cifs: get mode bits from special sid on stat

When mounting with "modefromsid" retrieve mode bits from
special SID (S-1-5-88-3) on stat.  Subsequent patch will fix
setattr (chmod) to save mode bits in S-1-5-88-3-<mode>

Note that when an ACE matching S-1-5-88-3 is not found, we
default the mode to an approximation based on the owner, group
and everyone permissions (as with the "cifsacl" mount option).

See See e.g.
    https://docs.microsoft.com/en-us/previous-versions/windows/it-pro/windows-server-2008-R2-and-2008/hh509017(v=ws.10)Signed-off-by: NSteve French <stfrench@microsoft.com>

fs/cifs/cifsacl.c

@@ -701,10 +701,9 @@ static void dump_ace(struct cifs_ace *pace, char *end_of_acl)
 }
 #endif
 
-
 static void parse_dacl(struct cifs_acl *pdacl, char *end_of_acl,
 		       struct cifs_sid *pownersid, struct cifs_sid *pgrpsid,
-		       struct cifs_fattr *fattr)
+		       struct cifs_fattr *fattr, bool mode_from_special_sid)
 {
 	int i;
 	int num_aces = 0;
@@ -757,22 +756,34 @@ static void parse_dacl(struct cifs_acl *pdacl, char *end_of_acl,
 #ifdef CONFIG_CIFS_DEBUG2
 			dump_ace(ppace[i], end_of_acl);
 #endif
-			if (compare_sids(&(ppace[i]->sid), pownersid) == 0)
+			if (mode_from_special_sid &&
+			    (compare_sids(&(ppace[i]->sid),
+					  &sid_unix_NFS_mode) == 0)) {
+				/*
+				 * Full permissions are:
+				 * 07777 = S_ISUID | S_ISGID | S_ISVTX |
+				 *         S_IRWXU | S_IRWXG | S_IRWXO
+				 */
+				fattr->cf_mode &= ~07777;
+				fattr->cf_mode |=
+					le32_to_cpu(ppace[i]->sid.sub_auth[2]);
+				break;
+			} else if (compare_sids(&(ppace[i]->sid), pownersid) == 0)
 				access_flags_to_mode(ppace[i]->access_req,
 						     ppace[i]->type,
 						     &fattr->cf_mode,
 						     &user_mask);
-			if (compare_sids(&(ppace[i]->sid), pgrpsid) == 0)
+			else if (compare_sids(&(ppace[i]->sid), pgrpsid) == 0)
 				access_flags_to_mode(ppace[i]->access_req,
 						     ppace[i]->type,
 						     &fattr->cf_mode,
 						     &group_mask);
-			if (compare_sids(&(ppace[i]->sid), &sid_everyone) == 0)
+			else if (compare_sids(&(ppace[i]->sid), &sid_everyone) == 0)
 				access_flags_to_mode(ppace[i]->access_req,
 						     ppace[i]->type,
 						     &fattr->cf_mode,
 						     &other_mask);
-			if (compare_sids(&(ppace[i]->sid), &sid_authusers) == 0)
+			else if (compare_sids(&(ppace[i]->sid), &sid_authusers) == 0)
 				access_flags_to_mode(ppace[i]->access_req,
 						     ppace[i]->type,
 						     &fattr->cf_mode,
@@ -851,7 +862,8 @@ static int parse_sid(struct cifs_sid *psid, char *end_of_acl)
 
 /* Convert CIFS ACL to POSIX form */
 static int parse_sec_desc(struct cifs_sb_info *cifs_sb,
-		struct cifs_ntsd *pntsd, int acl_len, struct cifs_fattr *fattr)
+		struct cifs_ntsd *pntsd, int acl_len, struct cifs_fattr *fattr,
+		bool get_mode_from_special_sid)
 {
 	int rc = 0;
 	struct cifs_sid *owner_sid_ptr, *group_sid_ptr;
@@ -900,7 +912,7 @@ static int parse_sec_desc(struct cifs_sb_info *cifs_sb,
 
 	if (dacloffset)
 		parse_dacl(dacl_ptr, end_of_acl, owner_sid_ptr,
-			   group_sid_ptr, fattr);
+			   group_sid_ptr, fattr, get_mode_from_special_sid);
 	else
 		cifs_dbg(FYI, "no ACL\n"); /* BB grant all or default perms? */
 
@@ -1128,8 +1140,8 @@ int set_cifs_acl(struct cifs_ntsd *pnntsd, __u32 acllen,
 /* Translate the CIFS ACL (similar to NTFS ACL) for a file into mode bits */
 int
 cifs_acl_to_fattr(struct cifs_sb_info *cifs_sb, struct cifs_fattr *fattr,
-		  struct inode *inode, const char *path,
-		  const struct cifs_fid *pfid)
+		  struct inode *inode, bool mode_from_special_sid,
+		  const char *path, const struct cifs_fid *pfid)
 {
 	struct cifs_ntsd *pntsd = NULL;
 	u32 acllen = 0;
@@ -1156,8 +1168,11 @@ cifs_acl_to_fattr(struct cifs_sb_info *cifs_sb, struct cifs_fattr *fattr,
 	if (IS_ERR(pntsd)) {
 		rc = PTR_ERR(pntsd);
 		cifs_dbg(VFS, "%s: error %d getting sec desc\n", __func__, rc);
+	} else if (mode_from_special_sid) {
+		rc = parse_sec_desc(cifs_sb, pntsd, acllen, fattr, true);
 	} else {
-		rc = parse_sec_desc(cifs_sb, pntsd, acllen, fattr);
+		/* get approximated mode from ACL */
+		rc = parse_sec_desc(cifs_sb, pntsd, acllen, fattr, false);
 		kfree(pntsd);
 		if (rc)
 			cifs_dbg(VFS, "parse sec desc failed rc = %d\n", rc);

fs/cifs/cifsproto.h

@@ -197,6 +197,7 @@ extern int cifs_rename_pending_delete(const char *full_path,
 				      const unsigned int xid);
 extern int cifs_acl_to_fattr(struct cifs_sb_info *cifs_sb,
 			      struct cifs_fattr *fattr, struct inode *inode,
+			      bool get_mode_from_special_sid,
 			      const char *path, const struct cifs_fid *pfid);
 extern int id_mode_to_cifs_acl(struct inode *inode, const char *path, __u64,
 					kuid_t, kgid_t);

fs/cifs/inode.c

@@ -893,8 +893,17 @@ cifs_get_inode_info(struct inode **inode, const char *full_path,
 	}
 
 	/* fill in 0777 bits from ACL */
-	if (cifs_sb->mnt_cifs_flags & CIFS_MOUNT_CIFS_ACL) {
-		rc = cifs_acl_to_fattr(cifs_sb, &fattr, *inode, full_path, fid);
+	if (cifs_sb->mnt_cifs_flags & CIFS_MOUNT_MODE_FROM_SID) {
+		rc = cifs_acl_to_fattr(cifs_sb, &fattr, *inode, true,
+				       full_path, fid);
+		if (rc) {
+			cifs_dbg(FYI, "%s: Get mode from SID failed. rc=%d\n",
+				__func__, rc);
+			goto cgii_exit;
+		}
+	} else if (cifs_sb->mnt_cifs_flags & CIFS_MOUNT_CIFS_ACL) {
+		rc = cifs_acl_to_fattr(cifs_sb, &fattr, *inode, false,
+				       full_path, fid);
 		if (rc) {
 			cifs_dbg(FYI, "%s: Getting ACL failed with error: %d\n",
 				 __func__, rc);