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#!/bin/sh
# SPDX-License-Identifier: GPL-2.0
#
# Measure kernel stack entropy by sampling via LKDTM's REPORT_STACK test.
set -e
samples="${1:-1000}"
TRIGGER=/sys/kernel/debug/provoke-crash/DIRECT
KSELFTEST_SKIP_TEST=4
# Verify we have LKDTM available in the kernel.
if [ ! -r $TRIGGER ] ; then
/sbin/modprobe -q lkdtm || true
if [ ! -r $TRIGGER ] ; then
echo "Cannot find $TRIGGER (missing CONFIG_LKDTM?)"
else
echo "Cannot write $TRIGGER (need to run as root?)"
fi
# Skip this test
exit $KSELFTEST_SKIP_TEST
fi
# Capture dmesg continuously since it may fill up depending on sample size.
log=$(mktemp -t stack-entropy-XXXXXX)
dmesg --follow >"$log" & pid=$!
report=-1
for i in $(seq 1 $samples); do
echo "REPORT_STACK" > $TRIGGER
if [ -t 1 ]; then
percent=$(( 100 * $i / $samples ))
if [ "$percent" -ne "$report" ]; then
/bin/echo -en "$percent%\r"
report="$percent"
fi
fi
done
kill "$pid"
# Count unique offsets since last run.
seen=$(tac "$log" | grep -m1 -B"$samples"0 'Starting stack offset' | \
grep 'Stack offset' | awk '{print $NF}' | sort | uniq -c | wc -l)
bits=$(echo "obase=2; $seen" | bc | wc -L)
echo "Bits of stack entropy: $bits"
rm -f "$log"
# We would expect any functional stack randomization to be at least 5 bits.
if [ "$bits" -lt 5 ]; then
echo "Stack entropy is low! Booted without 'randomize_kstack_offset=y'?"
exit 1
else
exit 0
fi
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