[PATCH iproute2-next 1/2] tc/tbf: enable use of 64 bit burst

Ioana Lazea
2026-03-23T09:31:44Z

Modify the TBF queueing discipline configuration to allow burst sizes up
to the limits supported by the kernel API, which may exceed 4 GB at
higher rates.

In the current implementation, the burst option is effectively limited
to 4 GB because it is stored in a 32-bit field. However, the kernel
internally represents the burst in psched ticks (64 nsec each). At
sufficiently high rates, this representation can correspond to burst
sizes larger than 4 GB without requiring kernel changes.

Overflows (burst values that exceed UINT_MAX psched ticks) are now
correctly detected, and flagged as an error, rather than passing
arbitrary psched tick values to the kernel.

The change to increase the burst size to 64-bit introduced an issue with
TCA_TBF_BURST. The kernel expects this attribute as NLA_U32 (a 32-bit
unsigned value), but the updated implementation may produce a 64-bit
value, causing the tbf_policy check in the kernel to fail. However, when
TCA_TBF_BURST is omitted (which occurs when buffer64 >= UINT_MAX), the
kernel falls back to using the value assigned to qopt->buffer, which
correctly supports large burst sizes. Omitting TCA_TBF_BURST for large
bursts preserves backward compatibility and aligns with existing kernel
behavior. This change therefore ensures correct handling of large burst
sizes while remaining compatible with kernel and iproute2 expectations.

Signed-off-by: Ioana Lazea <ioana.lazea@canonical.com>
---
tc/q_tbf.c | 24 ++++++++++++++++--------
1 file changed, 16 insertions(+), 8 deletions(-)

diff --git a/tc/q_tbf.c b/tc/q_tbf.c
index 9356dfd2..cbe11025 100644
--- a/tc/q_tbf.c
+++ b/tc/q_tbf.c
@@ -37,12 +37,12 @@ static int tbf_parse_opt(const struct qdisc_util *qu, int argc, char **argv,
struct tc_tbf_qopt opt = {};
__u32 rtab[256];
__u32 ptab[256];
- unsigned buffer = 0, mtu = 0, mpu = 0, latency = 0;
+ unsigned mtu = 0, mpu = 0, latency = 0;
int Rcell_log = -1, Pcell_log = -1;
unsigned short overhead = 0;
unsigned int linklayer = LINKLAYER_ETHERNET; /* Assume ethernet */
struct rtattr *tail;
- __u64 rate64 = 0, prate64 = 0;
+ __u64 rate64 = 0, prate64 = 0, buffer64 = 0;

while (argc > 0) {
if (matches(*argv, "limit") == 0) {
@@ -79,11 +79,11 @@ static int tbf_parse_opt(const struct qdisc_util *qu, int argc, char **argv,
const char *parm_name = *argv;

NEXT_ARG();
- if (buffer) {
+ if (buffer64) {
fprintf(stderr, "tbf: duplicate \"buffer/burst/maxburst\" specification\n");
return -1;
}
- if (get_size_and_cell(&buffer, &Rcell_log, *argv) < 0) {
+ if (get_size64_and_cell(&buffer64, &Rcell_log, *argv) < 0) {
explain1(parm_name, *argv);
return -1;
}
@@ -175,7 +175,7 @@ static int tbf_parse_opt(const struct qdisc_util *qu, int argc, char **argv,
fprintf(stderr, "tbf: the \"rate\" parameter is mandatory.\n");
verdict = -1;
}
- if (!buffer) {
+ if (!buffer64) {
fprintf(stderr, "tbf: the \"burst\" parameter is mandatory.\n");
verdict = -1;
}
@@ -200,7 +200,7 @@ static int tbf_parse_opt(const struct qdisc_util *qu, int argc, char **argv,
opt.peakrate.rate = (prate64 >= (1ULL << 32)) ? ~0U : prate64;

if (opt.limit == 0) {
- double lim = rate64*(double)latency/TIME_UNITS_PER_SEC + buffer;
+ double lim = rate64*(double)latency/TIME_UNITS_PER_SEC + buffer64;

if (prate64) {
double lim2 = prate64*(double)latency/TIME_UNITS_PER_SEC + mtu;
@@ -217,7 +217,11 @@ static int tbf_parse_opt(const struct qdisc_util *qu, int argc, char **argv,
fprintf(stderr, "tbf: failed to calculate rate table.\n");
return -1;
}
- opt.buffer = tc_calc_xmittime(opt.rate.rate, buffer);
+ opt.buffer = tc_calc_xmittime(opt.rate.rate, buffer64);
+ if (opt.buffer == UINT_MAX) {
+ fprintf(stderr, "tbf: burst out of range\n");
+ return -1;
+ }

if (opt.peakrate.rate) {
opt.peakrate.mpu = mpu;
@@ -231,7 +235,11 @@ static int tbf_parse_opt(const struct qdisc_util *qu, int argc, char **argv,

tail = addattr_nest(n, 1024, TCA_OPTIONS);
addattr_l(n, 2024, TCA_TBF_PARMS, &opt, sizeof(opt));
- addattr_l(n, 2124, TCA_TBF_BURST, &buffer, sizeof(buffer));
+ if (buffer64 < (1ULL << 32)) {
+ __u32 buffer32 = (__u32)buffer64;
+
+ addattr_l(n, 2124, TCA_TBF_BURST, &buffer32, sizeof(buffer32));
+ }
if (rate64 >= (1ULL << 32))
addattr_l(n, 2124, TCA_TBF_RATE64, &rate64, sizeof(rate64));
addattr_l(n, 3024, TCA_TBF_RTAB, rtab, 1024);
--
2.43.0