Re: Detecting the availability of VSYSCALL

On Tue, 25 Jun 2019, Florian Weimer wrote:

We're trying to create portable binaries which use VSYSCALL on older
kernels (to avoid performance regressions), but gracefully degrade to
full system calls on kernels which do not have VSYSCALL support compiled
in (or disabled at boot).

For technical reasons, we cannot use vDSO fallback.  Trying vDSO first
and only then use VSYSCALL is the way this has been tackled in the past,
which is why this userspace ABI breakage goes generally unnoticed.  But
we don't have a dynamic linker in our scenario.

I'm not following. On newer kernels which usually have vsyscall disabled
you need to use real syscalls anyway, so why are you so worried about
performance on older kernels. That doesn't make sense.

Is there any reliable way to detect that VSYSCALL is unavailable,
without resorting to parsing /proc/self/maps or opening file
descriptors?

Not that I'm aware of except

    sigaction(SIG_SEGV,....)

/me hides
 

Should we try mapping something at the magic address (without MAP_FIXED)
and see if we get back a different address?  Something in the auxiliary
vector would work for us, too, but nothing seems to exists there
unfortunately.

Would, but there is no such thing.

Thanks,

	tglx