linux-kernel - Re: [PATCH v2] arm64/mm: fix variable 'tag' set but not used

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Message-ID: <CAAeHK+xMQ5m-_eeQUPM2DoN=6OV-1uC6NX3dVnSKcmEqwSM5ZA@mail.gmail.com>
Date:   Mon, 5 Aug 2019 17:37:06 +0200
From:   Andrey Konovalov <andreyknvl@...gle.com>
To:     Qian Cai <cai@....pw>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Linux Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] arm64/mm: fix variable 'tag' set but not used

On Thu, Aug 1, 2019 at 4:47 PM Qian Cai <cai@....pw> 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 the same as
> arch_kasan_set_tag() in mm/kasan/kasan.h for consistency because there
> is a macro in arch/arm64/include/asm/kasan.h,
>
>  #define arch_kasan_set_tag(addr, tag) __tag_set(addr, tag)
>
> However, when CONFIG_DEBUG_VIRTUAL=n and CONFIG_SPARSEMEM_VMEMMAP=y,
> page_to_virt() will call __tag_set() with incorrect type of a
> parameter, so fix that as well. Also, still let page_to_virt() return
> "void *" instead of "const void *", so will not need to add a similar
> cast in lowmem_page_address().
>
> Signed-off-by: Qian Cai <cai@....pw>
> ---
>
> v2: Fix compilation warnings of CONFIG_DEBUG_VIRTUAL=n spotted by Will.
>
>  arch/arm64/include/asm/memory.h | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
>
> diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
> index b7ba75809751..fb04f10a78ab 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;
> +}
> +
>  #define __tag_reset(addr)      (addr)
>  #define __tag_get(addr)                0
>  #endif
> @@ -301,8 +305,8 @@ static inline void *phys_to_virt(phys_addr_t x)
>  #define page_to_virt(page)     ({                                      \
>         unsigned long __addr =                                          \
>                 ((__page_to_voff(page)) | PAGE_OFFSET);                 \
> -       unsigned long __addr_tag =                                      \
> -                __tag_set(__addr, page_kasan_tag(page));               \
> +       const void *__addr_tag =                                        \
> +               __tag_set((void *)__addr, page_kasan_tag(page));        \
>         ((void *)__addr_tag);                                           \
>  })
>
> --
> 1.8.3.1
>

Reviewed-by: Andrey Konovalov <andreyknvl@...gle.com>