cifs: Convert struct fealist away from 1-element array

The kernel is globally removing the ambiguous 0-length and 1-element
arrays in favor of flexible arrays, so that we can gain both compile-time
and run-time array bounds checking[1].

While struct fealist is defined as a "fake" flexible array (via a
1-element array), it is only used for examination of the first array
element. Walking the list is performed separately, so there is no reason
to treat the "list" member of struct fealist as anything other than a
single entry. Adjust the struct and code to match.

Additionally, struct fea uses the "name" member either as a dynamic
string, or is manually calculated from the start of the struct. Redefine
the member as a flexible array.

No machine code output differences are produced after these changes.

[1] For lots of details, see both:
    https://docs.kernel.org/process/deprecated.html#zero-length-and-one-element-arrays
    https://people.kernel.org/kees/bounded-flexible-arrays-in-c

Cc: Steve French <sfrench@samba.org>
Cc: Paulo Alcantara <pc@cjr.nz>
Cc: Ronnie Sahlberg <lsahlber@redhat.com>
Cc: Shyam Prasad N <sprasad@microsoft.com>
Cc: Tom Talpey <tom@talpey.com>
Cc: linux-cifs@vger.kernel.org
Cc: samba-technical@lists.samba.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/cifspdu.h

@@ -2583,7 +2583,7 @@ struct fea {
 	unsigned char EA_flags;
 	__u8 name_len;
 	__le16 value_len;
-	char name[1];
+	char name[];
 	/* optionally followed by value */
 } __attribute__((packed));
 /* flags for _FEA.fEA */
@@ -2591,7 +2591,7 @@ struct fea {
 
 struct fealist {
 	__le32 list_len;
-	struct fea list[1];
+	struct fea list;
 } __attribute__((packed));
 
 /* used to hold an arbitrary blob of data */

fs/cifs/cifssmb.c

@@ -5787,7 +5787,7 @@ CIFSSMBQAllEAs(const unsigned int xid, struct cifs_tcon *tcon,
 
 	/* account for ea list len */
 	list_len -= 4;
-	temp_fea = ea_response_data->list;
+	temp_fea = &ea_response_data->list;
 	temp_ptr = (char *)temp_fea;
 	while (list_len > 0) {
 		unsigned int name_len;
@@ -5902,7 +5902,7 @@ CIFSSMBSetEA(const unsigned int xid, struct cifs_tcon *tcon,
 	else
 		name_len = strnlen(ea_name, 255);
 
-	count = sizeof(*parm_data) + ea_value_len + name_len;
+	count = sizeof(*parm_data) + 1 + ea_value_len + name_len;
 	pSMB->MaxParameterCount = cpu_to_le16(2);
 	/* BB find max SMB PDU from sess */
 	pSMB->MaxDataCount = cpu_to_le16(1000);
@@ -5926,14 +5926,14 @@ CIFSSMBSetEA(const unsigned int xid, struct cifs_tcon *tcon,
 	byte_count = 3 /* pad */  + params + count;
 	pSMB->DataCount = cpu_to_le16(count);
 	parm_data->list_len = cpu_to_le32(count);
-	parm_data->list[0].EA_flags = 0;
+	parm_data->list.EA_flags = 0;
 	/* we checked above that name len is less than 255 */
-	parm_data->list[0].name_len = (__u8)name_len;
+	parm_data->list.name_len = (__u8)name_len;
 	/* EA names are always ASCII */
 	if (ea_name)
-		strncpy(parm_data->list[0].name, ea_name, name_len);
-	parm_data->list[0].name[name_len] = 0;
-	parm_data->list[0].value_len = cpu_to_le16(ea_value_len);
+		strncpy(parm_data->list.name, ea_name, name_len);
+	parm_data->list.name[name_len] = '\0';
+	parm_data->list.value_len = cpu_to_le16(ea_value_len);
 	/* caller ensures that ea_value_len is less than 64K but
 	we need to ensure that it fits within the smb */
 
@@ -5941,7 +5941,7 @@ CIFSSMBSetEA(const unsigned int xid, struct cifs_tcon *tcon,
 	     negotiated SMB buffer size BB */
 	/* if (ea_value_len > buffer_size - 512 (enough for header)) */
 	if (ea_value_len)
-		memcpy(parm_data->list[0].name+name_len+1,
+		memcpy(parm_data->list.name + name_len + 1,
 		       ea_value, ea_value_len);
 
 	pSMB->TotalDataCount = pSMB->DataCount;