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

[Bradley Morgan]

On 25 September 2026 12:45:27 BST, Joel Granados <joel.granados@xxxxxxxxxx>
wrote:
>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.
>> 

its because I think it could, but it isn't a complete patch breaker.

>> 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
>

--- Thanks!
"I'm not a very positive person" - Linus torvalds