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I'm currently trying to implement my own VDSO. I have seen this tutorial which explains how to do this.

However, this tutorial is made for Linux 2.6.x and I would like to use it on Linux 4.8.x. But the architectural structure of vdso changed a lot since this version (e.g. vextern.h has been suppressed) and I don't know how to adapt it to get it working.

Do you know how I could adapt this tutorial for a recent kernel or where I can find enough resources about vdso to implement my own?

Also, do you know whether I can simply recreate the behavior of an arbitrary syscall as a VDSO? I know it is a very bad idea from a security perspective (and the performance gain is negligible)

closed as off-topic by Rui F Ribeiro, Jeff Schaller, RalfFriedl, G-Man, Archemar Nov 21 '18 at 8:12

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  • Thanks I learnt something new. However the phrase “the kernel is divided into two primary segments of memory: userland and kernel land.” is inept. This is as silly as saying that apples are divided into two groups: apples and oranges. – ctrl-alt-delor Nov 20 '18 at 14:02
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I’m not sure there’s an updated tutorial in the same style, but the changes are limited:

  • arch/x86/vdso moved to arch/x86/entry/vdso;
  • variable declarations have been greatly simplified.

The latter means that you only need to declare your variable once, in arch/x86/include/asm/vvar.h, at a fixed offset, with the DECLARE_VVAR macro. To define it, use the DEFINE_VVAR macro; to access it, use the VVAR macro. The linked commit above has a number of examples (all the existing uses of shared variables).

You can’t recreate the behaviour of an arbitrary syscall in the vDSO, at least not without calling into the kernel in kernel mode. The vDSO is only really useful for calls which can be handled in user space; there aren’t many of those.

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