From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Mon, 22 Oct 2018 11:42:41 +0200 From: Christian Brauner To: Tycho Andersen Cc: Kees Cook , Jann Horn , Linux API , Linux Containers , Akihiro Suda , Oleg Nesterov , LKML , "Eric W . Biederman" , "linux-fsdevel@vger.kernel.org" , Christian Brauner , Andy Lutomirski Subject: Re: [PATCH v7 1/6] seccomp: add a return code to trap to userspace Message-ID: <20181022094240.mp7pd2m2abyttrfb@brauner.io> References: <20180927151119.9989-1-tycho@tycho.ws> <20180927151119.9989-2-tycho@tycho.ws> <20181017202933.GB14047@cisco> <20181021160437.GB25202@cisco> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20181021160437.GB25202@cisco> Sender: linux-kernel-owner@vger.kernel.org List-ID: On Sun, Oct 21, 2018 at 05:04:37PM +0100, Tycho Andersen wrote: > On Wed, Oct 17, 2018 at 03:21:02PM -0700, Kees Cook wrote: > > On Wed, Oct 17, 2018 at 1:29 PM, Tycho Andersen wrote: > > > On Thu, Sep 27, 2018 at 02:31:24PM -0700, Kees Cook wrote: > > >> On Thu, Sep 27, 2018 at 8:11 AM, Tycho Andersen wrote: > > >> > @@ -60,4 +62,29 @@ struct seccomp_data { > > >> > __u64 args[6]; > > >> > }; > > >> > > > >> > +struct seccomp_notif { > > >> > + __u16 len; > > >> > + __u64 id; > > >> > + __u32 pid; > > >> > + __u8 signaled; > > >> > + struct seccomp_data data; > > >> > +}; > > >> > + > > >> > +struct seccomp_notif_resp { > > >> > + __u16 len; > > >> > + __u64 id; > > >> > + __s32 error; > > >> > + __s64 val; > > >> > +}; > > >> > > >> So, len has to come first, for versioning. However, since it's ahead > > >> of a u64, this leaves a struct padding hole. pahole output: > > >> > > >> struct seccomp_notif { > > >> __u16 len; /* 0 2 */ > > >> > > >> /* XXX 6 bytes hole, try to pack */ > > >> > > >> __u64 id; /* 8 8 */ > > >> __u32 pid; /* 16 4 */ > > >> __u8 signaled; /* 20 1 */ > > >> > > >> /* XXX 3 bytes hole, try to pack */ > > >> > > >> struct seccomp_data data; /* 24 64 */ > > >> /* --- cacheline 1 boundary (64 bytes) was 24 bytes ago --- */ > > >> > > >> /* size: 88, cachelines: 2, members: 5 */ > > >> /* sum members: 79, holes: 2, sum holes: 9 */ > > >> /* last cacheline: 24 bytes */ > > >> }; > > >> struct seccomp_notif_resp { > > >> __u16 len; /* 0 2 */ > > >> > > >> /* XXX 6 bytes hole, try to pack */ > > >> > > >> __u64 id; /* 8 8 */ > > >> __s32 error; /* 16 4 */ > > >> > > >> /* XXX 4 bytes hole, try to pack */ > > >> > > >> __s64 val; /* 24 8 */ > > >> > > >> /* size: 32, cachelines: 1, members: 4 */ > > >> /* sum members: 22, holes: 2, sum holes: 10 */ > > >> /* last cacheline: 32 bytes */ > > >> }; > > >> > > >> How about making len u32, and moving pid and error above "id"? This > > >> leaves a hole after signaled, so changing "len" won't be sufficient > > >> for versioning here. Perhaps move it after data? > > > > > > Just to confirm my understanding; I've got these as: > > > > > > struct seccomp_notif { > > > __u32 len; /* 0 4 */ > > > __u32 pid; /* 4 4 */ > > > __u64 id; /* 8 8 */ > > > __u8 signaled; /* 16 1 */ > > > > > > /* XXX 7 bytes hole, try to pack */ > > > > > > struct seccomp_data data; /* 24 64 */ > > > /* --- cacheline 1 boundary (64 bytes) was 24 bytes ago --- */ > > > > > > /* size: 88, cachelines: 2, members: 5 */ > > > /* sum members: 81, holes: 1, sum holes: 7 */ > > > /* last cacheline: 24 bytes */ > > > }; > > > struct seccomp_notif_resp { > > > __u32 len; /* 0 4 */ > > > __s32 error; /* 4 4 */ > > > __u64 id; /* 8 8 */ > > > __s64 val; /* 16 8 */ > > > > > > /* size: 24, cachelines: 1, members: 4 */ > > > /* last cacheline: 24 bytes */ > > > }; > > > > > > in the next version. Since the structure has no padding at the end of > > > it, I think the Right Thing will happen. Note that this is slightly > > > different than what Kees suggested, if I add signaled after data, then > > > I end up with: > > > > > > struct seccomp_notif { > > > __u32 len; /* 0 4 */ > > > __u32 pid; /* 4 4 */ > > > __u64 id; /* 8 8 */ > > > struct seccomp_data data; /* 16 64 */ > > > /* --- cacheline 1 boundary (64 bytes) was 16 bytes ago --- */ > > > __u8 signaled; /* 80 1 */ > > > > > > /* size: 88, cachelines: 2, members: 5 */ > > > /* padding: 7 */ > > > /* last cacheline: 24 bytes */ > > > }; > > > > > > which I think will have the versioning problem if the next member > > > introduces is < 7 bytes. > > > > It'll be a problem in either place. What I was thinking was that > > specific versioning is required instead of just length. > > Euh, so I implemented this, and it sucks :). It's ugly, and generally > feels bad. > > What if instead we just get rid of versioning all together, and > instead introduce a u32 flags? We could have one flag right now > (SECCOMP_NOTIF_FLAG_SIGNALED), and use introduce others as we add more > information to the response. Then we can add > SECCOMP_NOTIF_FLAG_EXTRA_FOO, and add another SECCOMP_IOCTL_GET_FOO to > grab the info? > > FWIW, it's not really clear to me that we'll ever add anything to the > response since hopefully we'll land PUT_FD, so maybe this is all moot > anyway. I guess the only argument against a flag would be that you run out of bits quickly if your interface grows (cf. mount, netlink etc.). But this is likely not a concern here. I actually think that the way vfs capabilities are done is pretty nice. By accident or design they allow transparent translation between old and new formats in-kernel. So would be cool if we can have the same guarantee for this interface. Christian