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

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

On Thu, Oct 14, 2021 at 10:09:43PM IST, Song Liu wrote:
>
>
> > 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?
>

Ok, will put both into the same function in the next version. Though the part
between:

> > +{
> > +	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));

this and ...

> > +		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)));

... this won't overlap (so it will have to jump into the else branch to clear_src_reg).

e.g. it looks something like this:

if (kdesc->ref > 1) {
	move...
	if (!relo->is_typeless)
		...
		goto clear_src_reg;
}
kdesc->insn = insn;
...
if (relo->is_typeless) {
	...
} else {
	...
clear_src_reg:
	...
}

so it looked better to split into separate functions (maybe we can just move the
logging part to common helper? the rest is just duplicating the inital get and
move_blob2blob).

> > +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
> >
>

--
Kartikeya