[PATCH bpf-next 1/3] riscv, bpf: Fix support for BPF_SDIV and BPF_SMOD in RV32 JIT

[Kuan-Wei Chiu]

The current rv32 bpf jit compiler incorrectly treats BPF_SDIV and
BPF_SMOD as unsigned operations. The BPF instruction set allows
signed division and modulo by reusing the BPF_DIV and BPF_MOD opcodes
with the instruction offset set to 1.

Update the emit_alu_r32() function to accept an 'is_sdiv' variable and
emit the correct div and rem instructions when the offset is 1.

Before this patch:
[   44.161771] test_bpf: #165 ALU_SDIV_X: -6 / 2 = -3 jited:1 ret 2147483645 != -3 (0x7ffffffd != 0xfffffffd)FAIL (1 times)
[   44.167385] test_bpf: #166 ALU_SDIV_K: -6 / 2 = -3 jited:1 ret 2147483645 != -3 (0x7ffffffd != 0xfffffffd)FAIL (1 times)
[   44.171053] test_bpf: #169 ALU_SMOD_X: -7 % 2 = -1 jited:1 ret 1 != -1 (0x1 != 0xffffffff)FAIL (1 times)
[   44.172081] test_bpf: #170 ALU_SMOD_K: -7 % 2 = -1 jited:1 ret 1 != -1 (0x1 != 0xffffffff)FAIL (1 times)

After this patch:
[   16.002192] test_bpf: #165 ALU_SDIV_X: -6 / 2 = -3 jited:1 95 PASS
[   16.002983] test_bpf: #166 ALU_SDIV_K: -6 / 2 = -3 jited:1 1059 PASS
[   16.017167] test_bpf: #169 ALU_SMOD_X: -7 % 2 = -1 jited:1 136 PASS
[   16.023002] test_bpf: #170 ALU_SMOD_K: -7 % 2 = -1 jited:1 109 PASS

Signed-off-by: Kuan-Wei Chiu <visitorckw at gmail.com>
---
 arch/riscv/net/bpf_jit_comp32.c | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/arch/riscv/net/bpf_jit_comp32.c b/arch/riscv/net/bpf_jit_comp32.c
index 592dd86fbf81..7396899ea276 100644
--- a/arch/riscv/net/bpf_jit_comp32.c
+++ b/arch/riscv/net/bpf_jit_comp32.c
@@ -509,7 +509,7 @@ static void emit_alu_r64(const s8 *dst, const s8 *src,
 }
 
 static void emit_alu_r32(const s8 *dst, const s8 *src,
-			 struct rv_jit_context *ctx, const u8 op)
+			 struct rv_jit_context *ctx, const u8 op, bool is_sdiv)
 {
 	const s8 *tmp1 = bpf2rv32[TMP_REG_1];
 	const s8 *tmp2 = bpf2rv32[TMP_REG_2];
@@ -539,10 +539,16 @@ static void emit_alu_r32(const s8 *dst, const s8 *src,
 		emit(rv_mul(lo(rd), lo(rd), lo(rs)), ctx);
 		break;
 	case BPF_DIV:
-		emit(rv_divu(lo(rd), lo(rd), lo(rs)), ctx);
+		if (is_sdiv)
+			emit(rv_div(lo(rd), lo(rd), lo(rs)), ctx);
+		else
+			emit(rv_divu(lo(rd), lo(rd), lo(rs)), ctx);
 		break;
 	case BPF_MOD:
-		emit(rv_remu(lo(rd), lo(rd), lo(rs)), ctx);
+		if (is_sdiv)
+			emit(rv_rem(lo(rd), lo(rd), lo(rs)), ctx);
+		else
+			emit(rv_remu(lo(rd), lo(rd), lo(rs)), ctx);
 		break;
 	case BPF_LSH:
 		emit(rv_sll(lo(rd), lo(rd), lo(rs)), ctx);
@@ -959,6 +965,7 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
 	u8 code = insn->code;
 	s16 off = insn->off;
 	s32 imm = insn->imm;
+	bool is_sdiv = false;
 
 	const s8 *dst = bpf2rv32[insn->dst_reg];
 	const s8 *src = bpf2rv32[insn->src_reg];
@@ -1041,7 +1048,9 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
 			emit_imm32(tmp2, imm, ctx);
 			src = tmp2;
 		}
-		emit_alu_r32(dst, src, ctx, BPF_OP(code));
+		if ((BPF_OP(code) == BPF_DIV || BPF_OP(code) == BPF_MOD) && insn->off == 1)
+			is_sdiv = true;
+		emit_alu_r32(dst, src, ctx, BPF_OP(code), is_sdiv);
 		break;
 
 	case BPF_ALU | BPF_MOV | BPF_K:
@@ -1065,7 +1074,7 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
 		 * src is ignored---choose tmp2 as a dummy register since it
 		 * is not on the stack.
 		 */
-		emit_alu_r32(dst, tmp2, ctx, BPF_OP(code));
+		emit_alu_r32(dst, tmp2, ctx, BPF_OP(code), false);
 		break;
 
 	case BPF_ALU | BPF_END | BPF_FROM_LE:
-- 
2.54.0.545.g6539524ca2-goog