[PATCH v3] arm64: enforce x1|x2|x3 == 0 upon kernel entry as per boot protocol

[Ard Biesheuvel]

On 20 March 2015 at 12:41, Mark Rutland <mark.rutland at arm.com> wrote:
>> >> +     if (boot_args[1] || boot_args[2] || boot_args[3]) {
>> >> +             pr_err("WARNING: boot protocol violation detected (x1 == %llx, x2 == %llx, x3 == %llx)\n",
>> >> +                     boot_args[1], boot_args[2], boot_args[3]);
>> >> +             pr_err("WARNING: your bootloader may fail to load newer kernels\n");
>> >
>> > If we ever decide to use x1-x3 for something, and try to boot an older
>> > kernel, that warning is going to be a bit misleading. That could matter
>> > for VMs where we're going to see old kernel images for a long time.
>> >
>> > I would like the warning to mention that could be the case.
>> >
>> > It would also be nice if the message were consistently spaced regardless
>> > of the values of x1-x3, so we should zero-pad them (and as that takes a
>> > resonable amount of space, let's give them a line each).
>> >
>> > So could we change the warning to be something like:
>> >
>> >         pr_err("WARNING: x1-x3 nonzero in violation of boot protocol:\n"
>> >                "\tx1: %016llx\n\tx2: %016llx\n\tx3: %016llx\n"
>> >                "This indicates a broken bootloader or old kernel\n",
>> >                boot_args[1], boot_args[2], boot_args[3]);
>> >
>>
>> OK, I have applied this change.
>>
>> But I would like to note that we should probably only extend the boot
>> protocol in a way that would not trigger this on older kernels in the
>> first place.
>> I.e., assign a bit in the flags field in the header, which indicates
>> whether some boot protocol enhancement is supported by the kernel
>> being loaded, and only allow x1/x2/x3 to be non-zero if said
>> enhancement defines that.
>
> Good point.
>
> Given that, if you want to restore your original last line, that would
> be fine with me (and my Ack still applies).
>

I think it's fine to leave it as is