slub - extended kmalloc redzone and dma alignment

[Vlastimil Babka <vbabka@suse.cz>]

Hi,

due to some off-list inquiry I have realized that since 946fa0dbf2d8
("mm/slub: extend redzone check to extra allocated kmalloc space than
requested")
we might be reporting false positives due to dma writing into the redzone.

It wasn't confirmed (yet) during the conversation but AFAICS it can be
happening. We have this ARCH_DMA_MINALIGN and kmalloc() will guarantee it,
but the redzone check doesn't take it into account. So if it's 8 bytes, I
think it's valid to do e.g. kmalloc(60), get a 64 byte object, do a dma
operation and it can write into the last 4 bytes. But the redzone check will
then report a buffer overflow.

Do you agree? If yes we should probably align the redzone with
ARCH_DMA_MINALIGN even if it means not reporting some non-dma buffer
overflows anymore.

Thanks,
Vlastimil