[Adding Alex Barcelo to the CC]
On Thu, Feb 27, 2014 at 6:20 AM, Michael Matz matz@suse.de wrote:
Hi,
On Wed, 26 Feb 2014, Dann Frazier wrote:
I've narrowed down the changes that seem to prevent both types of segfaults to the following changes that introduce a wrapper around sigprocmask:
https://github.com/susematz/qemu/commit/f1542ae9fe10d5a241fc2624ecaef5f0948e... https://github.com/susematz/qemu/commit/4e5e1607758841c760cda4652b0ee7a6bc6e... https://github.com/susematz/qemu/commit/63eb8d3ea58f58d5857153b0c632def1bbd0...
I'm not sure if this is a real fix or just papering over my issue -
It's fixing the issue, but strictly speaking introduces an QoI problem. SIGSEGV must not be controllable by the guest, it needs to be always deliverable to qemu; that is what's fixed.
The QoI problem introduced is that with the implementation as is, the fiddling with SIGSEGV is detectable by the guest. E.g. if it installs a segv handler, blocks segv, then forces a segfault, checks that it didn't arrive, then unblocks segv and checks that it now arrives, such testcase would be able to detect that in fact it couldn't block SIGSEGV.
Luckily for us, the effect of blocking SIGSEGV and then generating one in other ways than kill/sigqueue/raise (e.g. by writing to NULL) are undefined, so in practice that QoI issue doesn't matter.
To fix also that latter part it'd need a further per-thread flag segv_blocked_p which needs to be checked before actually delivering a guest-directed SIGSEGV (in comparison to a qemu-directed SEGV), and otherwise requeue it. That's made a bit complicated when the SEGV was process-directed (not thread-directed) because in that case it needs to be delivered as long as there's _any_ thread which has it unblocked. So given the above undefinedness for sane uses of SEGVs it didn't seem worth the complication of having an undetectable virtualization of SIGSEGV.
Thanks for the explanation.
but either way, are these changes reasonable for upstream submission?
IIRC the first two commits (from Alex Barcelo) were submitted once in the past, but fell through the cracks.
Alex: are you interested in resubmitting these - or would you like me to attempt to on your behalf?
-dann
Ciao, Michael.