-----Original Message----- From: Qian Cai [mailto:cai@xxxxxx] Sent: Wednesday, July 17, 2019 5:50 PM To: Nick Desaulniers <ndesaulniers@xxxxxxxxxx> Cc: Wysocki, Rafael J <rafael.j.wysocki@xxxxxxxxx>; Moore, Robert <robert.moore@xxxxxxxxx>; Schmauss, Erik <erik.schmauss@xxxxxxxxx>; jkim@xxxxxxxxxxx; Len Brown <lenb@xxxxxxxxxx>; linux-acpi@xxxxxxxxxxxxxxx; devel@xxxxxxxxxx; clang-built-linux <clang-built-linux@xxxxxxxxxxxxxxxx>; LKML <linux-kernel@xxxxxxxxxxxxxxx> Subject: Re: [PATCH] acpica: fix -Wnull-pointer-arithmetic warnings > On Jul 17, 2019, at 6:01 PM, Nick Desaulniers <ndesaulniers@xxxxxxxxxx> wrote: > > On Tue, Jul 16, 2019 at 8:38 PM Qian Cai <cai@xxxxxx> 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. ACPI_SUB_PTR is used in the iasl data table compiler. > >> >> Signed-off-by: Qian Cai <cai@xxxxxx> >> --- >> 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)) We have some questions concerning this change. If (a) is not cast to a u8, the addition will be in whatever units are appropriate for (a) i.e., the type of (a). However, we want ACPI_ADD_PTR (And ACPI_SUB_PTR) to simply perform a byte addition or subtraction - thus the cast to u8. I believe that is the original thinking behind the macros. >> #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
