RFR(s): 8076185: Provide SafeFetchX implementation for zero

[Severin Gehwolf]

Hi Thomas,

On Fri, 2015-03-27 at 19:00 +0100, Thomas Stüfe wrote:
> Hi all,
> 
> 
> Please review this change which provides a real SafeFetch
> implementation 
> on zero.
> 
> 
> webrev:
>  http://cr.openjdk.java.net/~stuefe/webrevs/8076185/webrev.01/webrev/
> bug:     https://bugs.openjdk.java.net/browse/JDK-8076185
> 
> 
> It works like this:
> 
> 
> Before a load is attempted from a potentially unsafe address, we set
> up a 
> jump buffer with sigsetjmp(). In the signal handler, for SIGSEGV and
> SIGBUS,
> we test whether there is a jump buffer set and if yes, take this as an
> indication
> that the crash was an attempted SafeFetch. In this case we jump back
> via longjmp().
> 
> 
> Coding is a bit more difficult because we need to be threadsafe. We
> keep the jump
> buffer on the thread stack - this is ok, because that stack never gets
> unwinded 
>  - either we crash, in which case signal handler stack frames are
> built up below 
> us, or we don't crash, in which case we never leave the SafeFetch
> function. In 
> both cases, we never loose the jump buffer.
> 
> 
> To communicate the jump buffer location to the signal handler, TLS is
> used. I 
> use POSIX tls, because that always works and is not dependend on VM
> infrastructure.
> 
> 
> ---
> 
> 
> I built and tested this on Linux x64 zero (Ubuntu 14.4). It works and
> the 
> initialization tests for SafeFetch in the stub routine initialization
> now work for 
> zero too.
> 
> 
> Note that I do not have a BSD system right now, so I cannot check
> whether this 
> change builds and works for BSD. But the change only requires POSIX
> Apis, so
> BSD should probably be ok.
> 
> 
> Someone from the zero team should definitly check and test this for
> other zero
> platforms, but the chances are good that this just works.

Thomas, thanks very much for this patch! It works fine for me on x86_64
F21, but I'd also like to test on non-x86. Zero is notoriously stack
hungry and having the jump buffer on the stack too is a bit of a concern
(but probably fine). Testing that too, though, will take a while, since
I don't have such hardware readily available.

As for the patch (I'm not a reviewer): It looks fine to me. It's
certainly a lot better than this unsafe SafeFetch that we currently use
for Zero.

Thanks,
Severin

> Thanks and Kind Regards, 
> 
> 
> Thomas Stuefe
> 
>