cifs: client: enforce consistent handling of multichannel and max_channels

Previously, the behavior of the multichannel and max_channels mount
options was inconsistent and order-dependent. For example, specifying
"multichannel,max_channels=1" would result in 2 channels, while
"max_channels=1,multichannel" would result in 1 channel. Additionally,
conflicting combinations such as "nomultichannel,max_channels=3" or
"multichannel,max_channels=1" did not produce errors and could lead to
unexpected channel counts.

This commit introduces two new fields in smb3_fs_context to explicitly
track whether multichannel and max_channels were specified during
mount. The option parsing and validation logic is updated to ensure:
- The outcome is no longer dependent on the order of options.
- Conflicting combinations (e.g., "nomultichannel,max_channels=3" or
"multichannel,max_channels=1") are detected and result in an error.
- The number of channels created is consistent with the specified
options.

This improves the reliability and predictability of mount option
handling for SMB3 multichannel support.

Reviewed-by: Shyam Prasad N <[e-mail]>
Signed-off-by: Rajasi Mandal <[e-mail]>
Signed-off-by: Steve French <[e-mail]>

fs/smb/client/cifsfs.c

···
 	} else {
 		cifs_info("Attempting to mount %s\n", old_ctx->source);
 	}
-
 	cifs_sb = kzalloc(sizeof(*cifs_sb), GFP_KERNEL);
 	if (!cifs_sb)
 		return ERR_PTR(-ENOMEM);

fs/smb/client/fs_context.c

···
 	return 0;
 }
 
+static int smb3_handle_conflicting_options(struct fs_context *fc)
+{
+	struct smb3_fs_context *ctx = smb3_fc2context(fc);
+
+	if (ctx->multichannel_specified) {
+		if (ctx->multichannel) {
+			if (!ctx->max_channels_specified) {
+				ctx->max_channels = 2;
+			} else if (ctx->max_channels == 1) {
+				cifs_errorf(fc,
+					    "max_channels must be greater than 1 when multichannel is enabled\n");
+				return -EINVAL;
+			}
+		} else {
+			if (!ctx->max_channels_specified) {
+				ctx->max_channels = 1;
+			} else if (ctx->max_channels > 1) {
+				cifs_errorf(fc,
+					    "max_channels must be equal to 1 when multichannel is disabled\n");
+				return -EINVAL;
+			}
+		}
+	} else {
+		if (ctx->max_channels_specified) {
+			if (ctx->max_channels > 1)
+				ctx->multichannel = true;
+			else
+				ctx->multichannel = false;
+		} else {
+			ctx->multichannel = false;
+			ctx->max_channels = 1;
+		}
+	}
+
+	//resetting default values as remount doesn't initialize fs_context again
+	ctx->multichannel_specified = false;
+	ctx->max_channels_specified = false;
+
+	return 0;
+}
+
 static void smb3_fs_context_free(struct fs_context *fc);
 static int smb3_fs_context_parse_param(struct fs_context *fc,
 				       struct fs_parameter *param);
···
 		if (ret < 0)
 			break;
 	}
+	ret = smb3_handle_conflicting_options(fc);
 
 	return ret;
 }
···
 		ctx->nodelete = 1;
 		break;
 	case Opt_multichannel:
-		if (result.negated) {
+		ctx->multichannel_specified = true;
+		if (result.negated)
 			ctx->multichannel = false;
-			ctx->max_channels = 1;
-		} else {
+		else
 			ctx->multichannel = true;
-			/* if number of channels not specified, default to 2 */
-			if (ctx->max_channels < 2)
-				ctx->max_channels = 2;
-		}
 		break;
 	case Opt_uid:
 		ctx->linux_uid = result.uid;
···
 		ctx->max_credits = result.uint_32;
 		break;
 	case Opt_max_channels:
+		ctx->max_channels_specified = true;
 		if (result.uint_32 < 1 || result.uint_32 > CIFS_MAX_CHANNELS) {
 			cifs_errorf(fc, "%s: Invalid max_channels value, needs to be 1-%d\n",
 				 __func__, CIFS_MAX_CHANNELS);
 			goto cifs_parse_mount_err;
 		}
 		ctx->max_channels = result.uint_32;
-		/* If more than one channel requested ... they want multichan */
-		if (result.uint_32 > 1)
-			ctx->multichannel = true;
 		break;
 	case Opt_max_cached_dirs:
 		if (result.uint_32 < 1) {
···
 		goto cifs_parse_mount_err;
 	}
 
-	/*
-	 * Multichannel is not meaningful if max_channels is 1.
-	 * Force multichannel to false to ensure consistent configuration.
-	 */
-	if (ctx->multichannel && ctx->max_channels == 1)
-		ctx->multichannel = false;
-
 	return 0;
 
  cifs_parse_mount_err:
···
 
 	/* default to no multichannel (single server connection) */
 	ctx->multichannel = false;
+	ctx->multichannel_specified = false;
+	ctx->max_channels_specified = false;
 	ctx->max_channels = 1;
 
 	ctx->backupuid_specified = false; /* no backup intent for a user */

fs/smb/client/fs_context.h

···
 	bool domainauto:1;
 	bool rdma:1;
 	bool multichannel:1;
+	bool multichannel_specified:1; /* true if user specified multichannel or nomultichannel */
+	bool max_channels_specified:1; /* true if user specified max_channels */
 	bool use_client_guid:1;
 	/* reuse existing guid for multichannel */
 	u8 client_guid[SMB2_CLIENT_GUID_SIZE];