Bug #13330


SMB read hangs on Mac Big Sur

Added by Andy Fiddaman about 1 year ago. Updated about 1 year ago.

cifs - CIFS server and client
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There have been several reports of problems with accessing illumos SMB shares from MacOS Big Sur, on both Intel and M1 macbooks.

The symptom is that the reading application (often Finder) hangs and there is a constant stream of network traffic between the client and server, with the client continually attempting to read data at the end of the file offset. The only way to stop it is to turn the network off and on again.

This is an example of the repeating pattern - this request and response pair just keeps happening.

    Read Request (0x08)
        StructureSize: 0x0031
            0000 0000 0011 000. = Fixed Part Length: 24
            .... .... .... ...1 = Dynamic Part: True
        Padding: 0x00
        Flags: 0x00
            .... ...0 = Unbuffered: Client is NOT asking for UNBUFFERED read
            .... ..0. = Compressed: Client is NOT asking for COMPRESSED data
        Read Length: 40960
        File Offset: 85438464
        GUID handle File:
            File Id: d65cc408-01e8-0000-2100-000000000000
            [Frame handle opened: 350910]
        Min Count: 1
        Channel: None (0x00000000)
        Remaining Bytes: 0
        Blob Offset: 0x00000000
        Blob Length: 0
        Channel Info Blob: NO DATA

    Read Response (0x08)
        StructureSize: 0x0011
            0000 0000 0001 000. = Fixed Part Length: 8
            .... .... .... ...1 = Dynamic Part: True
        Data Offset: 0x0050
        Read Length: 0
        Read Remaining: 0
        Reserved: 00000000
        Read Data: <MISSING>
Actions #1

Updated by Gordon Ross about 1 year ago

  • Status changed from New to In Progress
  • Assignee set to Gordon Ross

What's actually shown in the network capture is that MacOS "Big Sur" repeatedly sends
SMB2 read requests at an offset matching the end of file, specifying a "Minimum Count" of one,
and the native SMB server repeatedly returns success with a zero length result.

According to the MS-SMB2 spec. section that's not quite right:

[MS-SMB2] If the read returns fewer bytes than specified by
the MinimumCount field of the request, the server MUST fail
the request with STATUS_END_OF_FILE

Thanks to Matt Barden for noticing that part of the specification and
the fact that the mac was sending Minimum Count > 0.

Actions #2

Updated by Gordon Ross about 1 year ago

Thanks to Andy Fiddiman (and others) for testing this fix with Mac Big Sur clients.
Those clients no longer "hang" (infinite loop trying to read at EoF)

Actions #3

Updated by Electric Monk about 1 year ago

  • Status changed from In Progress to Closed
  • % Done changed from 0 to 100

git commit 4d2aba2f17eed4643aa996841698226ff2673205

commit  4d2aba2f17eed4643aa996841698226ff2673205
Author: Gordon Ross <>
Date:   2020-11-27T20:34:32.000Z

    13330 SMB read hangs on Mac Big Sur
    Reviewed by: Robert Mustacchi <>
    Reviewed by: Paul Winder <>
    Reviewed by: Andy Fiddaman <>
    Reviewed by: Matt Barden <>
    Approved by: Richard Lowe <>


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