Re: [PATCH] ARM: Add support for SHF_ARM_NOREAD section flag

[Christophe MONAT <christophe dot monat at st dot com>]

Hi Andre, Nick,

[Sorry that Mickael does not answer by himself, he was not subscribed to 
the list so far -- now corrected].

On 01/07/16 19:33, Andre Vieira wrote:
> Terry sent a patch upstream to handle the noread attribute in 2014:
> https://sourceware.org/ml/binutils/2014-04/msg00181.html
>
> Having seen this patch I believe the approach taken here to use section
> names to represent the noread attribute in assembly is inferior to
> Terry's approach.
>
> For the GCC implementation of either an attribute or compile option for
> execute-only we should not use section names to represent the noread
> attribute, since for instance that means it can not be combined with
> -ffunction-sections, or any other option that sets section names for
> functions.

We disagree with that specific point : the section names that we emit 
when gcc is using -ffunction-sections is in the form of:
.text.noread.*
which are perfectly caught and handled (it matters to us - we just 
checked this).

> I would like to rebase Terry's patch and make the necessary changes to
> it, slightly different attribute name and so on, and use that instead of
> this patch.
>
> Would there be any objections to this?

The binutils patch that we contributed was in the perspective of 
up-streaming the so-called PCROP support also in gcc also - the gcc 
proposal is completed on our side, but still not public.

For the matter of marking the .text sections read-only, we tried the two 
following strategies (and choose 2)):

1) keep the .text sections' names and emit the noread attribute in the 
assembly (with the very same 'y' key) : we failed doing that because of 
the specific treatment done by gas, that in the end *ignores* at some 
final point the custom attributes on the pure .text name.
This implies that the .text sections in noread mode cannot be called 
.text but must be called .text.something (which in your patch's tests 
appears as .text.foo, otherwise the noread attribute would not been have 
accounted for).
In addition from the gcc standpoint, this emission forces to duplicate 
emission code since the particular place we need to touch is hook-able, 
but requires a complete duplication of the section attribute emission 
(default_elf_asm_named_section) just to add two lines dealing with the 
'y' case.

2) emit some .text.noread sections (that work nicely with 
-ffunction-sections), without even requiring a section attribute change, 
and delegating the treatment to gas that handles specifically those 
sections.

This to write that your patch is certainly very good (just regretting 
that it was not contributed at that time) but leads to uglier code in 
gcc, which you may consider a problem.

Would it be possible to simply add the 'y' support from your patch, this 
would not break our changes and fulfill your purposes ?

Best regards,
--C