Comment 10 for bug 1810433

Proposed patch at https://patchwork.ozlabs.org/patch/1022092/

NB: my guess is that your pwrite on 32-bit arm test is behaving like that because it isn't going via the pwrite64 syscall, or possibly because glibc there is dealing with the NULL special case early. Use QEMU's -strace argument (or strace on real h/w) to see what libc is actually turning that pwrite() function call into at the syscall level.