Re: [PATCH v2] ACPI/IORT: Fix GCC 12 warning

Robin Murphy
2022-02-11T12:16:11Z

On 2022-02-11 11:43, Ard Biesheuvel wrote:
> On Fri, 11 Feb 2022 at 11:34, Robin Murphy <robin.murphy@arm.com> wrote:
>>
>> Hi Kees,
>>
>> On 2022-02-10 23:47, Kees Cook wrote:
>>> On Thu, Feb 10, 2022 at 08:41:51PM +0100, Ard Biesheuvel wrote:
>>>> On Thu, 10 Feb 2022 at 19:48, Victor Erminpour
>>>> <victor.erminpour@oracle.com> wrote:
>>>>>
>>>>> When building with automatic stack variable initialization, GCC 12
>>>>> complains about variables defined outside of switch case statements.
>>>>> Move the variable into the case that uses it, which silences the warning:
>>>>>
>>>>> ./drivers/acpi/arm64/iort.c:1670:59: error: statement will never be executed [-Werror=switch-unreachable]
>>>>> 1670 | struct acpi_iort_named_component *ncomp;
>>>>> | ^~~~~
>>>>>
>>>>> Signed-off-by: Victor Erminpour <victor.erminpour@oracle.com>
>>>>
>>>> Please cc people that commented on your v1 when you send a v2.
>>>>
>>>> Still NAK, for the same reasons.
>>>
>>> Let me see if I can talk you out of this. ;)
>>>
>>> So, on the face of it, I agree with you: this is a compiler bug. However,
>>> it's still worth fixing. Just because it's valid C isn't a good enough
>>> reason to leave it as-is: we continue to minimize the subset of the
>>> C language the kernel uses if it helps us get the most out of existing
>>> compiler features. We've eliminated all kinds of other "valid C" from the
>>> kernel because it improves robustness, security, etc. This is certainly
>>> nothing like removing VLAs or implicit fallthrough, but given that this
>>> is, I think, the only remaining case of it (I removed all the others a
>>> while ago when I had the same issues with the GCC plugins), I'd like to
>>> get it fixed.
>>
>> It concerns me if minimising the subset of the C language that the
>> kernel uses is achieved by converting more of the kernel to a
>> not-quite-C language that is not formally specified anywhere, by
>> prematurely adopting newly-invented compiler options that clearly don't
>> work properly (the GCC warning message quoted above may as well be
>> "error: giraffes are not purple" for all the sense it makes.)
>>
>>> And I should point out that Clang suffers[1] from the same problem (the
>>> variables will be missed for auto-initialization), but actually has a
>>> worse behavior: it does not even warn about it.
>>>
>>> And note that the problem isn't limited to -ftrivial-auto-var-init. This
>>> code pattern seems to also hide the variables from similar instrumentation
>>> like KASan, etc. (Which is similarly silent like above.)
>>
>> From your security standpoint (and believe me, I really do have faith
>> in your expertise here), which of these sounds better:
>>
>> 1: Being able to audit code based on well-defined language semantics
>>
>> 2: Playing whack-a-mole as issues are discovered empirically.
>>
>> 3: Neither of the above, but a warm fuzzy feeling because hey someone
>> said "security" in a commit message.
>>
>> AFAICS you're effectively voting against #1, and the examples you've
>> given demonstrate that #2 is nowhere near reliable enough either, so
>> where does that leave us WRT actual secure and robust code in Linux?
>>
>
> My concerns are more about:
> - The GCC version of the feature not being fully baked yet, which
> makes it hard to have full confidence in its implementation (surely,
> GCC has a test case or two with a switch scope variable declaration;
> - We waste the credit we have with other developers who care less
> about security on things that we could have fixed before they'd even
> notice. What will happen the next time around when we *really* need
> source level changes?
>
>>> In both compilers, it seems fixing this is not "easy", and given its
>>> corner-case nature and ease of being worked around in the kernel source,
>>> it isn't being highly prioritized. But since I both don't want these
>>> blinds spots with Clang (and GCC) var-init, and I don't want these
>>> warnings to suddenly appear once GCC 12 _does_ get released, so I'd like
>>> to get this case fixed as well.
>>>
>
> So how is this
>
> switch {
> var foo;
> case x:
> ...
> }
>
> fundamentally different from
>
> {
> var foo;
> switch {
> case x:
> ...
> }
> }
>
> Surely, some kind of transformation is possible where the var
> declaration is hoisted into a parent scope added around the entire
> switch {} statement?
>
>>> All that said, I think this patch could be improved.
>>>
>>> I'd recommend, instead, just simply:
>>>
>>> diff --git a/drivers/acpi/arm64/iort.c b/drivers/acpi/arm64/iort.c
>>> index f2f8f05662de..9e765d30da82 100644
>>> --- a/drivers/acpi/arm64/iort.c
>>> +++ b/drivers/acpi/arm64/iort.c
>>> @@ -1671,13 +1671,14 @@ phys_addr_t __init acpi_iort_dma_get_max_cpu_address(void)
>>> end = ACPI_ADD_PTR(struct acpi_iort_node, iort, iort->header.length);
>>>
>>> for (i = 0; i < iort->node_count; i++) {
>>> + struct acpi_iort_named_component *ncomp;
>>> + struct acpi_iort_root_complex *rc;
>>> + phys_addr_t local_limit;
>>> +
>>> if (node >= end)
>>> break;
>>>
>>> switch (node->type) {
>>> - struct acpi_iort_named_component *ncomp;
>>> - struct acpi_iort_root_complex *rc;
>>> - phys_addr_t local_limit;
>>>
>>> case ACPI_IORT_NODE_NAMED_COMPONENT:
>>> ncomp = (struct acpi_iort_named_component *)node->node_data;
>>>
>>> This results in no change in binary instruction output (when there is no
>>> auto-init).
>>
>> In fairness I'd have no objection to that patch if it came with a
>> convincing justification, but that is so far very much lacking. My aim
>> here is not to be a change-averse Luddite, but to try to find a
>> compromise where I can actually have some confidence in such changes
>> being made. Let's not start pretending that 3 100ml bottles of shampoo
>> are somehow "safer" than a 300ml bottle of shampoo...
>>
>
> Not sure I get the shampoo reference, but I just don't think this
> idiom meets the bar for code that really needs modification for the
> compiler to be able to do the right thing.

I was alluding to the same concern that you have - wasting developers'
time and goodwill with churn that lacks solid justification. For me the
security theatre of international air travel over the last decade has
successfully outweighed any desire to ever go to an airport again, and
I'd rather not be driven to take a similar attitude towards security
patches.

Cheers,
Robin.