cifs: Replace a couple of one-element arrays with flexible-array members

One-element arrays are deprecated, and we are replacing them with flexible
array members instead. So, replace one-element arrays with flexible-array
member in structs negotiate_req and extended_response, and refactor the
rest of the code, accordingly.

Also, make use of the DECLARE_FLEX_ARRAY() helper to declare flexible
array member EncryptionKey in union u. This new helper allows for
flexible-array members in unions.

Change pointer notation to proper array notation in a call to memcpy()
where flexible-array member DialectsArray is being used as destination
argument.

Important to mention is that doing a build before/after this patch results
in no binary output differences.

This helps with the ongoing efforts to tighten the FORTIFY_SOURCE
routines on memcpy() and help us make progress towards globally
enabling -fstrict-flex-arrays=3 [1].

Link: https://github.com/KSPP/linux/issues/79
Link: https://github.com/KSPP/linux/issues/229
Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101836 [1]
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/cifspdu.h

@@ -483,7 +483,7 @@ put_bcc(__u16 count, struct smb_hdr *hdr)
 typedef struct negotiate_req {
 	struct smb_hdr hdr;	/* wct = 0 */
 	__le16 ByteCount;
-	unsigned char DialectsArray[1];
+	unsigned char DialectsArray[];
 } __attribute__((packed)) NEGOTIATE_REQ;
 
 #define MIN_TZ_ADJ (15 * 60) /* minimum grid for timezones in seconds */
@@ -508,13 +508,14 @@ typedef struct negotiate_rsp {
 	__u8 EncryptionKeyLength;
 	__u16 ByteCount;
 	union {
-		unsigned char EncryptionKey[1];	/* cap extended security off */
+		/* cap extended security off */
+		DECLARE_FLEX_ARRAY(unsigned char, EncryptionKey);
 		/* followed by Domain name - if extended security is off */
 		/* followed by 16 bytes of server GUID */
 		/* then security blob if cap_extended_security negotiated */
 		struct {
 			unsigned char GUID[SMB1_CLIENT_GUID_SIZE];
-			unsigned char SecurityBlob[1];
+			unsigned char SecurityBlob[];
 		} __attribute__((packed)) extended_response;
 	} __attribute__((packed)) u;
 } __attribute__((packed)) NEGOTIATE_RSP;

fs/cifs/cifssmb.c

@@ -465,7 +465,7 @@ CIFSSMBNegotiate(const unsigned int xid,
 	for (i = 0; i < CIFS_NUM_PROT; i++) {
 		size_t len = strlen(protocols[i].name) + 1;
 
-		memcpy(pSMB->DialectsArray+count, protocols[i].name, len);
+		memcpy(&pSMB->DialectsArray[count], protocols[i].name, len);
 		count += len;
 	}
 	inc_rfc1001_len(pSMB, count);