Re: [PATCH] nptl: fix potential merge of __rseq_* relro symbols

[Florian Weimer <fweimer@redhat.com>]

* Michael Jeanson:

> While working on a patch to add support for the extensible rseq ABI, we
> came across an issue where a new 'const' variable would be merged with
> the existing '__rseq_size' variable. We tracked this to the use of
> '-fmerge-all-constants' which allows the compiler to merge identical
> constant variables. This means that all 'const' variables in a compile
> unit that are of the same size and are initialized to the same value can
> be merged.
>
> In this specific case, on 32 bit systems 'unsigned int' and 'ptrdiff_t'
> are both 4 bytes and initialized to 0 which should trigger the merge.
> However for reasons we haven't delved into when the attribute 'section
> (".data.rel.ro")' is added to the mix, only variables of the same exact
> types are merged. As far as we know this behavior is not specified
> anywhere and could change with a new compiler version, hence this patch.
>
> Declare both variables as regular static variables and add 'extern
> const' aliases for the ABI. This has the added bonus of removing the asm
> workaround to set the values on rseq registration.

I think we should move the definition of these variables into an
assembler file, like elf/dl-debug-symbols.S.  This way, we can also
create the desired hidden aliases more easily, too.

I can come up with a patch or review yours, please tell me what you
prefer.

Thanks,
Florian