Re: [PATCH] tc: fix build failure with Linux >= 6.8 kernel headers (CBQ removal)

On Mon, 17 Aug 2026 02:15:16 +0500
Muhammad Bilal via busybox <[email protected]> wrote:

> Linux commit 33241dca4862 ("net/sched: Remove uapi support for CBQ
> qdisc"), first released in v6.8-rc1 (2024-01-02), dropped all CBQ
> related definitions (TCA_CBQ_*, struct tc_cbq_*, TC_CBQ_MAXPRIO, ...)
> from include/uapi/linux/pkt_sched.h. The CBQ qdisc itself had already
> been retired from the kernel earlier, in v6.3 (commit 051d44209842,
> "net/sched: Retire CBQ qdisc").
> 
> Building tc.c against >= 6.8 kernel headers fails with:
> 
>     networking/tc.c: In function 'cbq_print_opt':
>     networking/tc.c:236:27: error: 'TCA_CBQ_MAX' undeclared (first
>     use in this function); did you mean 'TCA_CBS_MAX'?
>       236 |   struct rtattr *tb[TCA_CBQ_MAX+1];
>           |                     ^~~~~~~~~~~
>     (and further errors for TCA_CBQ_RATE, struct tc_cbq_lssopt, etc.)
> 
> Guard cbq_print_opt() with '#ifdef TCA_CBQ_MAX', the same pattern
> already used a few lines above for the deprecated TCA_PRIO_MAX
> multiqueue interface, and guard its two call sites in print_qdisc()
> and print_class(). print_rate() is only ever called from
> cbq_print_opt(), so move it under the same #ifdef too, to avoid a new
> "defined but not used" warning once CBQ support is compiled out.
> 
> This is essentially the same fix independently proposed by Uwe
> Kleine-Koenig on the mailing list in March 2024, which was never
> applied:
>   http://lists.busybox.net/pipermail/busybox/2024-March/090678.html
> 
> Already reported twice, but never fixed:
>   https://bugs.busybox.net/show_bug.cgi?id=15931
>     "CBQ support removed from Linux kernel"
>   https://bugs.busybox.net/show_bug.cgi?id=15934
>     "Busybox fails to build with linux kernels >= 6.8"
>   https://lists.busybox.net/pipermail/busybox-cvs/2024-January/041752.html
> 
> Root cause double-checked directly against a local clone of
> torvalds/linux: commit 33241dca486264193ed68167c8eeae1fb197f3df
> removes struct tc_cbq_*/TCA_CBQ_* from
> include/uapi/linux/pkt_sched.h, and `git describe --contains` places
> it at v6.8-rc1~131^2~60^2^2 (2024-01-02).
> 

Hi,

as far as I understand with the proposed fix the
tc applet will only work with the kernel it is compiled with,
if you use a busybox tc applet compiled with kernel 6.12.103
it will not work with 6.7.0 and cbq , this could be seen as a shortcoming.
Please take a look at my proposed patch which fixes building
busybox and allows use with all kernels at the expense
of a little size increase.

