On Fri, Jan 26, 2018 at 8:22 AM, Andy Lutomirski <luto@xxxxxxxxxx> wrote: > On Fri, Jan 26, 2018 at 7:36 AM, Dan Rue <dan.rue@xxxxxxxxxx> wrote: >> >> We've noticed that fsgsbase_64 can fail intermittently with the >> following error: >> >> [RUN] ARCH_SET_GS(0x0) and clear gs, then schedule to 0x1 >> Before schedule, set selector to 0x1 >> other thread: ARCH_SET_GS(0x1) -- sel is 0x0 >> [FAIL] GS/BASE changed from 0x1/0x0 to 0x0/0x0 >> >> This can be reliably reproduced by running fsgsbase_64 in a loop. i.e. >> >> for i in $(seq 1 10000); do ./fsgsbase_64 || break; done >> >> This problem isn't new - I've reproduced it on latest mainline and every >> release going back to v4.12 (I did not try earlier). This was tested on >> a Supermicro board with a Xeon E3-1220 as well as an Intel Nuc with an >> i3-5010U. >> > > Hmm, I can reproduce it, too. I'll look in a bit. I'm triggering a different error, and I think what's going on is that the kernel doesn't currently re-save GSBASE when a task switches out and that task has save gsbase != 0 and in-register GS == 0. This is arguably a bug, but it's not an infoleak, and fixing it could be a wee bit expensive. I'm not sure what, if anything, to do about this. I suppose I could add some gross perf hackery to the test to detect this case and suppress the error. I can also trigger the problem you're seeing, and I don't know what's up. It may be related to and old problem I've seen that causes signal delivery to sometimes corrupt %gs. It's deterministic, but it depends in some odd way on register state. I can currently reproduce that issue 100% of the time, and I'm trying to see if I can figure out what's happening. -- To unsubscribe from this list: send the line "unsubscribe linux-kselftest" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
