Not quite. The vDSO provides a general syscall-wrapper mechanism for certain types of system call interfaces. It also provides implementations of gettimeofday clock_gettime and 2 other system calls completely in userland and acts precisely as you've described.
Please see this[1] for a detailed explanation. For a shorter explanation, please see the vDSO man page[2]. Thanks for reading my blog post!
I'm aware of the high level about VDSO implementation, but I would still say that the Solaris implementation is more narrowly focused and as a result does not have the subtle issues / tradeoffs that VDSO does.
Also, I personally find VDSO disagreeable as do others although perhaps not in as dramatic terms as some:
I think Ian Lance Taylor's summary is the most balanced and thoughtful:
Basically you want the kernel to provide a mapping for a small number of magic symbols to addresses that can be called at runtime. In other words, you want to map a small number of indexes to addresses. I can think of many different ways to handle that in the kernel. I don't think the first mechanism I would reach for would be for the kernel to create an in-memory shared library. It's kind of a baroque mechanism for implementing a simple table.
It's true that dynamically linked programs can use the ELF loader. But the ELF loader needed special changes to support VDSOs. And so did gdb. And this approach doesn't help statically linked programs much. And glibc functions needed to be changed anyhow to be aware of the VDSO symbols. So as far as I can tell, all of this complexity really
didn't get anything for free. It just wound up being complex.
> Not quite; vdso is a general syscall-wrapper mechanism.
It's not. On 32-bit x86, it sort of is, but that's just because the 32-bit x86 fast syscall mechanism isn't really compatible with inline syscalls. Linux (and presumably most other kernels) provides a wrapper function that means "do a syscall". It's only accelerated insofar as it uses a faster hardware mechanism. It has nothing to do with fast timing.
On x86_64, there is no such mechanism.
> It's true that dynamically linked programs can use the ELF loader. But the ELF loader needed special changes to support VDSOs. And so did gdb. And this approach doesn't help statically linked programs much.
That's because the glibc ELF loader is a piece of, ahem, is baroque and overcomplicated. And there's no reason whatsoever that vDSO usage needs to be integrated with the dynamic linker at all.
I wrote a CC0-licensed standalone vDSO parser here:
It's 269 lines of code, including lots of comments, and it works in static binaries just fine. Go's runtime (which is static!) uses a vDSO loader based on it. I agree that a static table would be slightly simpler, but the tooling for debugging the vDSO is a heck of a lot simpler with the ELF approach.
This all seems predicated on the fact that Solaris doesn't support direct system calls and the fact that they ship their kernel and libc as one unified whole (like BSDs). Solaris is free to update the layout of their shared data structures whenever they want[1].
Because Linux kernel interfaces are distinct and separate from libc, and given Linus' policy on backwards compatibility, Linux had two choices for an _interface_: 1) export a data structure to userland that could never change, or 2) export a code linking mechanism to userland that could never change. In that light the latter choice seems far more reasonable.
[1] The shared data structures for this particular feature. There are other kernel data structures that leak through the libc interface and for which Solaris is bound to maintain compatibility.
Please see this[1] for a detailed explanation. For a shorter explanation, please see the vDSO man page[2]. Thanks for reading my blog post!
[1]: https://blog.packagecloud.io/eng/2016/04/05/the-definitive-g... [2]: http://man7.org/linux/man-pages/man7/vdso.7.html