Re: [PATCH v2] nptl: fix potential merge of __rseq_* relro symbols

[Michael Jeanson <mjeanson@efficios.com>]

On 2024-01-22 17:11, Michael Jeanson wrote:
> 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.
> 
> Move the definitions of these variables into an assembler file and add
> hidden writable aliases for internal use. This has the added bonus of
> removing the asm workaround to set the values on rseq registration.
> 
> Tested on Debian 12 with GCC 12.2.

Hi Florian,

I took a crack at it, hopefully it's close to what you had in mind.

Michael