Re: Why does _dl_protect_relro align the end address down?

["H.J. Lu" <hjl.tools@gmail.com>]

On Tue, Jan 17, 2023 at 6:30 AM Adhemerval Zanella Netto
<adhemerval.zanella@linaro.org> wrote:
>
>
>
> On 17/01/23 11:16, Florian Weimer wrote:
> > * Adhemerval Zanella Netto:
> >
> >> On 17/01/23 04:57, Florian Weimer via Libc-help wrote:
> >>> * Joel Molin via Libc-help:
> >>>
> >>>> Since mprotect will protect entire pages anyway, wouldn't it make a lot
> >>>> more sense here to say `end = ALIGN_UP(...)`? Apart from feeling more
> >>>> intuitive, it also seems like it would avoid page size inconsistencies
> >>>> between runtime and link editing like the one above.
> >>>
> >>> ALIGN_UP might make memory read-only that should not be.
> >>>
> >>> There's some discussion about the alignment here:
> >>>
> >>>   PT_GNU_RELRO is somewhat broken
> >>>   <https://sourceware.org/pipermail/libc-alpha/2022-May/138638.html>
> >>>
> >>> I still think this is fairly broken, but there does not seem to be much
> >>> desire to fix it.
> >>
> >> It was not clear to me from that discussion what you think it is still
> >> broken with PT_GNU_RELRO.
> >
> > Using ALIGN_DOWN for the start address can't be right.
> >
>
> Do we have a bug report for this? Because H.J explanation [1] only describes
> ld support for the end of the RO segment.
>
> [1] https://sourceware.org/pipermail/libc-alpha/2022-May/138642.html

If the end of the PT_GNU_RELRO segment isn't page aligned,
ALIGN_UP may change the executable page to read-only.

-- 
H.J.