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From:   Florian Weimer <fweimer@redhat.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        "Paul E . McKenney" <paulmck@kernel.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        "H . Peter Anvin" <hpa@zytor.com>, Paul Turner <pjt@google.com>,
        linux-api@vger.kernel.org, Christian Brauner <brauner@kernel.org>,
        David.Laight@ACULAB.COM, carlos@redhat.com,
        Peter Oskolkov <posk@posk.io>,
        Alexander Mikhalitsyn <alexander@mihalicyn.com>
Subject: Re: [PATCH v4 01/25] rseq: Introduce feature size and alignment ELF
 auxiliary vector entries
References: <20220922105941.237830-1-mathieu.desnoyers@efficios.com>
        <20220922105941.237830-2-mathieu.desnoyers@efficios.com>
Date:   Mon, 10 Oct 2022 14:42:42 +0200
In-Reply-To: <20220922105941.237830-2-mathieu.desnoyers@efficios.com> (Mathieu
        Desnoyers's message of "Thu, 22 Sep 2022 06:59:16 -0400")
Message-ID: <877d1726kd.fsf@oldenburg.str.redhat.com>
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* Mathieu Desnoyers:

> Export the rseq feature size supported by the kernel as well as the
> required allocation alignment for the rseq per-thread area to user-space
> through ELF auxiliary vector entries.
>
> This is part of the extensible rseq ABI.
>
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> ---
>  fs/binfmt_elf.c             | 5 +++++
>  include/uapi/linux/auxvec.h | 2 ++
>  include/uapi/linux/rseq.h   | 5 +++++
>  3 files changed, 12 insertions(+)
>
> diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
> index 63c7ebb0da89..04fca1e4cbd2 100644
> --- a/fs/binfmt_elf.c
> +++ b/fs/binfmt_elf.c
> @@ -46,6 +46,7 @@
>  #include <linux/cred.h>
>  #include <linux/dax.h>
>  #include <linux/uaccess.h>
> +#include <linux/rseq.h>
>  #include <asm/param.h>
>  #include <asm/page.h>
>=20=20
> @@ -288,6 +289,10 @@ create_elf_tables(struct linux_binprm *bprm, const s=
truct elfhdr *exec,
>  	if (bprm->have_execfd) {
>  		NEW_AUX_ENT(AT_EXECFD, bprm->execfd);
>  	}
> +#ifdef CONFIG_RSEQ
> +	NEW_AUX_ENT(AT_RSEQ_FEATURE_SIZE, offsetof(struct rseq, end));
> +	NEW_AUX_ENT(AT_RSEQ_ALIGN, __alignof__(struct rseq));
> +#endif
>  #undef NEW_AUX_ENT
>  	/* AT_NULL is zero; clear the rest too */
>  	memset(elf_info, 0, (char *)mm->saved_auxv +
> diff --git a/include/uapi/linux/auxvec.h b/include/uapi/linux/auxvec.h
> index c7e502bf5a6f..6991c4b8ab18 100644
> --- a/include/uapi/linux/auxvec.h
> +++ b/include/uapi/linux/auxvec.h
> @@ -30,6 +30,8 @@
>  				 * differ from AT_PLATFORM. */
>  #define AT_RANDOM 25	/* address of 16 random bytes */
>  #define AT_HWCAP2 26	/* extension of AT_HWCAP */
> +#define AT_RSEQ_FEATURE_SIZE	27	/* rseq supported feature size */
> +#define AT_RSEQ_ALIGN		28	/* rseq allocation alignment */
>=20=20
>  #define AT_EXECFN  31	/* filename of program */

Do we need the alignment?  Or can we keep it perpetually at 32?  Or we
could steal some bits from AT_RSEQ_FEATURE_SIZE?  (Not the lower
bits=E2=80=94they aren't unused due to the way the feature size works.)

Thanks,
Florian