Incorrect packet handling in SMBClient MacOS 26.

Question

Created Dec ’25

Replies 6

Boosts 7

Participants 2

SMBClient-593 introduces a crtitical bug.

When reading and writing data at high volume, the SMBClient no longer properly receives and handle responses from the server.

In some cases, the client mishandles the response packet and the following errors are seen in the logs:

2025-12-02 21:36:04.774772-0700  localhost kernel[0]: (smbfs) smb2_smb_parse_write_one: Bad struct size: 0
2025-12-02 21:36:04.774776-0700  localhost kernel[0]: (smbfs) smb2_smb_write: smb2_smb_read_write_async failed with an error 72
2025-12-02 21:36:04.774777-0700  localhost kernel[0]: (smbfs) smbfs_do_strategy: file.txt: WRITE failed with an error of 72

In other cases, the client mishandles the response packet and becomes completely unresponsive, unable to send or receive additional messages, and a forced shutdown of the computer is required to recover.

This bug is only present on macos 26. We believe the operative change is in the latest commit, SMBClient-593 beginning at line now 3011 in smb_iod.c. The issue seems to be a race, and occurs much more frequently once throughput exceeds around 10Gbps, and again more frequently above 20Gbps.

Answered by DTS Engineer in 868297022

I recommend that you report this officially by filing a bug.

Please post your bug number, just for the record.

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Quinn “The Eskimo!” @ Developer Technical Support @ Apple
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Boost

Answer 1

DTS Engineer OP

Apple

Dec ’25

Recommended

I recommend that you report this officially by filing a bug.

Please post your bug number, just for the record.

Share and Enjoy
—
Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"

0

Answer 2

mfh OP

Dec ’25

Thank you,

Yes, I have filed a bug report FB21249476.

2

Answer 3

mfh OP

2w

Hi, any update on this bug report?

0

Answer 4

DTS Engineer OP

Apple

2w

If I’m reading FB21249476 correctly, there should be improvements in this space in Xcode 26.3b2 (25D5101c) [1]. I recommend that you re-test with that build.

Share and Enjoy
—
Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"

[1] This only just started seeding, so Feedback Assistant hasn’t had time to notify you about it.

1

Answer 5

mfh OP

1w

Hi,

The issue persists.

We’re no longer seeing the “rpc struct is bad” errors, so perhaps one of the two cases is resolved, but we’re still able to consistently deadlock the SMBClient, requiring a forced restart of the computer to recover.

The reproduction steps are the same.

0

Answer 6

DTS Engineer OP

Apple

1w

We’re no longer seeing the “rpc struct is bad” errors

OK, that sounds like progress (-:

we’re still able to consistently deadlock the SMBClient

Bummer )-:

I recommend that you file a new bug about that. Make sure to reference FB21249476, confirm that you tested on Xcode 26.3b2 (25D5101c), and that you continue to see problems. Also, attach a sysdiagnose of the problem in action on 26.3b2.

Please post your new bug number as well.

Share and Enjoy
—
Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"

0