What is maximum SMB2 IOCTL(FSCTL_GET_SHADOW_COPY) response size.

[Hemanth Thummala]

I have modified one of smbtorture tests(smb2.ioctl.shadow_copy) to test
this case.
Test is passed. Linux client is able to received the response which is >
64KB.

No.     Time            Source                Destination
Protocol Length Info
175 15:31:33.621624 xx.xx.xx.xx       yy.yy.yy.yy           SMB2     160
 Ioctl Response FSCTL_GET_SHADOW_COPY_DATA File: \FOOBAR

Transmission Control Protocol, Src Port: 445 (445), Dst Port: 43314
(43314), Seq: 87971, Ack: 1194, Len: 94
[65 Reassembled TCP Segments (87134 bytes): #73(1360), #76(1360),
#78(1360), #80(1360), #82(1360), #84(1360), #86(1360), #88(1360),
#90(1360), #92(1360), #94(1360), #96(1360), #98(1360), #100(1360),
#102(1360), #104(1360), #106(1360), #108(]
NetBIOS Session Service
SMB2 (Server Message Block Protocol version 2)
    SMB2 Header
    Ioctl Response (0x0b)
        StructureSize: 0x0031
        unknown: 0000
        Function: FSCTL_GET_SHADOW_COPY_DATA (0x00144064)
        GUID handle File: \FOOBAR
        In Data: NO DATA
            Offset: 0x00000070
            Length: 0
        Out Data
            Offset: 0x00000070
            Length: 87018 =====> greater than 64KB.
            Num Volumes: 1740
            Num Labels: 1740
            Count: 87006
            Label: @GMT-2015.01.20-17.00.01
            Label: @GMT-2015.01.20-16.00.00
            Label: @GMT-2015.01.20-15.00.00
            ....

Based on this test, this issue looks very specific to windows client(smb
client) on all available OS versions.
Unfortunately, we would still have to provide patch from server end(to
limit the response to 64K) till MS fix this problem.

Thanks,
Hemanth.


On Tue, Jan 20, 2015 at 12:25 PM, Volker Lendecke <Volker.Lendecke at sernet.de
> wrote:

> On Tue, Jan 20, 2015 at 09:21:57AM -0800, Hemanth Thummala wrote:
> > Hi Volker,
> >
> > I found hard limit(512) on the number of shadow copies(MaxShadowCopies)
> > that windows server can keep. With 512 snapshots, IOCTL response is not
> > going to exceed 32K. I don't think client will behave differently in this
> > case even after enable or disable the large MTU support.
> > Please correct me if I am wrong.
>
> Sure. I did not know about that limit. Sorry, I don't have
> more ideas then except to test with latest Samba. You might
> want to test on a standard Linux and simulate the snapshots
> somehow. If that works, comparing the traces might point out
> some minimal patch that can be applied to 3.6.
>
> Volker
>
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