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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
This commit is contained in:
Johan Almbladh 2021-08-09 11:18:25 +02:00 committed by Daniel Borkmann
parent e5009b4636
commit 66e5eb8474
1 changed files with 43 additions and 0 deletions
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43
lib/test_bpf.c
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@ -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, ...",
{ },