Re: [PATCH v4 01/25] rseq: Introduce feature size and alignment ELF auxiliary vector entries

[Florian Weimer <fweimer@redhat.com>]

* 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>
>  
> @@ -288,6 +289,10 @@ create_elf_tables(struct linux_binprm *bprm, const struct 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 */
>  
>  #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—they aren't unused due to the way the feature size works.)

Thanks,
Florian