Commit 66e5eb84 authored by Johan Almbladh's avatar Johan Almbladh Committed by Daniel Borkmann
Browse files

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: default avatarJohan Almbladh <johan.almbladh@anyfinetworks.com>
Signed-off-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210809091829.810076-11-johan.almbladh@anyfinetworks.com
parent e5009b46

lib/test_bpf.c

+43 −0
Original line number Diff line number Diff line
@@ -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, ...",

		{ },