[PATCH v4 06/13] tpm: add buffer function to point to returned parameters

[James Bottomley <James.Bottomley@HansenPartnership.com>]

Introducing encryption sessions changes where the return parameters
are located in the buffer because if a return session is present
they're 4 bytes beyond the header with those 4 bytes showing the
parameter length.  If there is no return session, then they're in the
usual place immediately after the header.  The tpm_buf_parameters()
encapsulates this calculation and should be used everywhere
&buf.data[TPM_HEADER_SIZE] is used now.

Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>

---
v4: add kdoc
---
 drivers/char/tpm/tpm-buf.c | 27 +++++++++++++++++++++++++++
 include/linux/tpm.h        |  2 ++
 2 files changed, 29 insertions(+)

diff --git a/drivers/char/tpm/tpm-buf.c b/drivers/char/tpm/tpm-buf.c
index da0f6e725c3f..d107321bcdff 100644
--- a/drivers/char/tpm/tpm-buf.c
+++ b/drivers/char/tpm/tpm-buf.c
@@ -277,3 +277,30 @@ u32 tpm_get_inc_u32(const u8 **ptr)
 	return val;
 }
 EXPORT_SYMBOL_GPL(tpm_get_inc_u32);
+
+static u16 tpm_buf_tag(struct tpm_buf *buf)
+{
+	struct tpm_header *head = (struct tpm_header *)buf->data;
+
+	return be16_to_cpu(head->tag);
+}
+
+/**
+ * tpm_buf_parameters - return the parameters area of the tpm_buf
+ * @buf: tpm_buf to use
+ *
+ * Where the parameters are located depends on the tag of a TPM
+ * command. Evaluate this and return a pointer to the first byte of
+ * the parameters area.
+ *
+ * @return: pointer to parameters area
+ */
+u8 *tpm_buf_parameters(struct tpm_buf *buf)
+{
+	int offset = TPM_HEADER_SIZE;
+
+	if (tpm_buf_tag(buf) == TPM2_ST_SESSIONS)
+		offset += 4;
+
+	return &buf->data[offset];
+}
diff --git a/include/linux/tpm.h b/include/linux/tpm.h
index 845eadfed715..d2fea2afd37c 100644
--- a/include/linux/tpm.h
+++ b/include/linux/tpm.h
@@ -340,6 +340,8 @@ u8 tpm_get_inc_u8(const u8 **ptr);
 u16 tpm_get_inc_u16(const u8 **ptr);
 u32 tpm_get_inc_u32(const u8 **ptr);
 
+u8 *tpm_buf_parameters(struct tpm_buf *buf);
+
 /*
  * Check if TPM device is in the firmware upgrade mode.
  */
-- 
2.35.3