Vaibhav Jain [off-list ref] writes:

....

 +

+/* Papr-scm-header + payload expected with ND_CMD_CALL ioctl from libnvdimm */
+struct nd_pdsm_cmd_pkg {
+	struct nd_cmd_pkg hdr;	/* Package header containing sub-cmd */
+	__s32 cmd_status;	/* Out: Sub-cmd status returned back */
+	__u16 payload_offset;	/* In: offset from start of struct */
+	__u16 payload_version;	/* In/Out: version of the payload */
+	__u8 payload[];		/* In/Out: Sub-cmd data buffer */
+} __packed;

that payload_offset can be avoided if we prevent userspace to user a
different variant of nd_pdsm_cmd_pkg which different header. We can keep
things simpler if we can always find payload at
nd_pdsm_cmd_pkg->payload.

+
+/*
+ * Methods to be embedded in ND_CMD_CALL request. These are sent to the kernel
+ * via 'nd_pdsm_cmd_pkg.hdr.nd_command' member of the ioctl struct
+ */
+enum papr_scm_pdsm {
+	PAPR_SCM_PDSM_MIN = 0x0,
+	PAPR_SCM_PDSM_MAX,
+};
+
+/* Convert a libnvdimm nd_cmd_pkg to pdsm specific pkg */
+static inline struct nd_pdsm_cmd_pkg *nd_to_pdsm_cmd_pkg(struct nd_cmd_pkg *cmd)
+{
+	return (struct nd_pdsm_cmd_pkg *) cmd;
+}
+
+/* Return the payload pointer for a given pcmd */
+static inline void *pdsm_cmd_to_payload(struct nd_pdsm_cmd_pkg *pcmd)
+{
+	if (pcmd->hdr.nd_size_in == 0 && pcmd->hdr.nd_size_out == 0)
+		return NULL;
+	else
+		return (void *)((__u8 *) pcmd + pcmd->payload_offset);
+}
+

we need to make sure userspace is not passing a wrong payload_offset.

and in the next patch you do

+	/* Copy the health struct to the payload */
+	memcpy(pdsm_cmd_to_payload(pkg), &p->health, copysize);
+	pkg->hdr.nd_fw_size = copysize;
+

All this can be simplified if you can keep payload at
nd_pdsm_cmd_pkg->payload.

If you still want to have the ability to extend the header, then added a
reserved field similar to nd_cmd_pkg.


-aneesh