linux-kernel - Re: Aligning tcmalloc with glibc 2.35 rseq ABI

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Date:   Wed, 2 Feb 2022 17:28:55 -0500
From:   Carlos O'Donell <carlos@...hat.com>
To:     Florian Weimer <fweimer@...hat.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Chris Kennelly <ckennelly@...gle.com>,
        Peter Oskolkov <posk@...k.io>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        libc-alpha <libc-alpha@...rceware.org>,
        Paul Turner <pjt@...gle.com>
Subject: Re: Aligning tcmalloc with glibc 2.35 rseq ABI

On 2/2/22 12:31, Carlos O'Donell wrote:
> On 2/2/22 10:01, Florian Weimer via Libc-alpha wrote:
>> * Mathieu Desnoyers:
>>
>>> More to the point: is ptrdiff_t the correct type here ? I think so.
>>> Do we want to revert the ABI and wait another 6 months before we
>>> bring back rseq into glibc just for this ? I'm not sure this limitation
>>> justifies it.
>>>
>>> So if there is a quick way to fix that before the official 2.35 release,
>>> I'm all for it, otherwise I cannot say that __rseq_offset being an "int"
>>> rather than a "ptrdiff_t" will make much real-life difference (unless
>>> I'm proven wrong). But we will be stuck with this quirk forever.
>>
>> I'm going to post a patch.  It's fairly small.
> 
> I'll review this as glibc release manager for glibc 2.35.
> 
> We'll get this right before I cut the release.
 
I've reviewed the static linker relocation designs, and aside from the odd-duck
for ia64, we've used ptrdiff_t sized relocations, nominally 64-bits for the
64-bit targets (though on AArch64 -mtls-size admits at most 48-bits).

The ptrdiff_t change will be a part of the glibc 2.35 release:

commit 6c33b018438ee799c29486f21d43d8100bdbd597
Author: Florian Weimer <fweimer@...hat.com>
Date:   Wed Feb 2 22:37:20 2022 +0100

    Linux: Use ptrdiff_t for __rseq_offset
    
    This matches the data size initial-exec relocations use on most
    targets.
    
    Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
    Reviewed-by: Carlos O'Donell <carlos@...hat.com>

-- 
Cheers,
Carlos.