On Thu, Aug 13, 2015 at 9:28 AM, David Drysdale <[email protected]> wrote: > On Thu, Aug 13, 2015 at 4:17 PM, Denys Vlasenko <[email protected]> wrote: >> On 08/13/2015 10:30 AM, David Drysdale wrote: >>> Hi folks, >>> >>> I've got an odd regression with the v4.2 rc kernel, and I wondered if anyone >>> else could reproduce it. >>> >>> The problem occurs with a seccomp-bpf filter program that's set up to return >>> an errno value -- an errno of 1 is always returned instead of what's in the >>> filter, plus other oddities (selftest output below). >>> >>> The problem seems to need a combination of circumstances to occur: >>> >>> - The seccomp-bpf userspace program needs to be 32-bit, running against a >>> 64-bit kernel -- I'm testing with seccomp_bpf from >>> tools/testing/selftests/seccomp/, built via 'CFLAGS=-m32 make'. >> >> Does it work correctly when built as 64-bit program? > > Yep, 64-bit works fine (both at v4.2-rc6 and at commit 3f5159). > >>> >>> - The kernel needs to be running as a VM guest -- it occurs inside my >>> VMware Fusion host, but not if I run on bare metal. Kees tells me he >>> cannot repro with a kvm guest though. >>> >>> Bisecting indicates that the commit that induces the problem is >>> 3f5159a9221f19b0, "x86/asm/entry/32: Update -ENOSYS handling to match the >>> 64-bit logic", included in all the v4.2-rc* candidates. >>> >>> Apologies if I've just got something odd with my local setup, but the >>> bisection was unequivocal enough that I thought it worth reporting... >>> >>> Thanks, >>> David >>> >>> >>> seccomp_bpf failure outputs: > > [snip] > >> End result should be: >> pt_regs->ax = -E2BIG (via syscall_set_return_value()) >> pt_regs->orig_ax = -1 ("skip syscall") >> and syscall_trace_enter_phase1() usually returns with 0, >> meaning "re-execute syscall at once, no phase2 needed". >> >> This, in turn, is called from .S files, and when it returns there, >> execution loops back to syscall dispatch. >> >> Because of orig_ax = -1, syscall dispatch should skip calling syscall. >> So -E2BIG should survive and be returned... > > So I was just about to send: > > That makes sense, and given that exactly the same 32-bit binary > runs fine on a different machine, there's presumably something up > with my local setup. The failing machine is a VMware guest, but > maybe that's not the relevant interaction -- particularly if no-one > else can repro. > > But then I noticed some odd audit entries in the main log: > > Aug 13 16:52:56 ubuntu kernel: [ 20.687249] audit: type=1326 > audit(1439481176.034:62): auid=4294967295 uid=1000 gid=1000 > ses=4294967295 pid=2621 comm="secccomp_bpf.ke" > exe="/home/dmd/secccomp_bpf.kees.m32" sig=9 arch=40000003 syscall=172 > compat=1 ip=0xf773cc90 code=0x0 > Aug 13 16:52:56 ubuntu kernel: [ 20.691157] audit: type=1326 > audit(1439481176.038:63): auid=4294967295 uid=1000 gid=1000 > ses=4294967295 pid=2631 comm="secccomp_bpf.ke" > exe="/home/dmd/secccomp_bpf.kees.m32" sig=31 arch=40000003 syscall=20 > compat=1 ip=0xf773cc90 code=0x10000000 > ... > > I didn't think I had any audit stuff turned on, and indeed: > # auditctl -l > No rules > > But as soon as I'd run that auditctl command, the 32-bit > seccomp_bpf binary started running fine! > > So now I'm confused, and I can no longer reproduce the > problem. Which probably means this was a false alarm, in > which case, my apologies.
You might have triggered TIF_AUDIT or whatever it's called, which causes a whole different path through the asm tangle, so you might really have a problem. Try auditctl -a task,never. If that doesn't change anything, try rebooting the guest. --Andy > > D. -- Andy Lutomirski AMA Capital Management, LLC -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
