On Wed, Feb 27, 2019 at 1:28 AM Waldek Kozaczuk <jwkozac...@gmail.com> wrote:
> Recently I had a chance to explore if we can run Golang pie apps built > with --buildmode=pie options. I have actually added two examples of it - > https://github.com/cloudius-systems/osv-apps/tree/master/golang-pie-example > and > https://github.com/cloudius-systems/osv-apps/tree/master/golang-pie-httpserver > . > > Running Golang pie apps does not require the golang wrapper ( > https://github.com/cloudius-systems/osv/blob/master/modules/golang/go.cc) > which is used to launch apps built as shared libraries which is the > "official way" of running them on OSv. Relatedly Golang pie apps when run > trigger a warning about "a PIE using TLS" (see issue > https://github.com/cloudius-systems/osv/issues/352). Given all that I did > not expect them to run without crashes which also I reported couple of > months ago - > https://github.com/cloudius-systems/osv/issues/352#issuecomment-434692750 > . > > It turns out that original crash from 352 was caused by missing > sys_exit_group syscall fixed later by this commit - > https://github.com/cloudius-systems/osv/commit/a52e73ceec726378pie > shared object tls-modelf642f5621caf5966f96bad9c > <https://github.com/cloudius-systems/osv/commit/a52e73ceec726378f642f5621caf5966f96bad9c>. > So when I built and ran golang-pie-example recently, it would execute > without any problems. Encouraged with it I tried another more complex > example - golang-pie-httpserver - and I was also able to start it and > stress test it with ab. I did not see any issues until I tried to exit with > 'Ctrl-C' which caused a crash as reported by this new issue - > https://github.com/cloudius-systems/osv/issues/1026. > > Couple of questions: > > 1. How do these two apps run almost properly (except for that Ctrl-C > issue) given they are pie's with local-exec TLS which we do not support per > #352? > > > 1. > 2. Do the apps run properly because they actually do NOT use any > thread local variables? Or we simply do not see any bad side effects and > program just "happens' to work fine? I know that Golang language itself > does not have a concept of thread local variables like C++ or Java but > Golang runtime behind the scenes seems to use them given presence of TLS > section. Or maybe TLS section is empty? > > Maybe golang's implementation does not actually use these TLS variables, or is although PIE *can* use local-exec TLS mode, it was intentionally compiled to use any other TLS mode? You can see examples on how a program can choose its variables' TLS model on the whole compilation (-ftlst-model=...) or individual variables in tests/tst-tls.cc. I have no idea if this is the explanation, though. I'm also pretty rusty with all that TLS code, and why the mere existance of that section is a problem. > > 1. The C++ example from #352 executes on OSv but prints wrong value. > What does exactly mean that local-exec TLS is not supported? In what way? > Does code try to read/write data from/to wrong offsets in memory or thread > local variables do not get properly initialized? > > Again, I'm rusty in this, the issue probably has more information than I remember now. I vaguely remember that "local exec" means that the code which *uses* these variables assumes they are in the main per-thread area (%fs), but what we have there are actually OSv's own per-thread variables. > > 1. Is it possible that some patches (for example > > https://github.com/cloudius-systems/osv/commit/e60339de2676b186c1ec957102a5a384506d127e_ > committed to support Golang apps with initial-exec TLS mode helped support > Golang pies as well. > > This is only relevant if somehow the PIE uses initial-exec TLS mode instead of local-exec. Maybe it does? I really have no idea how Go uses TLS... I read some old comment in some random discussion which suggests that Go does something similar to *initial-exec*, but manually, and if this is the case for both the SO and PIE modes, than it should be fine. If it's really fine, I wonder how we can detect it and avoid the warning message. What does that TLS section contain? > -- > You received this message because you are subscribed to the Google Groups > "OSv Development" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to osv-dev+unsubscr...@googlegroups.com. > For more options, visit https://groups.google.com/d/optout. > -- You received this message because you are subscribed to the Google Groups "OSv Development" group. To unsubscribe from this group and stop receiving emails from it, send an email to osv-dev+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
