selftests: mptcp: join: avoid using 'cmp --bytes'

BusyBox's 'cmp' command doesn't support the '--bytes' parameter.

Some CIs -- i.e. LKFT -- use BusyBox and have the mptcp_join.sh test
failing [1] because their 'cmp' command doesn't support this '--bytes'
option:

    cmp: unrecognized option '--bytes=1024'
    BusyBox v1.35.0 () multi-call binary.

    Usage: cmp [-ls] [-n NUM] FILE1 [FILE2]

Instead, 'head --bytes' can be used as this option is supported by
BusyBox. A temporary file is needed for this operation.

Because it is apparently quite common to use BusyBox, it is certainly
better to backport this fix to impacted kernels.

Fixes: 6bf41020b72b ("selftests: mptcp: update and extend fastclose test-cases")
Cc: stable@vger.kernel.org
Link: https://qa-reports.linaro.org/lkft/linux-mainline-master/build/v6.3-rc5-5-g148341f0a2f5/testrun/16088933/suite/kselftest-net-mptcp/test/net_mptcp_userspace_pm_sh/log
Suggested-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

diff --git a/tools/testing/selftests/net/mptcp/mptcp_join.sh b/tools/testing/selftests/net/mptcp/mptcp_join.sh
index 26310c1..4f3fe45 100755 (executable)
--- a/tools/testing/selftests/net/mptcp/mptcp_join.sh
+++ b/tools/testing/selftests/net/mptcp/mptcp_join.sh
@@ -17,6 +17,7 @@ sout=""
 cin=""
 cinfail=""
 cinsent=""
+tmpfile=""
 cout=""
 capout=""
 ns1=""
@@ -175,6 +176,7 @@ cleanup()
 {
        rm -f "$cin" "$cout" "$sinfail"
        rm -f "$sin" "$sout" "$cinsent" "$cinfail"
+       rm -f "$tmpfile"
        rm -rf $evts_ns1 $evts_ns2
        cleanup_partial
 }
@@ -383,9 +385,16 @@ check_transfer()
                        fail_test
                        return 1
                fi
-               bytes="--bytes=${bytes}"
+
+               # note: BusyBox's "cmp" command doesn't support --bytes
+               tmpfile=$(mktemp)
+               head --bytes="$bytes" "$in" > "$tmpfile"
+               mv "$tmpfile" "$in"
+               head --bytes="$bytes" "$out" > "$tmpfile"
+               mv "$tmpfile" "$out"
+               tmpfile=""
        fi
-       cmp -l "$in" "$out" ${bytes} | while read -r i a b; do
+       cmp -l "$in" "$out" | while read -r i a b; do
                local sum=$((0${a} + 0${b}))
                if [ $check_invert -eq 0 ] || [ $sum -ne $((0xff)) ]; then
                        echo "[ FAIL ] $what does not match (in, out):"