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

Rémi Denis-Courmont
2023-05-22T10:06:33Z

Hi,

Le 22 mai 2023 11:28:28 GMT+03:00, Andy Chiu <andy.chiu@sifive.com> a écrit :
>On Sun, May 21, 2023 at 1:41 PM Rémi Denis-Courmont <remi@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.

I don't think the value of an auxillary vector entry can change in an existing process nor that we need that. If an application starts with V disabled, you can keep the V bit clear even if V gets enabled later on; that won't break existing userspace code, which simply won't use vectors.

What does break existing userspace is setting the V bit whilst vectors are disabled.

>
>>
>> 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
>