sys_linux: fix building with older compilers and some archs

The recent replacement of <termios.h> with <linux/termios.h> to get
TCGETS2 seems to work only with compilers (or C standards) that allow
the same structure to be defined multiple times. There is a conflict
between <sys/ioctl.h> and <linux/termios.h>.

Another problem is that TCGETS2 is not used on some archs like ppc64.

Switch back to <termios.h> and move TCGETS2 to a list in a separate
file where it can be compiled without <sys/ioctl.h>.

Fixes: 03875f1ea5 ("sys_linux: allow ioctl(TCGETS2) in seccomp filter")

configure

@@ -808,6 +808,7 @@ then
   # a time and the async resolver would block the main thread
   priv_ops="NAME2IPADDRESS RELOADDNS"
   EXTRA_LIBS="$EXTRA_LIBS -lseccomp"
+  EXTRA_OBJECTS="$EXTRA_OBJECTS sys_linux_scmp.o"
 fi
 
 if [ "x$priv_ops" != "x" ]; then

doc/chrony.conf.adoc

@@ -1133,23 +1133,29 @@ distances are in milliseconds.
 
 [[clockprecision]]*clockprecision* _precision_::
 The *clockprecision* directive specifies the precision of the system clock (in
-seconds). It is used by *chronyd* to estimate the minimum noise in NTP
-measurements and randomise low-order bits of timestamps in NTP responses. By
-default, the precision is measured on start-up as the minimum time to read the
-clock.
+seconds). This value is used by *chronyd* as the minimum expected error and
+amount of noise in NTP and refclock measurements, and to randomise low-order
+bits of timestamps in NTP responses to make them less predictable. The minimum
+value is 1 nanosecond and the maximum value is 1 second.
 +
-The measured value works well in most cases. It generally overestimates the
-precision and it can be sensitive to the CPU speed, however, which can
-change over time to save power. In some cases with a high-precision clocksource
-(e.g. the Time Stamp Counter of the CPU) and hardware timestamping, setting the
-precision on the server to a smaller value can improve stability of clients'
-NTP measurements. The server's precision is reported on clients by the
+By default, *chronyd* tries to determine the precision on start-up as the
+resolution of the clock. On Linux, it tries to measure the resolution by
+observing the minimum change in differences between consecutive readings of the
+clock. On other systems it relies on the *clock_getres(2)* system function.
++
+If the measurement fails, or the value provided by the system is too large, the
+minimum measured time needed to read the clock will be used instead. This value
+is typically larger than the resolution, and it is sensitive to the CPU speed,
+however, which can change over time to save power.
++
+The server's precision is reported on clients by the
 <<chronyc.adoc#ntpdata,*ntpdata*>> command.
 +
-An example setting the precision to 8 nanoseconds is:
+An example setting the precision to 1 nanosecond (e.g. when the system clock is
+using a Time Stamp Counter (TSC) updated at a rate of at least 1 GHz) is:
 +
 ----
-clockprecision 8e-9
+clockprecision 1e-9
 ----
 
 [[corrtimeratio]]*corrtimeratio* _ratio_::

leapdb.c

@@ -130,7 +130,7 @@ get_list_leap(time_t when, int *tai_offset)
     char *p;
 
     /* Ignore blank lines */
-    for (p = line; *p && isspace(*p); ++p)
+    for (p = line; *p && isspace((unsigned char)*p); ++p)
       ;
     if (!*p)
       continue;

local.c

@@ -97,8 +97,142 @@ static double precision_quantum;
 
 static double max_clock_error;
 
+#define NSEC_PER_SEC 1000000000
+
 /* ================================================== */
 
