Re: [PATCH 06/11] change memory_is_poisoned_16 for aligned error

[Russell King - ARM Linux]

On Thu, Oct 12, 2017 at 11:27:40AM +0000, Liuwenliang (Lamb) wrote:
> >> - I don't understand why this is necessary.  memory_is_poisoned_16()
> >>   already handles unaligned addresses?
> >>
> >> - If it's needed on ARM then presumably it will be needed on other
> >>   architectures, so CONFIG_ARM is insufficiently general.
> >>
> >> - If the present memory_is_poisoned_16() indeed doesn't work on ARM,
> >>   it would be better to generalize/fix it in some fashion rather than
> >>   creating a new variant of the function.
> 
> 
> >Yes, I think it will be better to fix the current function rather then
> >have 2 slightly different copies with ifdef's.
> >Will something along these lines work for arm? 16-byte accesses are
> >not too common, so it should not be a performance problem. And
> >probably modern compilers can turn 2 1-byte checks into a 2-byte check
> >where safe (x86).
> 
> >static __always_inline bool memory_is_poisoned_16(unsigned long addr)
> >{
> >        u8 *shadow_addr = (u8 *)kasan_mem_to_shadow((void *)addr);
> >
> >        if (shadow_addr[0] || shadow_addr[1])
> >                return true;
> >        /* Unaligned 16-bytes access maps into 3 shadow bytes. */
> >        if (unlikely(!IS_ALIGNED(addr, KASAN_SHADOW_SCALE_SIZE)))
> >                return memory_is_poisoned_1(addr + 15);
> >        return false;
> >}
> 
> Thanks for Andrew Morton and Dmitry Vyukov's review. 
> If the parameter addr=0xc0000008, now in function:
> static __always_inline bool memory_is_poisoned_16(unsigned long addr)
> {
>  ---     //shadow_addr = (u16 *)(KASAN_OFFSET+0x18000001(=0xc0000008>>3)) is not 
>  ---     // unsigned by 2 bytes.
>         u16 *shadow_addr = (u16 *)kasan_mem_to_shadow((void *)addr); 
> 
>         /* Unaligned 16-bytes access maps into 3 shadow bytes. */
>         if (unlikely(!IS_ALIGNED(addr, KASAN_SHADOW_SCALE_SIZE)))
>                 return *shadow_addr || memory_is_poisoned_1(addr + 15);
> ----      //here is going to be error on arm, specially when kernel has not finished yet.
> ----      //Because the unsigned accessing cause DataAbort Exception which is not
> ----      //initialized when kernel is starting. 
>         return *shadow_addr;
> }
> 
> I also think it is better to fix this problem. 

What about using get_unaligned() ?

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