Re: Detecting the availability of VSYSCALL

[Florian Weimer <fweimer@redhat.com>]

* Andy Lutomirski:

>> We want binaries that run fast on VSYSCALL kernels, but can fall back to
>> full system calls on kernels that do not have them (instead of
>> crashing).
>
> Define "VSYSCALL kernels."  On any remotely recent kernel (*all* new
> kernels and all kernels for the last several years that haven't
> specifically requested vsyscall=native), using vsyscalls is much, much
> slower than just doing syscalls.  I know a way you can tell whether
> vsyscalls are fast, but it's unreliable, and I'm disinclined to
> suggest it.  There are also at least two pending patch series that
> will interfere.

The fast path is for the benefit of the 2.6.32-based kernel in Red Hat
Enterprise Linux 6.  It doesn't have the vsyscall emulation code yet, I
think.

My hope is to produce (statically linked) binaries that run as fast on
that kernel as they run today, but can gracefully fall back to something
else on kernels without vsyscall support.

>> We could parse the vDSO and prefer the functions found there, but this
>> is for the statically linked case.  We currently do not have a (minimal)
>> dynamic loader there in that version of the code base, so that doesn't
>> really work for us.
>
> Is anything preventing you from adding a vDSO parser?  I wrote one
> just for this type of use:
>
> $ wc -l tools/testing/selftests/vDSO/parse_vdso.c
> 269 tools/testing/selftests/vDSO/parse_vdso.c
>
> (289 lines includes quite a bit of comment.)

I'm worried that if I use a custom parser and the binaries start
crashing again because something changed in the kernel (within the scope
permitted by the ELF specification), the kernel won't be fixed.

That is, we'd be in exactly the same situation as today.

Thanks,
Florian