> On Jul 17, 2019, at 6:01 PM, Nick Desaulniers <ndesaulni...@google.com> wrote:
>
> On Tue, Jul 16, 2019 at 8:38 PM Qian Cai <c...@lca.pw> wrote:
>>
>> Clang generate quite a few of those warnings.
>>
>> drivers/acpi/scan.c:759:28: warning: arithmetic on a null pointer
>> treated as a cast from integer to pointer is a GNU extension
>> [-Wnull-pointer-arithmetic]
>> status = acpi_get_handle(ACPI_ROOT_OBJECT,
>> obj->string.pointer,
>> ^~~~~~~~~~~~~~~~
>> ./include/acpi/actypes.h:458:56: note: expanded from macro
>> 'ACPI_ROOT_OBJECT'
>> #define ACPI_ROOT_OBJECT ((acpi_handle) ACPI_TO_POINTER
>> (ACPI_MAX_PTR))
>> ^~~~~~~~~~~~~~~
>> ./include/acpi/actypes.h:509:41: note: expanded from macro
>> 'ACPI_TO_POINTER'
>> #define ACPI_TO_POINTER(i) ACPI_ADD_PTR (void, (void *) 0,
>> (acpi_size) (i))
>> ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>> ./include/acpi/actypes.h:503:84: note: expanded from macro
>> 'ACPI_ADD_PTR'
>> #define ACPI_ADD_PTR(t, a, b) ACPI_CAST_PTR (t,
>> (ACPI_CAST_PTR (u8, (a)) + (acpi_size)(b)))
>> ^~~~~~~~~~~~~~~~~
>> ./include/acpi/actypes.h:501:66: note: expanded from macro
>> 'ACPI_CAST_PTR'
>> #define ACPI_CAST_PTR(t, p) ((t *) (acpi_uintptr_t) (p))
>> ^
>> This is because pointer arithmetic on a pointer not pointing to an array
>> is an undefined behavior. Fix it by doing an integer arithmetic
>> instead.
>
> Hi Qian, thanks for the patch. How do I reproduce this issue,
> precisely? I just tried:
> $ make CC=clang -j71 drivers/acpi/scan.o
> on linux-next today and don't observe the warning. My clang is ToT
> built sometime this week. It looks like drivers/acpi/scan.o when
> CONFIG_ACPI=y, which is set in the defconfig. Is there another set of
> configs to enable to observe the warning?
# make W=1 -j 256
With the config,
https://raw.githubusercontent.com/cailca/linux-mm/master/arm64.config
>
> Also, the fix is curious. Arithmetic on pointers to different
> "objects" (with one element passed the end) may lead to provence
> issues due to undefined behavior, but I would have expected some cases
> to uintptr_t, then arithmetic on that type, as the solution (which is
> what I suspect ACPI_CAST_PTR is doing).
>
> Further, you seem to have modified ACPI_ADD_PTR but not ACPI_SUB_PTR;
> I would have expected both to be afflicted together or not at all
> based on their existing implementations.
Yes, I thought about that, but ACPI_SUB_PTR does not seem used anywhere, so I
thought maybe just start a new discussion to remove it all together later.
>
>>
>> Signed-off-by: Qian Cai <c...@lca.pw>
>> ---
>> include/acpi/actypes.h | 4 ++--
>> 1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/acpi/actypes.h b/include/acpi/actypes.h
>> index ad6892a24015..25b4a32da177 100644
>> --- a/include/acpi/actypes.h
>> +++ b/include/acpi/actypes.h
>> @@ -500,13 +500,13 @@ typedef u64 acpi_integer;
>>
>> #define ACPI_CAST_PTR(t, p) ((t *) (acpi_uintptr_t) (p))
>> #define ACPI_CAST_INDIRECT_PTR(t, p) ((t **) (acpi_uintptr_t) (p))
>> -#define ACPI_ADD_PTR(t, a, b) ACPI_CAST_PTR (t, (ACPI_CAST_PTR
>> (u8, (a)) + (acpi_size)(b)))
>> +#define ACPI_ADD_PTR(t, a, b) ACPI_CAST_PTR (t, (a) +
>> (acpi_size)(b))
>> #define ACPI_SUB_PTR(t, a, b) ACPI_CAST_PTR (t, (ACPI_CAST_PTR
>> (u8, (a)) - (acpi_size)(b)))
>> #define ACPI_PTR_DIFF(a, b) ((acpi_size) (ACPI_CAST_PTR (u8,
>> (a)) - ACPI_CAST_PTR (u8, (b))))
>>
>> /* Pointer/Integer type conversions */
>>
>> -#define ACPI_TO_POINTER(i) ACPI_ADD_PTR (void, (void *) 0,
>> (acpi_size) (i))
>> +#define ACPI_TO_POINTER(i) ACPI_ADD_PTR (void, 0, (acpi_size)
>> (i))
>
> IIUC, these are adding `i` to NULL (or (void*)0)? X + 0 == X ?
> --
> Thanks,
> ~Nick Desaulniers
