Re: [PATCH bpf v2] riscv, bpf: Emit fixed-length instructions for BPF_PSEUDO_FUNC

[Björn Töpel]

Pu Lehui <pulehui@xxxxxxxxxxxxxxx> writes:

> Sorry for replying so late. For BPF_PSEUDO_FUNC instruction, verifier 
> will set insn[0].imm and insn[1].imm to 1 that make addr to 0x100000001 
> before extra pass, and also ctx->insns is NULL in iteration stage, all 
> of these make off out of range of AUIPC-ADDI range, and return failed. 
> We could add some special handling at different stages, but that seems a 
> little weird. By the way, I do not really like emit_addr function with 
> return value.

My rational is that *if* for some reason the jit is passed an address
that auipc/addi can't represent, we'd like to catch that and not emit
broken code.

> While a proper address is at least 2B alignment, and the valid address 
> is from 0xffffffff00000000 to 0xffffffffffffffff, we can make address 
> shifed 1 place to right, and addr >> 1 will always in the range of 
> AUIPC-ADDI range. We can get rid of the range detection. The 
> implementation is as follows:
>
> static void emit_addr(u8 rd, u64 addr, struct rv_jit_context *ctx)
> {
>           s64 imm = addr >> 1;
>           s64 upper = (imm + (1 << 11)) >> 12;
>           s64 lower = imm & 0xfff;
>
>           emit(rv_lui(rd, upper), ctx);
>           emit(rv_addi(rd, rd, lower), ctx);
>           emit(rv_slli(rd, rd, 1), ctx);
> }
>
> What do you think?

That's a code generation penalty, instead of catching it at code
gen. Don't like! :-) I much prefer the auipc/addi version.

What do you think about the diff (on-top of your work) below?

--8<--
diff --git a/arch/riscv/net/bpf_jit_comp64.c b/arch/riscv/net/bpf_jit_comp64.c
index aa9410eef77c..7acaf28cb3be 100644
--- a/arch/riscv/net/bpf_jit_comp64.c
+++ b/arch/riscv/net/bpf_jit_comp64.c
@@ -137,15 +137,21 @@ static bool in_auipc_jalr_range(s64 val)
 }
 
 /* Emit fixed-length instructions for address */
-static void emit_addr(u8 rd, u64 addr, struct rv_jit_context *ctx)
+static int emit_addr(u8 rd, u64 addr, bool extra_pass, struct rv_jit_context *ctx)
 {
 	u64 ip = (u64)(ctx->insns + ctx->ninsns);
 	s64 off = addr - ip;
 	s64 upper = (off + (1 << 11)) >> 12;
 	s64 lower = ((off & 0xfff) << 52) >> 52;
 
+	if (extra_pass && !in_auipc_jalr_range(off)) {
+		pr_err("bpf-jit: target offset 0x%llx is out of range\n", off);
+		return -ERANGE;
+	}
+
 	emit(rv_auipc(rd, upper), ctx);
 	emit(rv_addi(rd, rd, lower), ctx);
+	return 0;
 }
 
 /* Emit variable-length instructions for 32-bit and 64-bit imm */
@@ -1061,13 +1067,17 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
 	{
 		struct bpf_insn insn1 = insn[1];
 		u64 imm64;
+		int ret;
 
 		imm64 = (u64)insn1.imm << 32 | (u32)imm;
-		if (bpf_pseudo_func(insn))
+		if (bpf_pseudo_func(insn)) {
 			/* fixed-length insns for extra jit pass */
-			emit_addr(rd, imm64, ctx);
-		else
+			ret = emit_addr(rd, imm64, extra_pass, ctx);
+			if (ret)
+				return ret;
+		} else {
 			emit_imm(rd, imm64, ctx);
+		}
 
 		return 1;
 	}

--8<--

Wouldn't that work?


Björn