linux-kernel - Re: [PATCH v3 02/23] rseq: Introduce extensible rseq ABI

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Message-ID: <8735e4hajt.fsf@oldenburg.str.redhat.com>
Date:   Wed, 10 Aug 2022 08:33:58 +0200
From:   Florian Weimer <fweimer@...hat.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        "H . Peter Anvin" <hpa@...or.com>, Paul Turner <pjt@...gle.com>,
        linux-api@...r.kernel.org,
        Christian Brauner <christian.brauner@...ntu.com>,
        David.Laight@...LAB.COM, carlos@...hat.com,
        Peter Oskolkov <posk@...k.io>
Subject: Re: [PATCH v3 02/23] rseq: Introduce extensible rseq ABI

* Mathieu Desnoyers:

> Introduce the extensible rseq ABI, where the feature size supported by
> the kernel and the required alignment are communicated to user-space
> through ELF auxiliary vectors.
>
> This allows user-space to call rseq registration with a rseq_len of
> either 32 bytes for the original struct rseq size (which includes
> padding), or larger.
>
> If rseq_len is larger than 32 bytes, then it must be large enough to
> contain the feature size communicated to user-space through ELF
> auxiliary vectors.

I don't think this works with the glibc extension mechanism because
__rseq_size does not change until the padding is exhausted.

I think you'll need to add the suggested flags to the auxiliary vector,
and then we can use that during registration and also communicate these
flags via __rseq_flags.

Size and alignment can be stored in a single auxiliary vector entry.

Thanks,
Florian