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Re: [Libguestfs] [nbdkit PATCH v2 5/7] server: Allow longer NBD_OPT

On 9/30/19 11:30 AM, Eric Blake wrote:

  /* Maximum length of any option data (bytes). */
-#define MAX_OPTION_LENGTH 4096

I may have missed it - why was * 4 chosen?

NBD_OPT_SET_META_CONTEXT allows two strings plus a few glue bytes, so more than 8k of data from a compliant client.  16k is the next power of 2.  We can bump it larger if we want, especially since 16k pales in comparison to our 32M limit on NBD_CMD_WRITE, but for now, there is nothing in the NBD protocol larger than NBD_OPT_SET_META_CONTEXT.

Actually, having just written that, I now realize that NBD_OPT_SET_META_CONTEXT allows you to request more than one context at a time. It's very easy to provoke a request larger than 16k by requesting 3 contexts at once (nbdsh can do so), even if we only ever respond to a single recognized context. So I should probably just go whole-hog and cap this at the same limit as NBD_CMD_WRITE, rather than having two independent limits. And I probably ought to beef up the testsuite to actually demonstrate nbdsh provoking that large of an option.

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

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