linux-user: no implementation of fchmodat2 system call

Host environment

• Operating system: Arch Linux
• OS/kernel version: GNU/Linux / 6.13.4
• Architecture: x86_64
• QEMU flavor: qemu-aarch64-static
• QEMU version: 10.0.0
• QEMU command line:

  QEMU_LD_PREFIX=/<pathTo>/sysroot/ \
  qemu-aarch64-static \
    -d strace \
    /<pathTo>/sysroot/usr/lib/ld-linux-aarch64.so.1 \
    /<pathTo>/sysroot/usr/bin/chmod \
      +x \
      <anyFile>

Emulated/Virtualized environment

• Operating system: Linux
• OS/kernel version: 6.13.4
• Architecture: aarch64

Description of problem

chmod: changing permissions of '<file>': Function not implemented

Steps to reproduce

1. Cross-compile glibc (https://github.com/zatrazz/glibc/tree/azanella/clang-rtlib), bash, coreutils for arm{,64}
2. chroot into environment
3. run chmod +x <anyFile> (done by gperf autogen.sh)

Additional information

3322628 set_robust_list(0x7fa7d3c72f40,24) = -1 errno=38 (Function not implemented)
3322628 Unknown syscall 293

#define __NR_rseq 293 */usr/include/asm-generic/unistd.h

With -E GLIBC_TUNABLES=glibc.pthread.rseq=0 (thanks Arsen):

3323105 newfstatat(AT_FDCWD,"anyFile",0x000000000005c750,0) = 0
3323105 Unknown syscall 452

#define __NR_fchmodat2 452

It works for a musl-based system which is calling fchmodat (not 2) instead.