[PATCH v2 3/6] bpf/arm64: fix offset type to allow a negative jump

[Stephen Hemminger <stephen@networkplumber.org>]

From: Christophe Fontaine <cfontain@redhat.com>

The DPDK BPF JIT standalone test test_ld_mbuf1 fails on arm64.
It does:
	r6 = r1                    // mbuf
	r0 = *(u8 *)pkt[0]         // BPF_ABS
	if ((r0 & 0xf0) == 0x40)
		goto parse
	r0 = 0
	exit                       // epilogue E0
parse:
	r0 = *(u8 *)pkt[r0 + 3]    // BPF_IND
	...
	exit

emit_return_zero_if_src_zero() returns 0 by branching to a function
epilogue. The target maybe a previous epilogue so branch
might be backwards; therefore the offset needs to be negative.

The offset was stored in a uint16_t, so a negative value wrapped to a
large positive number; emit_b() then branched past the end of the
program and faulted at run time.

Fixes: 111e2a747a4f ("bpf/arm: add basic arithmetic operations")
Cc: stable@dpdk.org

Signed-off-by: Christophe Fontaine <cfontain@redhat.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
---
 lib/bpf/bpf_jit_arm64.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/lib/bpf/bpf_jit_arm64.c b/lib/bpf/bpf_jit_arm64.c
index a04ef33a9c..67e42015de 100644
--- a/lib/bpf/bpf_jit_arm64.c
+++ b/lib/bpf/bpf_jit_arm64.c
@@ -957,10 +957,12 @@ static void
 emit_return_zero_if_src_zero(struct a64_jit_ctx *ctx, bool is64, uint8_t src)
 {
 	uint8_t r0 = ebpf_to_a64_reg(ctx, EBPF_REG_0);
-	uint16_t jump_to_epilogue;
+	int32_t jump_to_epilogue;
 
 	emit_cbnz(ctx, is64, src, 3);
 	emit_mov_imm(ctx, is64, r0, 0);
+
+	/* maybe backwards branch to earlier epilogue */
 	jump_to_epilogue = (ctx->program_start + ctx->program_sz) - ctx->idx;
 	emit_b(ctx, jump_to_epilogue);
 }
-- 
2.53.0