The read and pread syscalls are permitted to return fewer bytes than requested (unlike the fread libc call which will only perform a short read on error or EOF). This is most likely to occur when binaries are located on slower filesystems (e.g. NFS or 9pfs)
ffab9d9 is a relatively recent regression here, but a lot of the code appears to be much older (e.g. the read call in 341b7dc)
In this MR I've focused on ntdll, but I see a lot of similar mishandling of the pread return value in `server/`. I will raise a bug as I don't intend to create a follow up MR at this time.