Ciao,
Tito

> Build tested on:
>   OS:      Fedora Linux 44 (Forty Four), x86_64
>   Kernel:  Linux 7.1.8-200.fc44.x86_64
>   Headers: kernel-headers-7.1.3-200.fc44.x86_64
>            (/usr/include/linux/pkt_sched.h has no CBQ symbols)
>   gcc:     gcc (GCC) 15.3.1 20260729
> 
> Before this patch, `make defconfig && make` failed while compiling
> networking/tc.c on the above system. With this patch, the build
> completes cleanly and "busybox tc" still builds and runs normally.
> 
> Fixes: https://bugs.busybox.net/show_bug.cgi?id=15931
> Fixes: https://bugs.busybox.net/show_bug.cgi?id=15934
> 
> Signed-off-by: Muhammad Bilal <[email protected]>
> ---
>  networking/tc.c | 55 ++++++++++++++++++++++++++++++-------------------
>  1 file changed, 34 insertions(+), 21 deletions(-)
> 
> diff --git a/networking/tc.c b/networking/tc.c
> index 3a79fd2d9..337d499b4 100644
> --- a/networking/tc.c
> +++ b/networking/tc.c
> @@ -176,27 +176,6 @@ static int get_tc_classid(uint32_t *h, const char *str)
>  	return 0;
>  }
>  
> -static void print_rate(char *buf, int len, uint32_t rate)
> -{
> -	double tmp = (double)rate*8;
> -
> -	if (use_iec) {
> -		if (tmp >= 1000*1024*1024)
> -			snprintf(buf, len, "%.0fMibit", tmp/(1024*1024));
> -		else if (tmp >= 1000*1024)
> -			snprintf(buf, len, "%.0fKibit", tmp/1024);
> -		else
> -			snprintf(buf, len, "%.0fbit", tmp);
> -	} else {
> -		if (tmp >= 1000*1000000)
> -			snprintf(buf, len, "%.0fMbit", tmp/1000000);
> -		else if (tmp >= 1000*1000)
> -			snprintf(buf, len, "%.0fKbit", tmp/1000);
> -		else
> -			snprintf(buf, len, "%.0fbit",  tmp);
> -	}
> -}
> -
>  #if 0
>  /* This is "pfifo_fast".  */
>  static int prio_parse_opt(int argc, char **argv, struct nlmsghdr *n)
> @@ -231,6 +210,35 @@ static int cbq_parse_opt(int argc, char **argv, struct nlmsghdr *n)
>  	return 0;
>  }
>  #endif
> +
> +#ifndef TCA_CBQ_MAX
> +/*
> + * Linux v6.8-rc1~131^2~60^2^2 removed the uapi definitions for CBQ.
> + * See https://git.kernel.org/linus/33241dca48626
> + */
> +#else
> +/* Used only by CBQ; other qdiscs print rates without this helper. */
> +static void print_rate(char *buf, int len, uint32_t rate)
> +{
> +	double tmp = (double)rate*8;
> +
> +	if (use_iec) {
> +		if (tmp >= 1000*1024*1024)
> +			snprintf(buf, len, "%.0fMibit", tmp/(1024*1024));
> +		else if (tmp >= 1000*1024)
> +			snprintf(buf, len, "%.0fKibit", tmp/1024);
> +		else
> +			snprintf(buf, len, "%.0fbit", tmp);
> +	} else {
> +		if (tmp >= 1000*1000000)
> +			snprintf(buf, len, "%.0fMbit", tmp/1000000);
> +		else if (tmp >= 1000*1000)
> +			snprintf(buf, len, "%.0fKbit", tmp/1000);
> +		else
> +			snprintf(buf, len, "%.0fbit",  tmp);
> +	}
> +}
> +
>  static int cbq_print_opt(struct rtattr *opt)
>  {
>  	struct rtattr *tb[TCA_CBQ_MAX+1];
> @@ -322,6 +330,7 @@ static int cbq_print_opt(struct rtattr *opt)
>   done:
>  	return 0;
>  }
> +#endif
>  
>  static FAST_FUNC int print_qdisc(
>  		const struct sockaddr_nl *who UNUSED_PARAM,
> @@ -372,8 +381,10 @@ static FAST_FUNC int print_qdisc(
>  		int qqq = index_in_strings(_q_, name);
>  		if (qqq == 0) { /* pfifo_fast aka prio */
>  			prio_print_opt(tb[TCA_OPTIONS]);
> +#ifdef TCA_CBQ_MAX
>  		} else if (qqq == 1) { /* class based queuing */
>  			cbq_print_opt(tb[TCA_OPTIONS]);
> +#endif
>  		} else {
>  			/* don't know how to print options for this qdisc */
>  			printf("(options for %s)", name);
> @@ -442,9 +453,11 @@ static FAST_FUNC int print_class(
>  		int qqq = index_in_strings(_q_, name);
>  		if (qqq == 0) { /* pfifo_fast aka prio */
>  			/* nothing. */ /*prio_print_opt(tb[TCA_OPTIONS]);*/
> +#ifdef TCA_CBQ_MAX
>  		} else if (qqq == 1) { /* class based queuing */
>  			/* cbq_print_copt() is identical to cbq_print_opt(). */
>  			cbq_print_opt(tb[TCA_OPTIONS]);
> +#endif
>  		} else {
>  			/* don't know how to print options for this class */
>  			printf("(options for %s)", name);