Re: [PATCH v7 1/6] seccomp: add a return code to trap to userspace

Tycho Andersen
2018-10-17T20:29:39Z

On Thu, Sep 27, 2018 at 02:31:24PM -0700, Kees Cook wrote:
> On Thu, Sep 27, 2018 at 8:11 AM, Tycho Andersen <tycho@tycho.ws> 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.

Tycho