smb: client: disable path remapping with POSIX extensions

JIRA: https://issues.redhat.com/browse/RHEL-96872

commit 9331005
Author: Philipp Kerling <pkerling@casix.org>
Date:   Sun Jun 8 20:59:00 2025 +0200

    smb: client: disable path remapping with POSIX extensions
    
    If SMB 3.1.1 POSIX Extensions are available and negotiated, the client
    should be able to use all characters and not remap anything. Currently, the
    user has to explicitly request this behavior by specifying the "nomapposix"
    mount option.
    
    Link: https://lore.kernel.org/4195bb677b33d680e77549890a4f4dd3b474ceaf.camel@rx2.rx-server.de
    Signed-off-by: Philipp Kerling <pkerling@casix.org>
    Reviewed-by: Namjae Jeon <linkinjeon@kernel.org>
    Signed-off-by: Steve French <stfrench@microsoft.com>

Signed-off-by: Paulo Alcantara <paalcant@redhat.com>

Author: pcacjr

Documentation/admin-guide/cifs/usage.rst

@@ -270,6 +270,8 @@ configured for Unix Extensions (and the client has not disabled
 illegal Windows/NTFS/SMB characters to a remap range (this mount parameter
 is the default for SMB3). This remap (``mapposix``) range is also
 compatible with Mac (and "Services for Mac" on some older Windows).
+When POSIX Extensions for SMB 3.1.1 are negotiated, remapping is automatically
+disabled.
 
 CIFS VFS Mount Options
 ======================

fs/smb/client/connect.c

@@ -3718,9 +3718,15 @@ int cifs_mount_get_tcon(struct cifs_mount_ctx *mnt_ctx)
 		goto out;
 	}
 
-	/* if new SMB3.11 POSIX extensions are supported do not remap / and \ */
-	if (tcon->posix_extensions)
+	/*
+	 * if new SMB3.11 POSIX extensions are supported, do not change anything in the
+	 * path (i.e., do not remap / and \ and do not map any special characters)
+	 */
+	if (tcon->posix_extensions) {
 		cifs_sb->mnt_cifs_flags |= CIFS_MOUNT_POSIX_PATHS;
+		cifs_sb->mnt_cifs_flags &= ~(CIFS_MOUNT_MAP_SFM_CHR |
+					     CIFS_MOUNT_MAP_SPECIAL_CHR);
+	}
 
 #ifdef CONFIG_CIFS_ALLOW_INSECURE_LEGACY
 	/* tell server which Unix caps we support */