Re: [PATCH 2/2] selftests: sysctl: Check the sign of a negative jiffies read

[Joel Granados]

On Wed, Sep 23, 2026 at 04:53:54PM +0100, Bradley Morgan wrote:
> On 23 September 2026 15:49:37 BST, Zhan Xusheng <zhanxusheng1024@xxxxxxxxx>
> wrote:
> >proc_int_k2u_conv_kop() returns the sign through *negp and the magnitude
> >through *u_ptr.  For a negative value it used to convert the
> >sign-extended int and negate afterwards, so the magnitude came back as a
> >wrapped 64-bit number.  Nothing noticed, because no test wrote a
> >negative value to a jiffies sysctl.
> >
> >Write one to each of the three int converters and require the value to
> >read back unchanged.  Each value is chosen to convert exactly at every
> >CONFIG_HZ, so the test does not depend on it:
> >
> >	converter		write	jiffies stored	reads back
> >	jiffies			-1	-HZ		-1
> >	ms_jiffies		-1000	-HZ		-1000
> >	userhz_jiffies		-100	-HZ		-100
> >
> >On a kernel without the fix, at CONFIG_HZ=250:
> >
> >  # int_jiffies: got '-18372957097414713411', want '-1'
> >  # int_ms_jiffies: got '-18446744069414585320', want '-1000'
> >  # int_userhz_jiffies: got '-18446742229035144346', want '-100'
> >  not ok 1 test_int_jiffies_negative
> >
> >(paths shortened; the runner prints them under
> >/proc/sys/debug/test_sysctl)
> >
> >Signed-off-by: Zhan Xusheng <zhanxusheng@xxxxxxxxxx>
> >---
> > tools/testing/selftests/sysctl/sysctl.sh | 23 +++++++++++++++++++++++
> > 1 file changed, 23 insertions(+)
> >
> >diff --git a/tools/testing/selftests/sysctl/sysctl.sh b/tools/testing/selftests/sysctl/sysctl.sh
> >index ceb5cfa0cfd4..ad1ee0fccd7c 100755
> >--- a/tools/testing/selftests/sysctl/sysctl.sh
> >+++ b/tools/testing/selftests/sysctl/sysctl.sh
> >@@ -328,6 +328,29 @@ test_int_plain()
> > 	assert_int_range "$t" "$baseline" || RET=$KSFT_FAIL
> > }
> > 
> >+# The jiffies converters report the sign separately from the magnitude, so a
> >+# negative value must survive the round trip through the converter.  Each
> >+# value below is chosen to convert exactly at every CONFIG_HZ.
> 
> I have a feeling this could be decreased, but I don't care.

If you don't care, don't comment.
> 
> Reviewed-by: Bradley Morgan <brads@xxxxxxxxxxxxxx>
> 
> 
> Tested on PowerPC: 
> 
> Tested-by: Bradley Morgan <brads@xxxxxxxxxxxxxx> # POWER10

Please don't suffix anything after the tag as b4 picks it up.

Best

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