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