Re: [PATCH v1 1/2] RISC-V: skip parsing multi-letter extensions starting with caps

[Conor Dooley <conor.dooley@microchip.com>]

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On Wed, Apr 26, 2023 at 02:18:52PM +0200, Andrew Jones wrote:
> On Wed, Apr 26, 2023 at 11:43:24AM +0100, Conor Dooley wrote:
> > Yangyu Chen reported that if an multi-letter extension begins with a
> > capital letter the parser will treat the remainder of that multi-letter
> > extension as single-letter extensions.
> 
> I think the problem is that the parser doesn't completely abort when
> it sees something it doesn't understand. Continuing is risky since
> it may be possible to compose an invalid string that gets the parser
> to run off the rails.

Usually I am of the opinion that we should not seek the validate the dt
in the kernel, since there are tools for doing so *cough* dt-validate
*cough*. This one seemed like low hanging fruit though, since the parser
handles having capital letters in any of the other places after the
rv##, but falls over pretty badly for this particular issue.

In general, I don't think we need to be concerned about anything that
fails dt-validate though, you kinda need to trust that that is correct.
I'd argue that we might even do too much validation in the parser at
present.
Is there some attack vector, or ACPI related consideration, that I am
unaware of that makes this risky?

> How about completely aborting, noisily, when the string doesn't match
> expectations, falling back to a default string such as rv64ima instead.
> That also ought to get faster corrections of device trees.

I did this first actually, but I was afraid that it would cause
regressions?

If you have riscv,isa = "rv64imafdc_Zifencei_zicbom", yes that is
invalid and dt-validate would have told you so, but at present that
would be parsed as "rv64imafdc_zicbom" which is a perfect description of
the hardware in question (since the meaning of i was set before RVI made
a hames of things).

So that's why I opted to not do some sort of pr_err/BUG()/WARN() and
try to keep processing the string. I'm happy to abort entirely on
reaching a capital if people feel there's unlikely to be a fallout from
that.

Cheers,
Conor.

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