[Regression v4.2 ?] 32-bit seccomp-BPF returned errno values wrong in VM?

From: David Drysdale
Date: Thu Aug 13 2015 - 04:30:55 EST

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'.

- 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...


seccomp_bpf failure outputs:

seccomp_bpf.c:533:global.ERRNO_valid:Expected 7 (7) ==
(*__errno_location ()) (1)
seccomp_bpf.c:560:global.ERRNO_zero:Expected 0 (0) == read(0, ((void
*)0), 0) (4294967295)
seccomp_bpf.c:587:global.ERRNO_capped:Expected 4095 (4095) ==
(*__errno_location ()) (1)
seccomp_bpf.c:905:precedence.errno_is_third:Expected 0 (0) ==
syscall(20) (4294967295)
seccomp_bpf.c:925:precedence.errno_is_third_in_any_order:Expected 0
(0) == syscall(20) (4294967295)
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