[PATCH -next v20 20/26] riscv: Add prctl controls for userspace vector management

[Andy Chiu]

On Sun, May 21, 2023 at 1:41 PM Rémi Denis-Courmont <remi at remlab.net> wrote:
>
>         Hi all,
>
> Le torstaina 18. toukokuuta 2023 19.19.43 EEST, vous avez écrit :
> > This patch add two riscv-specific prctls, to allow usespace control the
> > use of vector unit:
> >
> >  * PR_RISCV_V_SET_CONTROL: control the permission to use Vector at next,
> >    or all following execve for a thread. Turning off a thread's Vector
> >    live is not possible since libraries may have registered ifunc that
> >    may execute Vector instructions.
> >  * PR_RISCV_V_GET_CONTROL: get the same permission setting for the
> >    current thread, and the setting for following execve(s).
>
> So far the story was that if the nth bit in the ELF HWCAP auxillary vector was
> set, then the nth single lettered extension was supported. There is already
> userspace code out there that expects this of the V bit. (I know I have
> written such code, and I also know others did likewise.) This is how it
> already works for the D and F bits.

Yes, the V bit in ELF_HWCAP becomes vague in this series.

>
> Admittedly, upstream Linux has never ever set that bit to this day. But still,
> if we end up with the bit set in a process that has had V support disabled by
> the parent (or the sysctl), existing userspace will encounter SIGILL and
> break.
>
> IMO, the bit must be masked not only whence the kernel lacks V support (as
> PATCH 02 does), but also if the process starts with V disabled.

This is going to change ELF_HWCAP from a macro to a function. The
function will turn on COMPAT_HWCAP_ISA_V iff V is supported and
allowed. I am going to do this in v21 If this looks sane. i.e.
Currently I don't see other architectures which give different
ELF_HWCAP values on each execve. If ELF_HWCAP is not a right place to
encode the information then userspace has to make the prctl() call to
be certain on whether V is usable.

>
> There are two ways to achieve this:
> 1) V is never ever set, and userspace is forced to use hwprobe() instead.
> 2) V is set only in processes starting with V enabled (and it's their own
> fault if they disabled it in future child threads).

The prctl() interface does not allow processes to turn off V once it
is enabled in its current (execve) context. The process can only
disable V when the next execve() happens. Then, if we implement
ELF_HWCAP as mentioned above, the kernel will reload a new HWCAP for
the process. By then, the new HWCAP will have V masked since it is not
allowed.

>
> Br,
>
> --
> レミ・デニ-クールモン
> http://www.remlab.net/
>
>
>
>
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Thanks,
Andy