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

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).

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);
-- 
2.55.0