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SubjectGCC bug ? Re: [PATCH v2 10/10] powerpc/32s: Implement Kernel Userspace Access Protection
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On 04/18/2019 06:55 AM, Michael Ellerman wrote:
> Christophe Leroy <christophe.leroy@c-s.fr> writes:
>> diff --git a/arch/powerpc/include/asm/book3s/32/kup.h b/arch/powerpc/include/asm/book3s/32/kup.h
>> index 5f97c742ca71..b3560b2de435 100644
>> --- a/arch/powerpc/include/asm/book3s/32/kup.h
>> +++ b/arch/powerpc/include/asm/book3s/32/kup.h
>> @@ -37,6 +37,113 @@
> ...
>> +
>> +static inline void allow_user_access(void __user *to, const void __user *from, u32 size)
>> +{
>> + u32 addr = (__force u32)to;
>> + u32 end = min(addr + size, TASK_SIZE);
>> +
>> + if (!addr || addr >= TASK_SIZE || !size)
>> + return;
>> +
>> + current->thread.kuap = (addr & 0xf0000000) | ((((end - 1) >> 28) + 1) & 0xf);
>> + kuap_update_sr(mfsrin(addr) & ~SR_KS, addr, end); /* Clear Ks */
>> +}
>
> When rebasing on my v6 I changed the above to:
>
> static inline void allow_user_access(void __user *to, const void __user *from, u32 size)
> {
> u32 addr, end;
>
> if (__builtin_constant_p(to) && to == NULL)
> return;

Look like the above doesn't work: gcc bug ?

#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);
}

static inline int f2(int x)
{
if (__builtin_constant_p(x) && x == 0)
return 7;
return 9;
}

int g2(void)
{
return f2(0);
}



toto.o: file format elf32-powerpc


Disassembly of section .text:

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

00000008 <g2>:
8: 38 60 00 07 li r3,7
c: 4e 80 00 20 blr



It works for the int const, but not for the pointer const:

g1() should return 3, not 5. GCC bug ?

Christophe

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