musl is strictly POSIX conformant, which means it uses int for msg_iovlen and socklen_t for msg_controllen in the msghdr struct.  (See http://git.musl-libc.org/cgit/musl/commit/arch/x86_64/bits/socket.h?id=7168790763cdeb794df52be6e3b39fbb021c5a64 for more information.)

POSIX defines msghdr as containing "at least" the standard members.  (See http://pubs.opengroup.org/onlinepubs/009696699/basedefs/sys/socket.h.html for citation.)

Since the new shared fd code in KIO (non-conformantly) assumes the members and layout of msghdr in its ctor, it FTBFS on 64-bit musl platforms (including x86_64, ppc64, aarch64):

In file included from /usr/src/packages/user/kio/src/kio-5.43.0/src/ioslaves/file/fdreceiver.cpp:23:0:
/usr/src/packages/user/kio/src/kio-5.43.0/src/ioslaves/file/sharefd_p.h: In constructor 'FDMessageHeader::FDMessageHeader()':
/usr/src/packages/user/kio/src/kio-5.43.0/src/ioslaves/file/sharefd_p.h:78:37: error: invalid conversion from 'char (*)[24]' to 'int' [-fpermissive]
         , msg{nullptr, 0,  &io, 1,  &cmsg_buf, sizeof cmsg_buf, 0}
                                     ^~~~~~~~~
/usr/src/packages/user/kio/src/kio-5.43.0/src/ioslaves/file/sharefd_p.h:78:55: error: invalid conversion from 'long unsigned int' to 'void*' [-fpermissive]
         , msg{nullptr, 0,  &io, 1,  &cmsg_buf, sizeof cmsg_buf, 0}
                                                       ^~~~~~~~
/usr/src/packages/user/kio/src/kio-5.43.0/src/ioslaves/file/sharefd_p.h:78:66: warning: missing initializer for member 'msghdr::__pad2' [-Wmissing-field-initializers]
         , msg{nullptr, 0,  &io, 1,  &cmsg_buf, sizeof cmsg_buf, 0}
                                                                  ^
/usr/src/packages/user/kio/src/kio-5.43.0/src/ioslaves/file/sharefd_p.h:78:66: warning: missing initializer for member 'msghdr::msg_flags' [-Wmissing-field-initializers]