Setcap (libcap) - Failed to set capabilities on file: Not supported

[HectoPascal]

Hallo,

i have a x86-64 custom build that needs 'setcap'.

Any idea why it throws 'not supported'?

OpenWrt SNAPSHOT, r22454-9186da4027
Linux OpenWrt 5.15.104 #0 SMP Wed Mar 29 17:36:33 2023 x86_64 GNU/Linux

# setcap cap_sys_nice=eip /bin/chrt
Failed to set capabilities on file '/bin/chrt': Not supported

Kconfig includes:

CONFIG_USE_FS_ACL_ATTR=y
CONFIG_KERNEL_EXT4_FS_POSIX_ACL=y
CONFIG_PACKAGE_libcap-bin=y
CONFIG_PACKAGE_tinyionice=y

Filesystem (ext4):

# tune2fs -l /dev/vda2 |grep -A2 "Filesystem features:"
Filesystem features:      has_journal ext_attr resize_inode dir_index orphan_file filetype needs_recovery extent 64bit flex_bg metadata_csum_seed sparse_super large_file huge_file dir_nlink extra_isize metadata_csum orphan_present
Filesystem flags:         signed_directory_hash 
Default mount options:    journal_data_writeback user_xattr acl

[HectoPascal]

CONFIG_KERNEL_EXT4_FS_SECURITY=y is needed for setcap/setxattr "security.capability" namespace prefix.

strace shows:
setxattr("/bin/chrt", "security.capability", "\1\0\0\2\0\1\0\0\0\1\0\0\0\0\0\0\0\0\0", 20, 0) = -1 EOPNOTSUPP (Not supported)

setxattr(2) says:

ENOTSUP
       The namespace prefix of name is not valid.

ENOTSUP
       Extended attributes are not supported by the filesystem,
       or are disabled