gABI extension proposal: PT_SHMMAP

[Cary Coutant]

> Why is MAP_SHARED even needed for the linux vdso use-case?
>
> All the mappings created for the vdso are read-only mappings, right?
> And on Linux, MAP_PRIVATE mappings already reflect changes made to the
> underlying file, so long as the process hasn't written to the page, I
> think?
>
> So, if you're intending to use this to implement the "vvar" mapping,
> isn't the default MAP_PRIVATE already sufficient?

He mentioned that this was for the kernel-provided *data* page,
implying to me that he wants a writable, but still sharable (i.e., not
copy-on-write) mapping.

It seems generally useful to me. I probably wouldn't object to a new
gABI bit, but a little more background would be helpful in deciding.

-cary