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

On 9/28/19 3:07 PM, Richard W.M. Jones wrote:
On Fri, Sep 27, 2019 at 11:48:47PM -0500, Eric Blake wrote:
Fixes the fact that clients could not request the maximum string
length except with NBD_OPT_EXPORT_LEN.  Updates the testsuite to

Signed-off-by: Eric Blake <eblake redhat com>
  server/protocol-handshake-newstyle.c | 12 +++++++-----
  tests/test-long-name.sh              | 10 ++++------
  2 files changed, 11 insertions(+), 11 deletions(-)

diff --git a/server/protocol-handshake-newstyle.c b/server/protocol-handshake-newstyle.c
index 34958360..3b5d144e 100644
--- a/server/protocol-handshake-newstyle.c
+++ b/server/protocol-handshake-newstyle.c
@@ -48,7 +48,7 @@
  #define MAX_NR_OPTIONS 32

  /* 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.

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

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