Re: [PATCH] arm64/mm: fix variable 'tag' set but not used

[Qian Cai <cai@lca.pw>]

> On Aug 1, 2019, at 8:01 AM, Will Deacon <will@kernel.org> wrote:
> 
> On Wed, Jul 31, 2019 at 04:38:18PM -0400, Qian Cai wrote:
>> When CONFIG_KASAN_SW_TAGS=n, set_tag() is compiled away. GCC throws a
>> warning,
>> 
>> mm/kasan/common.c: In function '__kasan_kmalloc':
>> mm/kasan/common.c:464:5: warning: variable 'tag' set but not used
>> [-Wunused-but-set-variable]
>>  u8 tag = 0xff;
>>     ^~~
>> 
>> Fix it by making __tag_set() a static inline function.
>> 
>> Signed-off-by: Qian Cai <cai@lca.pw>
>> ---
>> arch/arm64/include/asm/memory.h | 6 +++++-
>> 1 file changed, 5 insertions(+), 1 deletion(-)
>> 
>> diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
>> index b7ba75809751..9645b1340afe 100644
>> --- a/arch/arm64/include/asm/memory.h
>> +++ b/arch/arm64/include/asm/memory.h
>> @@ -210,7 +210,11 @@ static inline unsigned long kaslr_offset(void)
>> #define __tag_reset(addr)	untagged_addr(addr)
>> #define __tag_get(addr)		(__u8)((u64)(addr) >> 56)
>> #else
>> -#define __tag_set(addr, tag)	(addr)
>> +static inline const void *__tag_set(const void *addr, u8 tag)
>> +{
>> +	return addr;
>> +}
> 
> Why doesn't this trigger a warning in page_to_virt(), which passes an
> unsigned long for the address parameter?

#define page_to_virt(page) … __tag_set(__addr, page_kasan_tag(page)); …

static inline u8 page_kasan_tag(const struct page *page)
{	
	return 0xff;
}

GCC will see that “page” is used.