Linux: reject read/write mapping to immutable file only on VM_SHARED

Private read/write mapping can't be used to modify the mapped files, so
they will remain be immutable. Private read/write mappings are usually
used to load the data segment of executable files, rejecting them will
rendering immutable executable files to stop working.

Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: WHR <[email protected]>
Closes #15344

Author: Low-power

module/os/linux/zfs/zfs_vnops_os.c

@@ -4078,8 +4078,8 @@ zfs_map(struct inode *ip, offset_t off, caddr_t *addrp, size_t len,
 	if ((error = zfs_enter_verify_zp(zfsvfs, zp, FTAG)) != 0)
 		return (error);
 
-	if ((vm_flags & VM_WRITE) && (zp->z_pflags &
-	    (ZFS_IMMUTABLE | ZFS_READONLY | ZFS_APPENDONLY))) {
+	if ((vm_flags & VM_WRITE) && (vm_flags & VM_SHARED) &&
+	    (zp->z_pflags & (ZFS_IMMUTABLE | ZFS_READONLY | ZFS_APPENDONLY))) {
 		zfs_exit(zfsvfs, FTAG);
 		return (SET_ERROR(EPERM));
 	}