newer SYSCALL/SYSENTER or int 0x80 depending on machine capability or other settings.
Actually "lib/modules/vdso" are part of kernel, but made accessible from user space.

Roland MCGrath's VDSO Linux Patch is located in
http://linux.derkeiler.com/Mailing-Lists/Kernel/2007-01/msg03739.html website.
This patch makes CONFIG_COMPAT_VDSO disable the real vma and  use the fixmap area consistently.
This makes it actually compatible with what the old vdso implementation did.
If "/proc/sys/vm/vdso_enabled" turned on, the kernel randomizes the vDSO address as early as possible.

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