[PATCH 0/6] ld: Add "-z textonly" option to ELF linker

[H.J. Lu]

On Mon, Jan 8, 2018 at 7:26 AM, Michael Matz <matz@suse.de> wrote:
> Hi,
>
> On Mon, 8 Jan 2018, H.J. Lu wrote:
>
>> On Tue, Nov 14, 2017 at 6:41 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
>> > On Tue, Nov 14, 2017 at 5:58 AM, Michael Matz <matz@suse.de> wrote:
>> >> Hi,
>> >>
>> >> On Tue, 14 Nov 2017, H.J. Lu wrote:
>> >>
>> >>> > This is the result of the above problem, but it's not necessary.  Like in
>> >>> > a traditional two-LOAD-segment file, which also isn't larger than 2MB,
>> >>> > just because the page size is 2MB.  It achieves this by mapping the same
>> >>> > file bytes multiple times, and you could do the same.  E.g. for the file
>> >>> > above the better layout would be:
>> >>> >
>> >>> >   LOAD           0x000000 0x00000000 0x00000000 0x00200 0x00200 R   0x200000
>> >>> >   LOAD           0x000fd0 0x00200fd0 0x00200fd0 0x0002b 0x0002b R E 0x200000
>> >>> >   LOAD           0x001000 0x00201000 0x00201000 0x00058 0x00058 R   0x200000
>> >>> >   LOAD           0x001060 0x00401060 0x00401060 0x000a0 0x000a0 RW  0x200000
>> >>> >
>> >>> > But even then something is wrong: The RE and the second R page are both
>> >>> > mapped onto 0x0020xxxx, i.e. the same 2MB page, so can't have different
>> >>> > protections.  (You'd need 4k pages again, which defeats the whole purpose
>> >>> > of having 2MB pages in the file to start with).
>> >>>
>> >>> You have found out yourself that it is impossible.
>> >>
>> >> Well, as is the above makes not much sense.  It wants to stay at 2MB
>> >> alignment but can't actually make use of it due to having protection
>> >> changes within one 2MB page (from RE to R).  Hmm, which means the very
>> >> introduction of GNU_RELRO already broke the whole 2MB pages optimization
>> >> :-/ You're just increasing this detrimental effect to now also cause
>> >> uselessly huge files.
>> >
>> > GNU_RELRO is intended for common page size, which is 4K on x86-64.  If
>> > the page size is 2MB at run-time, GNU_RELRO isn't applied.
>> >
>> >> Probably 4k pages should be default with "-z textonly"  Hmpf :(
>> >
>> > We can discuss this after "-z textonly" is checked in.
>> >
>>
>> Given what happened, I'd like to check this patch set into binutils
>> 2.30.
>
> I'm assuming you mean Spectre and Meltdown?  I'm not sure what those have

See:

https://gcc.gnu.org/ml/gcc-patches/2018-01/msg00484.html

> to do with your patch.  I don't see what changed since November; I still
> think 4k pages should be default for this new option (and eventually also
> for GNU_RELRO, as even there 2MB pages are effectively useless).  Note

Page size should be a separate issue.

> that I haven't anything against the page per se (and don't have any say
> anyway); just the name is a bit generic, "textonly" ... what's that
> supposed to mean? :)  -z separate-text-segment perhaps?

-z codeonly -or z insnonly?


-- 
H.J.