netdev - [PATCH iproute2] tc: Fix rounding in tc_calc_xmittime and tc_calc

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [day] [month] [year] [list]

Message-Id: <85371219-A098-4873-B3B9-0E881E812F2A@8x8.com>
Date: Wed, 5 Feb 2025 16:16:21 -0500
From: Jonathan Lennox <jonathan.lennox@....com>
To: netdev@...r.kernel.org
Subject: [PATCH iproute2] tc: Fix rounding in tc_calc_xmittime and
 tc_calc_xmitsize.

The logic in tc that converts between sizes and times for a given rate (the
functions tc_calc_xmittime and tc_calc_xmitsize) suffers from double rounding,
with intermediate values getting cast to unsigned int.

As a result, for example, on my test system (where tick_in_usec=15.625,
clock_factor=1, and hz=1000000000) for a bitrate of 1Gbps, all tc htb burst
values between 0 and 999 get encoded as 0; all values between 1000 and 1999
get encoded as 15 (equivalent to 960 bytes); all values between 2000 and 2999
as 31 (1984 bytes); etc.

The attached patch changes this so these calculations are done entirely in
floating-point, and only rounded to integer values when the value is returned.
It also changes tc_calc_xmittime to round its calculated value up, rather than
down, to ensure that the calculated time is actually sufficient for the requested
size.

This is a userspace-only fix to tc; no kernel changes are necessary.

(Please let me know if anything is wrong with this patch, this is my first
time submitting to any Linux kernel mailing lists.)

---
tc/tc_core.c | 11 ++++++++---
1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/tc/tc_core.c b/tc/tc_core.c
index 37547e9b..ba8d5bf0 100644
--- a/tc/tc_core.c
+++ b/tc/tc_core.c
@@ -23,11 +23,16 @@
static double tick_in_usec = 1;
static double clock_factor = 1;

-static unsigned int tc_core_time2tick(unsigned int time)
+static double tc_core_time2tick_d(double time)
{
	return time * tick_in_usec;
}

+static double tc_core_tick2time_d(double tick)
+{
+	return tick / tick_in_usec;
+}
+
unsigned int tc_core_tick2time(unsigned int tick)
{
	return tick / tick_in_usec;
@@ -45,12 +50,12 @@ unsigned int tc_core_ktime2time(unsigned int ktime)

unsigned int tc_calc_xmittime(__u64 rate, unsigned int size)
{
-	return tc_core_time2tick(TIME_UNITS_PER_SEC*((double)size/(double)rate));
+	return ceil(tc_core_time2tick_d(TIME_UNITS_PER_SEC*((double)size/(double)rate)));
}

unsigned int tc_calc_xmitsize(__u64 rate, unsigned int ticks)
{
-	return ((double)rate*tc_core_tick2time(ticks))/TIME_UNITS_PER_SEC;
+	return ((double)rate*tc_core_tick2time_d(ticks))/TIME_UNITS_PER_SEC;
}

/*