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From:   =?utf-8?B?QmrDtnJuIFTDtnBlbA==?= <bjorn@kernel.org>
To:     Pu Lehui <pulehui@huaweicloud.com>, bpf@vger.kernel.org,
        linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org
Cc:     Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <martin.lau@linux.dev>,
        Song Liu <song@kernel.org>, Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>,
        Stanislav Fomichev <sdf@google.com>,
        Hao Luo <haoluo@google.com>, Jiri Olsa <jolsa@kernel.org>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Pu Lehui <pulehui@huawei.com>
Subject: Re: [PATCH bpf v2] riscv, bpf: Emit fixed-length instructions for
 BPF_PSEUDO_FUNC
In-Reply-To: <0ade59ea-6863-4d68-607c-22e4b9405a0d@huaweicloud.com>
References: <20221202094837.3872444-1-pulehui@huaweicloud.com>
 <87y1rq848x.fsf@all.your.base.are.belong.to.us>
 <0ade59ea-6863-4d68-607c-22e4b9405a0d@huaweicloud.com>
Date:   Tue, 06 Dec 2022 08:55:42 +0100
Message-ID: <87359t3r0h.fsf@all.your.base.are.belong.to.us>
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Pu Lehui <pulehui@huaweicloud.com> writes:

> Sorry for replying so late. For BPF_PSEUDO_FUNC instruction, verifier=20
> will set insn[0].imm and insn[1].imm to 1 that make addr to 0x100000001=20
> before extra pass, and also ctx->insns is NULL in iteration stage, all=20
> of these make off out of range of AUIPC-ADDI range, and return failed.=20
> We could add some special handling at different stages, but that seems a=
=20
> little weird. By the way, I do not really like emit_addr function with=20
> 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=20
> is from 0xffffffff00000000 to 0xffffffffffffffff, we can make address=20
> shifed 1 place to right, and addr >> 1 will always in the range of=20
> AUIPC-ADDI range. We can get rid of the range detection. The=20
> implementation is as follows:
>
> static void emit_addr(u8 rd, u64 addr, struct rv_jit_context *ctx)
> {
>           s64 imm =3D addr >> 1;
>           s64 upper =3D (imm + (1 << 11)) >> 12;
>           s64 lower =3D 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_comp6=
4.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)
 }
=20
 /* 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_conte=
xt *ctx)
 {
 	u64 ip =3D (u64)(ctx->insns + ctx->ninsns);
 	s64 off =3D addr - ip;
 	s64 upper =3D (off + (1 << 11)) >> 12;
 	s64 lower =3D ((off & 0xfff) << 52) >> 52;
=20
+	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;
 }
=20
 /* 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 =3D insn[1];
 		u64 imm64;
+		int ret;
=20
 		imm64 =3D (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 =3D emit_addr(rd, imm64, extra_pass, ctx);
+			if (ret)
+				return ret;
+		} else {
 			emit_imm(rd, imm64, ctx);
+		}
=20
 		return 1;
 	}

--8<--

Wouldn't that work?


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