syscalls using __syscall but only actually returning 32bits, such as mmap(), specially : they set the return value in td->td_retval[0], but the userland functions will expect this in r1, and not in r0 as it is normally done, as it is the LSB. So add a special case for all these syscalls (all except lseek, which truly returns 64bits). Many thanks to Peter Grehan for his patience while explaining me the issue.