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SubjectRe: GCC bug ? Re: [PATCH v2 10/10] powerpc/32s: Implement Kernel Userspace Access Protection
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Le 22/01/2020 à 14:36, Segher Boessenkool a écrit :
> On Wed, Jan 22, 2020 at 07:52:02AM +0100, Christophe Leroy wrote:
>> Le 21/01/2020 à 20:55, Segher Boessenkool a écrit :
>>> On Tue, Jan 21, 2020 at 05:22:32PM +0000, Christophe Leroy wrote:
>>>> g1() should return 3, not 5.
>>>
>>> What makes you say that?
>>
>> What makes me say that is that NULL is obviously a constant pointer and
>> I think we are all expecting gcc to see it as a constant during kernel
>> build, ie at -O2
>
> But apparently at the point where the builtin was checked it did not
> yet know it is passed a null pointer.
>
> Please make a self-contained test case if we need further investigation?

The test in my original mail is self-contained:


#define NULL (void*)0

static inline int f1(void *to)
{
if (__builtin_constant_p(to) && to == NULL)
return 3;
return 5;
}

int g1(void)
{
return f1(NULL);
}


Build the above with -O2 then objdump:

00000000 <g1>:
0: 38 60 00 05 li r3,5
4: 4e 80 00 20 blr

It returns 5 so that shows __builtin_constant_p(to) was evaluated as false.


>
>>> "A return of 0 does not indicate that the
>>> value is _not_ a constant, but merely that GCC cannot prove it is a
>>> constant with the specified value of the '-O' option."
>>>
>>> (And the rules it uses for this are *not* the same as C "constant
>>> expressions" or C "integer constant expression" or C "arithmetic
>>> constant expression" or anything like that -- which should be already
>>> obvious from that it changes with different -Ox).
>>>
>>> You can use builtin_constant_p to have the compiler do something better
>>> if the compiler feels like it, but not anything more. Often people
>>> want stronger guarantees, but when they see how much less often it then
>>> returns "true", they do not want that either.
>
>> If GCC doesn't see NULL as a constant, then the above doesn't work as
>> expected.
>
> That's not the question. Of course GCC sees it as a null pointer
> constant, because it is one. But this builtin does its work very
> early, during preprocessing already. Its concept of "constant" is
> very different.
>
> Does it work if you write just "0" instead of "NULL", btw? "0" is
> also a null pointer constant eventually (here, that is).

No it doesn't.

It works if you change the 'void *to' to 'unsigned long to'

>
> The question is why (and if, it still needs verification after all)
> builtin_constant_p didn't return true.

I sent a patch to overcome the problem. See
https://patchwork.ozlabs.org/patch/1227249/

Christophe

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