Re: [PATCH] rombios: Work around GCC issue 99578

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 18/08/2023 11:09 am, Jan Beulich wrote:
> On 18.08.2023 11:44, Andrew Cooper wrote:
>> On 18/08/2023 7:50 am, Jan Beulich wrote:
>>> On 17.08.2023 22:45, Andrew Cooper wrote:
>>>> GCC 12 objects to pointers derived from a constant:
>>>>
>>>>   util.c: In function 'find_rsdp':
>>>>   util.c:429:16: error: array subscript 0 is outside array bounds of 
>>>> 'uint16_t[0]' {aka 'short unsigned int[]'} [-Werror=array-bounds]
>>>>     429 |     ebda_seg = *(uint16_t *)ADDR_FROM_SEG_OFF(0x40, 0xe);
>>>>   cc1: all warnings being treated as errors
>>>>
>>> Yet supposedly the bug was fixed in 12.1 (and the fix also backported to
>>> 11.3). Did you spot anything in ADDR_FROM_SEG_OFF() and this particular
>>> use of it that is different from the patterns mentioned in that bug?
>> $ gcc --version
>> gcc (GCC) 12.2.1 20221121
>>
>> At a guess, only a partial fix was backported into 12.1.  AIUI, it was
>> an area of logic which had already seen significant development in 13
>> before the behaviour was reverted.
> Hmm, for 12 I didn't think there was any backporting involved.
>
>> The only thing interesting about ADDR_FROM_SEG_OFF() is the constant
>> folding required for the expression to become *(uint16_t *)0x40e, which
>> (I suspect) is why it compiles fine at -Og but fails at -O2.
> Oh, the relevant aspect may be that is is below 4k (which I think is
> their heuristic offset to guess "almost NULL" dereferencing).

Ah yes, I remember that aspect now you've pointed it out.

Yeah, that's probably it.

>>>> This is a GCC bug, but work around it rather than turning array-bounds
>>>> checking off generally.
>>> I certainly agree here. I guess it's not worth trying to restrict the
>>> workaround for rombios (I will want to try doing so in the hypervisor).
>> Can I translate this to an ack?
> You can now:
> Acked-by: Jan Beulich <jbeulich@xxxxxxxx>

Thanks.