riscv, bpf: Emit fixed-length instructions for BPF_PSEUDO_FUNC

For BPF_PSEUDO_FUNC instruction, verifier will refill imm with
correct addresses of bpf_calls and then run last pass of JIT.
Since the emit_imm of RV64 is variable-length, which will emit
appropriate length instructions accorroding to the imm, it may
broke ctx->offset, and lead to unpredictable problem, such as
inaccurate jump. So let's fix it with fixed-length instructions.

Fixes: 69c087ba6225 ("bpf: Add bpf_for_each_map_elem() helper")
Suggested-by: Björn Töpel <[e-mail]>
Signed-off-by: Pu Lehui <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Reviewed-by: Björn Töpel <[e-mail]>
Acked-by: Björn Töpel <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

arch/riscv/net/bpf_jit_comp64.c

···
 		val < ((1L << 31) - (1L << 11));
 }
 
+/* Emit fixed-length instructions for address */
+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;
+
+	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 */
 static void emit_imm(u8 rd, s64 val, struct rv_jit_context *ctx)
 {
 	/* Note that the immediate from the add is sign-extended,
···
 		u64 imm64;
 
 		imm64 = (u64)insn1.imm << 32 | (u32)imm;
-		emit_imm(rd, imm64, ctx);
+		if (bpf_pseudo_func(insn)) {
+			/* fixed-length insns for extra jit pass */
+			ret = emit_addr(rd, imm64, extra_pass, ctx);
+			if (ret)
+				return ret;
+		} else {
+			emit_imm(rd, imm64, ctx);
+		}
+
 		return 1;
 	}