1#!/bin/bash 2# SPDX-License-Identifier: GPL-2.0 3# 4# A function's checksum must not depend on where the function sits. 5# 6# A jump or call without a relocation encodes its target as an offset from the 7# instruction. Hashing those bytes makes the checksum change whenever anything 8# ahead of the function changes size -- so an unrelated edit elsewhere in the 9# file reports this function as changed too, and the patch grows to include it 10# and everything it references. Nothing fails; the livepatch is just larger and 11# riskier than the patch it came from. 12# 13# Here the "patch" moves target() away from callee() and changes nothing else: 14# both are aligned to 64 in the patched build, which shifts them apart without 15# touching a byte of either. See the fixture for why it is done that way. 16 17. "$(dirname "$0")/../lib.sh" 18 19setup 20 21# -fno-function-sections, or each function is at offset 0 of its own section 22# and target() never moves. 23build_pair checksum_position.c -fno-function-sections 24 25assert_input_symbol target 26 27# The fixture is only meaningful if the displacement target's calls encode 28# actually changed, and that is the distance to callee() -- not target's own 29# offset. A compiler which shifted the two by the same amount would move 30# target and leave the distance alone, and then the bytes are identical and 31# the checksum matches for the uninteresting reason. Ask about the distance. 32sym_off() # $1 object, $2 symbol 33{ 34 in_symbols "$1" | awk -v n="$2" '$NF == n { print $2; exit }' 35} 36 37orig_t="$(sym_off orig.o target)"; orig_c="$(sym_off orig.o callee)" 38new_t="$(sym_off patched.o target)"; new_c="$(sym_off patched.o callee)" 39 40[ -n "$orig_t" ] && [ -n "$orig_c" ] && [ -n "$new_t" ] && [ -n "$new_c" ] || 41 fail "target or callee missing from one of the objects" 42 43orig_gap=$(( 16#$orig_t - 16#$orig_c )) 44new_gap=$(( 16#$new_t - 16#$new_c )) 45[ "$orig_gap" != "$new_gap" ] || 46 probe_skip "this compiler kept target() and callee() the same distance" \ 47 "apart; the call displacement did not change" 48 49assert_checksum_matches target 50 51pass "checksum unchanged when the function only moves" 52