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Re: [PATCH v2 1/2] nbd: Don't send oversize strings

From: Eric Blake
Subject: Re: [PATCH v2 1/2] nbd: Don't send oversize strings
Date: Wed, 13 Nov 2019 09:47:45 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.1.1

On 10/15/19 11:16 AM, Vladimir Sementsov-Ogievskiy wrote:

@@ -1561,6 +1569,8 @@ NBDExport *nbd_export_new(BlockDriverState *bs, uint64_t 
            exp->export_bitmap = bm;
            exp->export_bitmap_context = g_strdup_printf("qemu:dirty-bitmap:%s",
+        /* See BME_MAX_NAME_SIZE in block/qcow2-bitmap.c */

Hmm. BME_MAX_NAME_SIZE is checked only when creating persistent bitmaps. But 
for non-persistent
name length is actually unlimited. So, we should either limit all bitmap names 
to 1023 (hope,
this will not break existing scenarios) or error out here (or earlier) instead 
of assertion.

I'm seriously doubting that any existing scenarios try to use a name that long. If no one was relying on a long name (especially since it was inconsistent between persistent being limited to qcow2 constraints and non-persistent having no limit), we can consider it as a bug-fix rather than something needing a deprecation period.

I'm leaning towards limiting ALL bitmaps to the same length (as we've already 
debated the idea of being able to convert an existing bitmap from transient to 

Agreed, but ..

sizeof("qemu:dirty-bitmap:") - 1)

Except that BME_MAX_NAME_SIZE is not (currently) in a public .h file.

.. I think, than it should be new BLOCK_DIRTY_BITMAP_MAX_NAME_SIZE.. And we'll 
have to note it in qapi doc..
Should this change go through deprecation? Or we consider non-persistent 
bitmaps as something not really useful?

I'm preparing a v3 patch that just goes ahead and adds the limit on bitmap names everywhere, as a separate patch.

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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