+/* Ask the system for the resolution of the system clock.  The Linux
+   clock_getres() is not usable, because it reports the internal timer
+   resolution, which is 1 ns when high-resolution timers are enabled,
+   even when using a lower-resolution clocksource. */
+
+static int
+get_clock_resolution(void)
+{
+#if defined(HAVE_CLOCK_GETTIME) && !defined(LINUX)
+  struct timespec res;
+
+  if (clock_getres(CLOCK_REALTIME, &res) < 0)
+    return 0;
+
+  return NSEC_PER_SEC * res.tv_sec + res.tv_nsec;
+#else
+  return 0;
+#endif
+}
+
+/* ================================================== */
+
+#if defined(LINUX) && defined(HAVE_CLOCK_GETTIME) && defined(CLOCK_MONOTONIC_RAW)
+
+static int
+compare_ints(const void *a, const void *b)
+{
+  return *(const int *)a - *(const int *)b;
+}
+
+#define READINGS 64
+
+/* On Linux, try to measure the actual resolution of the system
+   clock by performing a varying amount of busy work between clock
+   readings and finding the minimum change in the measured interval.
+   Require a change of at least two nanoseconds to ignore errors
+   caused by conversion to timespec.  Use the raw monotonic clock
+   to avoid the impact of potential frequency changes due to NTP
+   adjustments made by other processes, and the kernel dithering of
+   the 32-bit multiplier. */
+
+static int
+measure_clock_resolution(void)
+{
+  int i, j, b, busy, diffs[READINGS - 1], diff2, min;
+  struct timespec start_ts, ts[READINGS];
+  uint32_t acc;
+
+  if (clock_gettime(CLOCK_MONOTONIC_RAW, &start_ts) < 0)
+    return 0;
+
+  for (acc = 0, busy = 1; busy < 100000; busy = busy * 3 / 2 + 1) {
+    for (i = 0, b = busy * READINGS; i < READINGS; i++, b -= busy) {
+      if (clock_gettime(CLOCK_MONOTONIC_RAW, &ts[i]) < 0)
+        return 0;
+
+      for (j = b; j > 0; j--)
+        acc += (acc & 1) + (uint32_t)ts[i].tv_nsec;
+    }
+
+    /* Give up after 0.1 seconds */
+    if (UTI_DiffTimespecsToDouble(&ts[READINGS - 1], &start_ts) > 0.1) {
+      DEBUG_LOG("Measurement too slow");
+      return 0;
+    }
+
+    for (i = 0; i < READINGS - 1; i++) {
+      diffs[i] = NSEC_PER_SEC * (ts[i + 1].tv_sec - ts[i].tv_sec) +
+                 (ts[i + 1].tv_nsec - ts[i].tv_nsec);
+
+      /* Make sure the differences are sane.  A resolution larger than the
+         reading time will be measured in measure_clock_read_delay(). */
+      if (diffs[i] <= 0 || diffs[i] > NSEC_PER_SEC)
+        return 0;
+    }
+
+    /* Sort the differences and keep values unique within 1 ns from the
+       first half of the array, which are less likely to be impacted by CPU
+       interruptions */
+    qsort(diffs, READINGS - 1, sizeof (diffs[0]), compare_ints);
+    for (i = 1, j = 0; i < READINGS / 2; i++) {
+      if (diffs[j] + 1 < diffs[i])
+        diffs[++j] = diffs[i];
+    }
+    j++;
+
+#if 0
+    for (i = 0; i < j; i++)
+      DEBUG_LOG("busy %d diff %d %d", busy, i, diffs[i]);
+#endif
+
+    /* Require at least three unique differences to be more confident
+       with the result */
+    if (j < 3)
+      continue;
+
+    /* Find the smallest difference between the unique differences */
+    for (i = 1, min = 0; i < j; i++) {
+      diff2 = diffs[i] - diffs[i - 1];
+      if (min == 0 || min > diff2)
+        min = diff2;
+    }
+
+    if (min == 0)
+      continue;
+
+    /* Prevent the compiler from optimising the busy work out */
+    if (acc == 0)
+      min += 1;
+
+    return min;
+  }
+
+  return 0;
+}
+
+#else
+static int
+measure_clock_resolution(void)
+{
+  return 0;
+}
+#endif
+
+/* ================================================== */
+
+/* As a fallback, measure how long it takes to read the clock.  It
+   typically takes longer than the resolution of the clock (and it
+   depends on the CPU speed), i.e. every reading gives a different
+   value, but handle also low-resolution clocks that might give
+   the same reading multiple times. */
+
 /* Define the number of increments of the system clock that we want
    to see to be fairly sure that we've got something approaching
    the minimum increment.  Even on a crummy implementation that can't
@@ -106,10 +240,8 @@ static double max_clock_error;
    under 1s of busy waiting. */
 #define NITERS 100
 
-#define NSEC_PER_SEC 1000000000
-
-static double
-measure_clock_precision(void)
+static int
+measure_clock_read_delay(void)
 {
   struct timespec ts, old_ts;
   int iters, diff, best;
@@ -135,7 +267,28 @@ measure_clock_precision(void)
 
   assert(best > 0);
 
