SMB3.1.1: remove confusing mount warning when no SPNEGO info on negprot rsp

Azure does not send an SPNEGO blob in the negotiate protocol response,
so we shouldn't assume that it is there when validating the location
of the first negotiate context.  This avoids the potential confusing
mount warning:

   CIFS: Invalid negotiate context offset

CC: Stable <stable@vger.kernel.org>
Reviewed-by: Pavel Shilovsky <pshilov@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/smb2misc.c

@@ -94,6 +94,8 @@ static const __le16 smb2_rsp_struct_sizes[NUMBER_OF_SMB2_COMMANDS] = {
 	/* SMB2_OPLOCK_BREAK */ cpu_to_le16(24)
 };
 
+#define SMB311_NEGPROT_BASE_SIZE (sizeof(struct smb2_sync_hdr) + sizeof(struct smb2_negotiate_rsp))
+
 static __u32 get_neg_ctxt_len(struct smb2_sync_hdr *hdr, __u32 len,
 			      __u32 non_ctxlen)
 {
@@ -109,11 +111,17 @@ static __u32 get_neg_ctxt_len(struct smb2_sync_hdr *hdr, __u32 len,
 
 	/* Make sure that negotiate contexts start after gss security blob */
 	nc_offset = le32_to_cpu(pneg_rsp->NegotiateContextOffset);
-	if (nc_offset < non_ctxlen) {
-		pr_warn_once("Invalid negotiate context offset\n");
+	if (nc_offset + 1 < non_ctxlen) {
+		pr_warn_once("Invalid negotiate context offset %d\n", nc_offset);
 		return 0;
-	}
-	size_of_pad_before_neg_ctxts = nc_offset - non_ctxlen;
+	} else if (nc_offset + 1 == non_ctxlen) {
+		cifs_dbg(FYI, "no SPNEGO security blob in negprot rsp\n");
+		size_of_pad_before_neg_ctxts = 0;
+	} else if (non_ctxlen == SMB311_NEGPROT_BASE_SIZE)
+		/* has padding, but no SPNEGO blob */
+		size_of_pad_before_neg_ctxts = nc_offset - non_ctxlen + 1;
+	else
+		size_of_pad_before_neg_ctxts = nc_offset - non_ctxlen;
 
 	/* Verify that at least minimal negotiate contexts fit within frame */
 	if (len < nc_offset + (neg_count * sizeof(struct smb2_neg_context))) {