netdev - [tc] Invalid JSON output from hfsc qdisc

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Message-ID:
 <SI2PPF4F82E9256898C9826AF17C3AE8AD9F467A@SI2PPF4F82E9256.apcprd04.prod.outlook.com>
Date: Sat, 7 Feb 2026 02:01:01 +0000
From: Deren Teo <deren.teo@...look.com>
To: "jhs@...atatu.com" <jhs@...atatu.com>
CC: "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: [tc] Invalid JSON output from hfsc qdisc

Dear Jamal,

I apologise if you have received this email twice. My first attempt mistakenly contained an HTML part and so was not delivered to the netdev mailing list. The original content follows.

The JSON output for the hfsc qdisc is not parseable when a default class is specified; `hfsc_print_opt` uses `fprintf` to print the default class.

Environment:

- Fedora 42
- Linux 6.19.0-rc4+
- iproute-tc-6.12.0-3.fc42.x86_64

Reproduce:

1. Add a hfsc qdisc with any default class. For example, from the tc-hfsc man page:
    # tc qdisc add dev eth0 root handle 1:0 hfsc default 1

2. Show the qdisc with the `-j` flag:
    # tc -j qdisc show dev eth0

Actual results:

[{"kind":"hfsc","handle":"1:","root":true,"refcnt":17,"options":{default 1 }}]

Expected results:

[{"kind":"hfsc","handle":"1:","root":true,"refcnt":17,"options":{"default":1}}]

Additional info:

A similar problem was previously resolved for the u32 filter by replacing `fprintf` with `print_uint`, etc: https://bugzilla.redhat.com/show_bug.cgi?id=1989591.

Best regards,
Deren Teo