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Re: [Qemu-block] [Qemu-devel] [PATCH 2/3] block/qcow2: fix the corruptio

From: Kevin Wolf
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH 2/3] block/qcow2: fix the corruption when rebasing luks encrypted files
Date: Mon, 9 Sep 2019 12:56:55 +0200
User-agent: Mutt/1.12.1 (2019-06-15)

Am 06.09.2019 um 21:17 hat Eric Blake geschrieben:
> > -        assert((offset_in_cluster & ~BDRV_SECTOR_MASK) == 0);
> > +        assert((guest_offset & ~BDRV_SECTOR_MASK) == 0);
> > +        assert((host_offset & ~BDRV_SECTOR_MASK) == 0);
> >          assert((bytes & ~BDRV_SECTOR_MASK) == 0);
> Pre-existing, but we could use QEMU_IS_ALIGNED(x, BDRV_SECTOR_SIZE) for
> slightly more legibility than open-coding the bit operation.
> Neat trick about power-of-2 alignment checks:
> assert(QEMU_IS_ALIGNED(offset_in_cluster | guest_offset |
>                        host_offset | bytes, BDRV_SECTOR_SIZE));
> gives the same result in one assertion.  (I've used it elsewhere in the
> code base, but I'm not opposed to one assert per variable if you think
> batching is too dense.)

A possible downside of this is that if a user reports an assertion
failure, you can't tell any more which of the variables ended up in a
bad state.

If you're lucky, you can still tell in gdb at least if the bug is
reproducible, but I wouldn't be surprised if in release builds, half of
the variables were actually optimised away, so that even this wouldn't


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