netdev - [PATCH bpf-next v2 10/14] bpf/tests: Add branch conversion JIT test

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Date:   Mon,  9 Aug 2021 11:18:25 +0200
From:   Johan Almbladh <johan.almbladh@...finetworks.com>
To:     ast@...nel.org, daniel@...earbox.net, andrii@...nel.org
Cc:     kafai@...com, songliubraving@...com, yhs@...com,
        john.fastabend@...il.com, kpsingh@...nel.org,
        Tony.Ambardar@...il.com, netdev@...r.kernel.org,
        bpf@...r.kernel.org,
        Johan Almbladh <johan.almbladh@...finetworks.com>
Subject: [PATCH bpf-next v2 10/14] bpf/tests: Add branch conversion JIT test

Some JITs may need to convert a conditional jump instruction to
to short PC-relative branch and a long unconditional jump, if the
PC-relative offset exceeds offset field width in the CPU instruction.
This test triggers such branch conversion on the 32-bit MIPS JIT.

Signed-off-by: Johan Almbladh <johan.almbladh@...finetworks.com>
---
 lib/test_bpf.c | 43 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 43 insertions(+)

diff --git a/lib/test_bpf.c b/lib/test_bpf.c
index 402c199cc119..896d37f4f4b3 100644
--- a/lib/test_bpf.c
+++ b/lib/test_bpf.c
@@ -461,6 +461,41 @@ static int bpf_fill_stxdw(struct bpf_test *self)
 	return __bpf_fill_stxdw(self, BPF_DW);
 }
 
+static int bpf_fill_long_jmp(struct bpf_test *self)
+{
+	unsigned int len = BPF_MAXINSNS;
+	struct bpf_insn *insn;
+	int i;
+
+	insn = kmalloc_array(len, sizeof(*insn), GFP_KERNEL);
+	if (!insn)
+		return -ENOMEM;
+
+	insn[0] = BPF_ALU64_IMM(BPF_MOV, R0, 1);
+	insn[1] = BPF_JMP_IMM(BPF_JEQ, R0, 1, len - 2 - 1);
+
+	/*
+	 * Fill with a complex 64-bit operation that expands to a lot of
+	 * instructions on 32-bit JITs. The large jump offset can then
+	 * overflow the conditional branch field size, triggering a branch
+	 * conversion mechanism in some JITs.
+	 *
+	 * Note: BPF_MAXINSNS of ALU64 MUL is enough to trigger such branch
+	 * conversion on the 32-bit MIPS JIT. For other JITs, the instruction
+	 * count and/or operation may need to be modified to trigger the
+	 * branch conversion.
+	 */
+	for (i = 2; i < len - 1; i++)
+		insn[i] = BPF_ALU64_IMM(BPF_MUL, R0, (i << 16) + i);
+
+	insn[len - 1] = BPF_EXIT_INSN();
+
+	self->u.ptr.insns = insn;
+	self->u.ptr.len = len;
+
+	return 0;
+}
+
 static struct bpf_test tests[] = {
 	{
 		"TAX",
@@ -6895,6 +6930,14 @@ static struct bpf_test tests[] = {
 		{ },
 		{ { 0, 1 } },
 	},
+	{	/* Mainly checking JIT here. */
+		"BPF_MAXINSNS: Very long conditional jump",
+		{ },
+		INTERNAL | FLAG_NO_DATA,
+		{ },
+		{ { 0, 1 } },
+		.fill_helper = bpf_fill_long_jmp,
+	},
 	{
 		"JMP_JA: Jump, gap, jump, ...",
 		{ },
-- 
2.25.1