-  return 1.0e-9 * best;
+  return best;
+}
+
+/* ================================================== */
+
+static double
+measure_clock_precision(void)
+{
+  int res, delay, prec;
+
+  res = get_clock_resolution();
+  if (res <= 0)
+    res = measure_clock_resolution();
+
+  delay = measure_clock_read_delay();
+
+  if (res > 0)
+    prec = MIN(res, delay);
+  else
+    prec = delay;
+
+  return prec / 1.0e9;
 }
 
 /* ================================================== */

sys_linux.c

@@ -63,6 +63,7 @@
 #endif
 
 #include "sys_linux.h"
+#include "sys_linux_scmp.h"
 #include "sys_timex.h"
 #include "conf.h"
 #include "local.h"
@@ -728,6 +729,14 @@ SYS_Linux_EnableSystemCallFilter(int level, SYS_ProcessContext context)
                            SCMP_A1(SCMP_CMP_EQ, ioctls[i])) < 0)
         goto add_failed;
     }
+
+    /* Allow selected ioctls that need to be specified in a separate
+       file to avoid conflicting headers (e.g. TCGETS2) */
+    for (i = 0; SYS_Linux_GetExtraScmpIoctl(i) != 0; i++) {
+      if (seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(ioctl), 1,
+                           SCMP_A1(SCMP_CMP_EQ, SYS_Linux_GetExtraScmpIoctl(i))) < 0)
+        goto add_failed;
+    }
   }
 
   if (seccomp_load(ctx) < 0)

sys_linux_scmp.c

@@ -0,0 +1,44 @@
+/*
+  chronyd/chronyc - Programs for keeping computer clocks accurate.
+
+ **********************************************************************
+ * Copyright (C) Miroslav Lichvar  2025
+ * 
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of version 2 of the GNU General Public License as
+ * published by the Free Software Foundation.
+ * 
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ * 
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+ * 
+ **********************************************************************
+
+  =======================================================================
+
+  Lists of values that are needed in seccomp filters but need to
+  be compiled separately from sys_linux.c due to conflicting headers.
+  */
+
+#include <linux/termios.h>
+
+#include "sys_linux_scmp.h"
+
+unsigned long
+SYS_Linux_GetExtraScmpIoctl(int index)
+{
+  const unsigned long ioctls[] = {
+#ifdef TCGETS2
+    /* Conflict between <linux/termios.h> and <sys/ioctl.h> */
+    TCGETS2,
+#endif
+    0
+  };
+
+  return ioctls[index];
+}

sys_linux_scmp.h

@@ -0,0 +1,28 @@
+/*
+  chronyd/chronyc - Programs for keeping computer clocks accurate.
+
+ **********************************************************************
+ * Copyright (C) Miroslav Lichvar  2025
+ * 
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of version 2 of the GNU General Public License as
+ * published by the Free Software Foundation.
+ * 
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ * 
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+ * 
+ **********************************************************************
+
+  =======================================================================
+
+  Header file for lists that are needed in seccomp filters but need to
+  be compiled separately from sys_linux.c due to conflicting headers.
+  */
+
+extern unsigned long SYS_Linux_GetExtraScmpIoctl(int index);

test/unit/socket.c

@@ -191,6 +191,7 @@ test_unit(void)
         s3 = SCK_AcceptConnection(s1, &sa2);
         TEST_CHECK(UTI_CompareIPs(&sa1.ip_addr, &sa2.ip_addr, NULL) == 0);
 
+        fcntl(s3, F_SETFL, fcntl(s3, F_GETFL) & ~O_NONBLOCK);
         send_and_recv(SCK_ADDR_IP, 1, 1, s3, s2);
 
         SCK_ShutdownConnection(s2);
@@ -227,6 +228,7 @@ test_unit(void)
     s3 = SCK_AcceptConnection(s1, &sa2);
     TEST_CHECK(sa2.ip_addr.family == IPADDR_UNSPEC);
 
+    fcntl(s3, F_SETFL, fcntl(s3, F_GETFL) & ~O_NONBLOCK);
     send_and_recv(SCK_ADDR_UNIX, 1, i % 2, s3, s2);
 
     if (i % 4)