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@anyfinetworks.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210809091829.810076-11-johan.almbladh@anyfinetworks.com

diff --git a/lib/test_bpf.c b/lib/test_bpf.c
index 402c199cc119d0882bb5988fe4973a981ea50164..896d37f4f4b3b388c33f62112bc3e1eed9d74f7f 100644 (file)
--- 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, ...",
                { },