On Tue, 2021-06-22 at 17:03 +0300, Andrey Konovalov wrote:

On Mon, Jun 21, 2021 at 6:45 PM [off-list ref] wrote:

quoted

From: Yee Lee <redacted>

This patch adds a memset to initialize object of unaligned size.
Duing to the MTE granulrity, the integrated initialization using
hwtag instruction will force clearing out bytes in granular size,
which may cause undesired effect, such as overwriting to the
redzone
of SLUB debug. In this patch, for the unaligned object size,
function
uses memset to initailize context instead of the hwtag instruction.

Signed-off-by: Yee Lee <redacted>
---
 mm/kasan/kasan.h | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
index 8f450bc28045..d8faa64614b7 100644
--- a/mm/kasan/kasan.h
+++ b/mm/kasan/kasan.h

@@ -387,8 +387,11 @@ static inline void kasan_unpoison(const void

*addr, size_t size, bool init)

        if (WARN_ON((unsigned long)addr & KASAN_GRANULE_MASK))
                return;
+       if (init && ((unsigned long)size & KASAN_GRANULE_MASK)) {
+               init = false;
+               memset((void *)addr, 0, size);
+       }

With this implementation, we loose the benefit of setting tags and
initializing memory with the same instructions.

Perhaps a better implementation would be to call
hw_set_mem_tag_range() with the size rounded down, and then
separately
deal with the leftover memory.

Yes, this fully takes the advantage of hw instruction. 
However, the leftover memory needs one more hw_set_mem_tag_range() for
protection as well.

If the extra path is only executed as CONFIG_SLUB_DEBUG, the
performance lost would be less concerned.

quoted

        size = round_up(size, KASAN_GRANULE_SIZE);
-
        hw_set_mem_tag_range((void *)addr, size, tag, init);
 }

--
2.18.0

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