truncate on MAP_SHARED files in ramfs filesystems on no-mmu

[Mike Frysinger <vapier.adi@gmail.com>]

reviewing LTP tests shows mmap09 failing.  this test creates a file of
a certain length, opens it and creates a shared mapping, and then
tries various truncate tests:
truncate to a smaller size
truncate to a larger size
truncate to size 0

the first and last fail on no-mmu due to
file-nommu.c:ramfs_nommu_resize() rejecting attempts to shrink on a
shared mapping:
    /* check that a decrease in size doesn't cut off any shared mappings */
    if (newsize < size) {
        ret = ramfs_nommu_check_mappings(inode, newsize, size);
        if (ret < 0)
            return ret;
    }

my question is why ?  if an application maps a fd with MAP_SHARED,
truncates it, and then it or someone else who has that fd mapped tries
to access the now-invalid tail end, that is a bug in the application.
i dont see why we should be protecting users here from their own buggy
code ?
-mike