Write-only mapping a O_WRONLY opened file supposed to work?

Question

Is mmap() supposed to be able to create a write-only mapping of a O_WRONLY opened file?

I am asking because following fails on a Linux 4.0.

Answer 1

I don't think the x86 hardware supports write-only pages, so write access implies read. But it seems to be a more general requirement than just x86 - mm/mmap.c contains this code in do_mmap_pgoff():

    case MAP_SHARED:
        if ((prot&PROT_WRITE) && !(file->f_mode&FMODE_WRITE))
            return -EACCES;
        ....
        /* fall through */
    case MAP_PRIVATE:
        if (!(file->f_mode & FMODE_READ))
            return -EACCES;

I think that explains what you're seeing.