Re: [PATCH bpf-next v2 2/8] libbpf: Add typeless ksym support to gen_loader

From: Song Liu <songliubraving@fb.com>
To: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Cc: "bpf@vger.kernel.org" <bpf@vger.kernel.org>,
	"Alexei Starovoitov" <ast@kernel.org>,
	"Daniel Borkmann" <daniel@iogearbox.net>,
	"Andrii Nakryiko" <andrii@kernel.org>,
	"Martin Lau" <kafai@fb.com>, "Yonghong Song" <yhs@fb.com>,
	"Jesper Dangaard Brouer" <brouer@redhat.com>,
	"Toke Høiland-Jørgensen" <toke@redhat.com>,
	"netdev@vger.kernel.org" <netdev@vger.kernel.org>
Subject: Re: [PATCH bpf-next v2 2/8] libbpf: Add typeless ksym support to gen_loader
Date: Thu, 14 Oct 2021 16:39:43 +0000	[thread overview]
Message-ID: <F745CD84-E520-4DCB-B9F1-0C4F0014CBFF@fb.com> (raw)
In-Reply-To: <20211013073348.1611155-3-memxor@gmail.com>

> On Oct 13, 2021, at 12:33 AM, Kumar Kartikeya Dwivedi <memxor@gmail.com> wrote:
> 
> This uses the bpf_kallsyms_lookup_name helper added in previous patches
> to relocate typeless ksyms. The return value ENOENT can be ignored, and
> the value written to 'res' can be directly stored to the insn, as it is
> overwritten to 0 on lookup failure. For repeating symbols, we can simply
> copy the previously populated bpf_insn.
> 
> Also, we need to take care to not close fds for typeless ksym_desc, so
> reuse the 'off' member's space to add a marker for typeless ksym and use
> that to skip them in cleanup_relos.
> 
> Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
[...]
> }
> 
> +/* Expects:
> + * BPF_REG_8 - pointer to instruction
> + */
> +static void emit_relo_ksym_typeless(struct bpf_gen *gen,
> +				    struct ksym_relo_desc *relo, int insn)

This function has quite some duplicated logic as emit_relo_ksym_btf().
I guess we can somehow reuse the code here. Say, we pull changes from
3/8 first to handle weak type. Then we extend the function to handle 
typeless. Would this work? 

> +{
> +	struct ksym_desc *kdesc;
> +
> +	kdesc = get_ksym_desc(gen, relo);
> +	if (!kdesc)
> +		return;
> +	/* try to copy from existing ldimm64 insn */
> +	if (kdesc->ref > 1) {
> +		move_blob2blob(gen, insn + offsetof(struct bpf_insn, imm), 4,
> +			       kdesc->insn + offsetof(struct bpf_insn, imm));
> +		move_blob2blob(gen, insn + sizeof(struct bpf_insn) + offsetof(struct bpf_insn, imm), 4,
> +			       kdesc->insn + sizeof(struct bpf_insn) + offsetof(struct bpf_insn, imm));
> +		goto log;
> +	}
> +	/* remember insn offset, so we can copy ksym addr later */
> +	kdesc->insn = insn;
> +	/* skip typeless ksym_desc in fd closing loop in cleanup_relos */
> +	kdesc->typeless = true;
> +	emit_bpf_kallsyms_lookup_name(gen, relo);
> +	emit(gen, BPF_JMP_IMM(BPF_JEQ, BPF_REG_7, -ENOENT, 1));
> +	emit_check_err(gen);
> +	/* store lower half of addr into insn[insn_idx].imm */
> +	emit(gen, BPF_STX_MEM(BPF_W, BPF_REG_8, BPF_REG_9, offsetof(struct bpf_insn, imm)));
> +	/* store upper half of addr into insn[insn_idx + 1].imm */
> +	emit(gen, BPF_ALU64_IMM(BPF_RSH, BPF_REG_9, 32));
> +	emit(gen, BPF_STX_MEM(BPF_W, BPF_REG_8, BPF_REG_9,
> +		      sizeof(struct bpf_insn) + offsetof(struct bpf_insn, imm)));
> +log:
> +	if (!gen->log_level)
> +		return;
> +	emit(gen, BPF_LDX_MEM(BPF_W, BPF_REG_7, BPF_REG_8,
> +			      offsetof(struct bpf_insn, imm)));
> +	emit(gen, BPF_LDX_MEM(BPF_H, BPF_REG_9, BPF_REG_8, sizeof(struct bpf_insn) +
> +			      offsetof(struct bpf_insn, imm)));
> +	debug_regs(gen, BPF_REG_7, BPF_REG_9, " var t=0 w=%d (%s:count=%d): imm[0]: %%d, imm[1]: %%d",
> +		   relo->is_weak, relo->name, kdesc->ref);
> +	emit(gen, BPF_LDX_MEM(BPF_B, BPF_REG_9, BPF_REG_8, offsetofend(struct bpf_insn, code)));
> +	debug_regs(gen, BPF_REG_9, -1, " var t=0 w=%d (%s:count=%d): insn.reg",
> +		   relo->is_weak, relo->name, kdesc->ref);
> 
[...]

> +++ b/tools/lib/bpf/libbpf.c
> @@ -6355,17 +6355,14 @@ static int bpf_program__record_externs(struct bpf_program *prog)
> 		case RELO_EXTERN_VAR:
> 			if (ext->type != EXT_KSYM)
> 				continue;
> -			if (!ext->ksym.type_id) {
> -				pr_warn("typeless ksym %s is not supported yet\n",
> -					ext->name);
> -				return -ENOTSUP;
> -			}
> -			bpf_gen__record_extern(obj->gen_loader, ext->name, ext->is_weak,
> +			bpf_gen__record_extern(obj->gen_loader, ext->name,
> +					       ext->is_weak, !ext->ksym.type_id,
> 					       BTF_KIND_VAR, relo->insn_idx);
> 			break;
> 		case RELO_EXTERN_FUNC:
> -			bpf_gen__record_extern(obj->gen_loader, ext->name, ext->is_weak,
> -					       BTF_KIND_FUNC, relo->insn_idx);
> +			bpf_gen__record_extern(obj->gen_loader, ext->name,
> +					       ext->is_weak, 0, BTF_KIND_FUNC,

nit: Prefer use "false" for bool arguments. 

> +					       relo->insn_idx);
> 			break;
> 		default:
> 			continue;
> -- 
> 2.33.0
> 