Messages in this thread | | | Date | Mon, 14 Sep 2015 16:19:24 +0300 | Subject | Re: [PATCH] kasan: use IS_ALIGNED in memory_is_poisoned_8() | From | Andrey Ryabinin <> |
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2015-09-12 1:47 GMT+03:00 Andrew Morton <akpm@linux-foundation.org>: > On Fri, 11 Sep 2015 10:02:29 +0800 Xishi Qiu <qiuxishi@huawei.com> wrote: >> - if (likely(((addr + 7) & KASAN_SHADOW_MASK) >= 7)) >> + if (likely(IS_ALIGNED(addr, 8))) >> return false; > > Wouldn't IS_ALIGNED(addr, KASAN_SHADOW_SCALE_SIZE) be more appropriate? > > But I'm not really sure what the original code is trying to do. >
Original code is trying to estimate whether we should check 2 shadow bytes or just 1 should be enough.
> if ((addr + 7) & 7) >= 7) > > can only evaluate true if ((addr + 7) & 7) equals 7, so the ">=" could > be "==". >
Yes, it could be "==". ">=" is just for consistency with similar code in memory_is_poisoned_2/4.
If I'm not mistaken generic formula for such check looks like this: ((addr + size - 1) & KASAN_SHADOW_MASK) >= ((size - 1) & KASAN_SHADOW_MASK)
But when size >= KASAN_SHADOW_SCALE_SIZE we could just check for alignment.
> I think. The code looks a bit weird. A code comment would help. > > And how come memory_is_poisoned_16() does IS_ALIGNED(addr, 8)? Should > it be 16? >
No, If 16 bytes are 8-byte aligned, then shadow is 2-bytes.
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