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

[Bradley Morgan]

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.

Reviewed-by: Bradley Morgan <brads@xxxxxxxxxxxxxx>


Tested on PowerPC: 

Tested-by: Bradley Morgan <brads@xxxxxxxxxxxxxx> # POWER10

>+test_int_jiffies_negative()
>+{
>+	local t="${SYSCTL}/int_jiffies"
>+	local m="${SYSCTL}/int_ms_jiffies"
>+	local u="${SYSCTL}/int_userhz_jiffies"
>+
>+	[[ -f "$t" && -f "$m" && -f "$u" ]] || { RET=$KSFT_SKIP; return; }
>+
>+	printf '%s' "-1" > "$t"
>+	assert_content "$t" "-1" || RET=$KSFT_FAIL
>+	printf '%s' "-1000" > "$m"
>+	assert_content "$m" "-1000" || RET=$KSFT_FAIL
>+	printf '%s' "-100" > "$u"
>+	assert_content "$u" "-100" || RET=$KSFT_FAIL
>+
>+	# The positive path was never broken; keep it covered.
>+	printf '%s' "1" > "$t"
>+	assert_content "$t" "1" || RET=$KSFT_FAIL
>+}
>+
> test_uint_plain()
> {
> 	local t="${SYSCTL}/uint_0001"
>

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