[musl] Prefer monotonic clock for DNS lookup timeouts

[musl] Prefer monotonic clock for DNS lookup timeouts

[Gregor Jasny]

Hello,

while looking for a reason for a failed DNS resolve I noticed that
the mtime function which is used to calculate and decide on the timeout 
uses the wall clock instead of a monotonic clock:

static unsigned long mtime()
{
	struct timespec ts;
	clock_gettime(CLOCK_REALTIME, &ts);
	return (unsigned long)ts.tv_sec * 1000
		+ ts.tv_nsec / 1000000;
}

http://git.musl-libc.org/cgit/musl/tree/src/network/res_msend.c#n28

Is this a bug or intentional?

Thanks,
Gregor

Re: [musl] Prefer monotonic clock for DNS lookup timeouts

[Rich Felker]

On Thu, Dec 01, 2022 at 12:24:47PM +0100, Gregor Jasny wrote:
> Hello,
> 
> while looking for a reason for a failed DNS resolve I noticed that
> the mtime function which is used to calculate and decide on the
> timeout uses the wall clock instead of a monotonic clock:
> 
> static unsigned long mtime()
> {
> 	struct timespec ts;
> 	clock_gettime(CLOCK_REALTIME, &ts);
> 	return (unsigned long)ts.tv_sec * 1000
> 		+ ts.tv_nsec / 1000000;
> }
> 
> http://git.musl-libc.org/cgit/musl/tree/src/network/res_msend.c#n28
> 
> Is this a bug or intentional?

It was intentional, based on a belief that the monotonic clock might
not be present on all kernels. That seems to be incorrect for the
range of versions we "support" (>=2.6.0) but some archs unofficially
work back to mid 2.4.x or earlier with limited functionality (no
threads). Note for example that clock_gettime has fallback to the
gettimeofday syscall despite all kernels >=2.6.0 having clock_gettime
(though was it perhaps gated under some CONFIG_ for "realtime
features" at some point? this probably calls for some research...)

Switching to monotonic here has been on my radar for a while. I see
two decent ways to do it without any possibility of regression:

1. Have the above mtime() function fall back to CLOCK_REALTIME on
   ENOSYS, or

2. Go through with integrating a fallback for CLOCK_MONOTONIC I've had
   in draft for a long time that works on ancient kernels. It works by
   combining the seconds-resolution time from SYS_sysinfo uptime with
   the finer-grained-but-wrapping jiffy count from SYS_times too get a
   monotonic jiffies-resolution uptime.

The latter is cute/fun but a little bit of work to get right and I'm
not sure it's sufficiently useful to justify doing it. Option 1 seems
very reasonable.

Rich

Re: [musl] Prefer monotonic clock for DNS lookup timeouts

[A. Wilcox]

On Dec 1, 2022, at 9:11 AM, Rich Felker <dalias@libc.org> wrote:
> 
>> http://git.musl-libc.org/cgit/musl/tree/src/network/res_msend.c#n28
>> 
>> Is this a bug or intentional?
> 
> It was intentional, based on a belief that the monotonic clock might
> not be present on all kernels. That seems to be incorrect for the
> range of versions we "support" (>=2.6.0) but some archs unofficially
> work back to mid 2.4.x or earlier with limited functionality (no
> threads). Note for example that clock_gettime has fallback to the
> gettimeofday syscall despite all kernels >=2.6.0 having clock_gettime
> (though was it perhaps gated under some CONFIG_ for "realtime
> features" at some point? this probably calls for some research...)
> 
> Switching to monotonic here has been on my radar for a while. I see
> two decent ways to do it without any possibility of regression:
> 
> 1. Have the above mtime() function fall back to CLOCK_REALTIME on
>   ENOSYS, or
> 
> 2. Go through with integrating a fallback for CLOCK_MONOTONIC I've had
>   in draft for a long time that works on ancient kernels. It works by
>   combining the seconds-resolution time from SYS_sysinfo uptime with
>   the finer-grained-but-wrapping jiffy count from SYS_times too get a
>   monotonic jiffies-resolution uptime.
> 
> The latter is cute/fun but a little bit of work to get right and I'm
> not sure it's sufficiently useful to justify doing it. Option 1 seems
> very reasonable.
> 
> Rich

clock_gettime was added in 2.5.63 and 2.6.0 and was always obj-y.

There is no CONFIG knob that controls it in early 2.6, though I stopped
my search at 2.6.16.

I have written this patch out, but I don’t have a box without monotonic
clock, so I can’t test the validity of the patch.  I will send the patch
to the list for review.

Best,
-A.

[musl] [PATCH] dns: Prefer monotonic clock for DNS lookup timeouts

[A. Wilcox]

Before this commit, DNS timeouts always used CLOCK_REALTIME, which could
produce errors if wall time changed for whatever reason.

Now we try CLOCK_MONOTONIC and only fall back to CLOCK_REALTIME when it
is unavailable.
---
 src/network/res_msend.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/src/network/res_msend.c b/src/network/res_msend.c
index 11c6aa0e..fef7e3a2 100644
--- a/src/network/res_msend.c
+++ b/src/network/res_msend.c
@@ -25,7 +25,8 @@ static void cleanup(void *p)
 static unsigned long mtime()
 {
 	struct timespec ts;
-	clock_gettime(CLOCK_REALTIME, &ts);
+	if (clock_gettime(CLOCK_MONOTONIC, &ts) < 0 && errno == ENOSYS)
+		clock_gettime(CLOCK_REALTIME, &ts);
 	return (unsigned long)ts.tv_sec * 1000
 		+ ts.tv_nsec / 1000000;
 }
-- 
2.37.1 (Apple Git-137.1)