On Thu, Sep 27, 2018 at 02:31:24PM -0700, Kees Cook wrote:

On Thu, Sep 27, 2018 at 8:11 AM, Tycho Andersen [off-list ref] wrote:

quoted

@@ -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