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

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Message-ID: <20190801121216.gsvneelth6scj57s@willie-the-truck>
Date:   Thu, 1 Aug 2019 13:12:17 +0100
From:   Will Deacon <will@...nel.org>
To:     Qian Cai <cai@....pw>
Cc:     Catalin Marinas <catalin.marinas@....com>, andreyknvl@...gle.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64/mm: fix variable 'tag' set but not used

On Thu, Aug 01, 2019 at 08:07:12AM -0400, Qian Cai wrote:
> 
> 
> > On Aug 1, 2019, at 8:01 AM, Will Deacon <will@...nel.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@....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.

No, I mean because you're making addr 'const void *'.

/me finds a toolchain.

Look:


In file included from ./arch/arm64/include/asm/pgtable-hwdef.h:8,
                 from ./arch/arm64/include/asm/processor.h:35,
                 from ./include/linux/mutex.h:19,
                 from ./include/linux/kernfs.h:12,
                 from ./include/linux/sysfs.h:16,
                 from ./include/linux/kobject.h:20,
                 from ./include/linux/of.h:17,
                 from ./include/linux/irqdomain.h:35,
                 from ./include/linux/acpi.h:13,
                 from ./include/acpi/apei.h:9,
                 from ./include/acpi/ghes.h:5,
                 from ./include/linux/arm_sdei.h:14,
                 from arch/arm64/kernel/asm-offsets.c:10:
./include/linux/mm.h: In function ‘lowmem_page_address’:
./arch/arm64/include/asm/memory.h:309:14: warning: passing argument 1 of ‘__tag_set’ makes pointer from integer without a cast [-Wint-conversion]
    __tag_set(__addr, page_kasan_tag(page));  \
              ^~~~~~
./include/linux/mm.h:1302:9: note: in expansion of macro ‘page_to_virt’
  return page_to_virt(page);
         ^~~~~~~~~~~~
./arch/arm64/include/asm/memory.h:213:49: note: expected ‘const void *’ but argument is of type ‘long unsigned int’
 static inline const void *__tag_set(const void *addr, u8 tag)
                                     ~~~~~~~~~~~~^~~~
./arch/arm64/include/asm/memory.h:309:4: warning: initialization of ‘long unsigned int’ from ‘const void *’ makes integer from pointer without a cast [-Wint-conversion]
    __tag_set(__addr, page_kasan_tag(page));  \
    ^~~~~~~~~
./include/linux/mm.h:1302:9: note: in expansion of macro ‘page_to_virt’
  return page_to_virt(page);
         ^~~~~~~~~~~~


If you're going to send patches removing compiler warnings, please have the
courtesy to test them first.

Will