From mboxrd@z Thu Jan 1 00:00:00 1970 From: Rich Felker Date: Thu, 03 Sep 2020 05:46:18 +0000 Subject: Re: [PATCH 3/4] sh: Add SECCOMP_FILTER Message-Id: <20200903054617.GW3265@brightrain.aerifal.cx> List-Id: References: <20200722231322.419642-1-kernel@mkarcher.dialup.fu-berlin.de> <20200722231322.419642-3-kernel@mkarcher.dialup.fu-berlin.de> <20200828155024.GX3265@brightrain.aerifal.cx> <20200828163057.GY3265@brightrain.aerifal.cx> <82b625c2-23cb-69a4-7495-39427430c306@physik.fu-berlin.de> <20200828170259.GZ3265@brightrain.aerifal.cx> <20200829004939.GB3265@brightrain.aerifal.cx> <20200903035603.GV3265@brightrain.aerifal.cx> In-Reply-To: <20200903035603.GV3265@brightrain.aerifal.cx> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: John Paul Adrian Glaubitz Cc: Michael Karcher , linux-sh@vger.kernel.org, linux-kernel@vger.kernel.org, Yoshinori Sato On Wed, Sep 02, 2020 at 11:56:04PM -0400, Rich Felker wrote: > On Sat, Aug 29, 2020 at 01:09:43PM +0200, John Paul Adrian Glaubitz wrote: > > Hi! > > > > On 8/29/20 2:49 AM, Rich Felker wrote: > > > This restored my ability to use strace > > > > I can confirm that. However ... > > > > > and I've written and tested a minimal strace-like hack using > > > SECCOMP_RET_USER_NOTIF that works as > > > expected on both j2 and qemu-system-sh4, so I think the above is > > > correct. > > > > The seccomp live testsuite has regressed. > > > > [...] > > Test 58-live-tsync_notify%%001-00001 result: FAILURE 58-live-tsync_notify 6 ALLOW rc > > This is similar to 51. > > I think the commonality of all the failures is that they deal with > return values set by seccomp filters for blocked syscalls, which are > getting clobbered by ENOSYS from the failed syscall here. So I do need > to keep the code path that jumps over the actual syscall if > do_syscall_trace_enter returns -1, but that means > do_syscall_trace_enter must now be responsible for setting the return > value in non-seccomp failure paths. > > I'll experiment to see what's still needed if that change is made. OK, I think I have an explanation for the mechanism of the bug, and it really is a combination of the 2008 bug (confusion of r0 vs r3) and the SECCOMP_FILTER commit. When the syscall_trace_entry code path is in use, a syscall with argument 5 having value -1 causes do_syscall_trace_enter to return -1 (because it returns regs[0], which contains argument 5), which the change in entry-common.S interprets as a sign to skip the syscall and jump to syscall_exit, and things blow up from there. In particular, SYS_mmap2 is almost always called with -1 as the 5th argument (fd), and this is even more common on nommu where SYS_brk does not work. I'll follow up with a new proposed patch. Rich