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

[Mathieu Desnoyers]

On 2022-10-10 08:42, Florian Weimer wrote:
> * 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@xxxxxxxxxxxx>
> > ---
> >   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.)

I cannot imagine a use-case that would require us to bump the alignment 
requirement over 32 bytes, so we may very well leave it at 32. But 
perhaps someone else has a better imagination than mine ?

Thanks,

Mathieu

> Thanks,
> Florian

--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com