linux-kernel - Re: [PATCH] m68k: virt: pass RNG seed via bootinfo block

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Message-ID: <4b0eb04d-564e-3111-a460-27934976084e@vivier.eu>
Date:   Sat, 25 Jun 2022 18:27:42 +0200
From:   Laurent Vivier <laurent@...ier.eu>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        linux-m68k <linux-m68k@...ts.linux-m68k.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] m68k: virt: pass RNG seed via bootinfo block

Le 25/06/2022 à 18:26, Jason A. Donenfeld a écrit :
> On Sat, Jun 25, 2022 at 6:24 PM Laurent Vivier <laurent@...ier.eu> wrote:
>>
>> Le 25/06/2022 à 18:19, Jason A. Donenfeld a écrit :
>>> On Sat, Jun 25, 2022 at 6:08 PM Laurent Vivier <laurent@...ier.eu> wrote:
>>>>
>>>> Le 25/06/2022 à 17:38, Jason A. Donenfeld a écrit :
>>>>> Other virt VMs can pass RNG seeds via the "rng-seed" device tree
>>>>> property or via UEFI, but m68k doesn't have either. Instead it has its
>>>>> own bootinfo protocol. So this commit adds support for receiving a RNG
>>>>> seed from it, which will be used at the earliest possible time in boot,
>>>>> just like device tree.
>>>>>
>>>>> Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
>>>>> ---
>>>>>     arch/m68k/include/uapi/asm/bootinfo-virt.h | 1 +
>>>>>     arch/m68k/virt/config.c                    | 4 ++++
>>>>>     2 files changed, 5 insertions(+)
>>>>>
>>>>> diff --git a/arch/m68k/include/uapi/asm/bootinfo-virt.h b/arch/m68k/include/uapi/asm/bootinfo-virt.h
>>>>> index e4db7e2213ab..7c3044acdf4a 100644
>>>>> --- a/arch/m68k/include/uapi/asm/bootinfo-virt.h
>>>>> +++ b/arch/m68k/include/uapi/asm/bootinfo-virt.h
>>>>> @@ -12,6 +12,7 @@
>>>>>     #define BI_VIRT_GF_TTY_BASE 0x8003
>>>>>     #define BI_VIRT_VIRTIO_BASE 0x8004
>>>>>     #define BI_VIRT_CTRL_BASE   0x8005
>>>>> +#define BI_VIRT_RNG_SEED     0x8006
>>>>>
>>>>>     #define VIRT_BOOTI_VERSION  MK_BI_VERSION(2, 0)
>>>>>
>>>>> diff --git a/arch/m68k/virt/config.c b/arch/m68k/virt/config.c
>>>>> index 632ba200ad42..ad71af8273ec 100644
>>>>> --- a/arch/m68k/virt/config.c
>>>>> +++ b/arch/m68k/virt/config.c
>>>>> @@ -2,6 +2,7 @@
>>>>>
>>>>>     #include <linux/reboot.h>
>>>>>     #include <linux/serial_core.h>
>>>>> +#include <linux/random.h>
>>>>>     #include <clocksource/timer-goldfish.h>
>>>>>
>>>>>     #include <asm/bootinfo.h>
>>>>> @@ -92,6 +93,9 @@ int __init virt_parse_bootinfo(const struct bi_record *record)
>>>>>                 data += 4;
>>>>>                 virt_bi_data.virtio.irq = be32_to_cpup(data);
>>>>>                 break;
>>>>> +     case BI_VIRT_RNG_SEED:
>>>>> +             add_bootloader_randomness(data + 4, be32_to_cpup(data));
>>>>
>>>> In fact, why don't you use the record->size to get the size of the buffer?
>>>>
>>>> It seems useless to encode twice the length of the buffer, the second time on a 32bit while the
>>>> length cannot exceed a 16bit value.
>>>
>>> Doesn't that make the length ambiguous because of required alignment?
>>
>> I agree, it's why I understand reviewing the QEMU part of your patch.
>>
>>> Would rather keep this general. As is, it's also much more like the
>>> others and more uniform to keep it that way. You were able to review
>>> it and see that it was right after glancing for a second. That seems
>>> superior to any imaginary gains we'd get by overloading the record
>>> size.
>>
>> And what about using a 16bit field rather than a 32bit field as the encoded length cannot be greater
>> than the record length?
> 
> I guess but that's different from all other length fields, and means
> we can't expand past 65k if somebody wants to use this for something
> more interesting later. Again I wonder what stinginess here gets us.
> This is just a boot parameter... No need to go crazy optimizing it.

I agree too.

Thanks,
Laurent