Re: [PATCH bpf-next v1 2/6] libbpf: Add typeless and weak ksym support to gen_loader

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

On Fri, Oct 08, 2021 at 03:15:10AM IST, Song Liu wrote:
> On Tue, Oct 5, 2021 at 5:29 PM Kumar Kartikeya Dwivedi <memxor@gmail.com> wrote:
> >
> > This patch adds typeless and weak ksym support to BTF_KIND_VAR
> > relocation code in gen_loader. For typeless ksym, we use the newly added
> > bpf_kallsyms_lookup_name helper.
> >
> > For weak ksym, we simply skip error check, and fix up the srg_reg for
> > the insn, as keeping it as BPF_PSEUDO_BTF_ID for weak ksym with its
> > insn[0].imm and insn[1].imm set as 0 will cause a failure.  This is
> > consistent with how libbpf relocates these two cases of BTF_KIND_VAR.
> >
> > We also modify cleanup_relos to check for typeless ksyms in fd closing
> > loop, since those have no fd associated with the ksym. For this we can
> > reuse the unused 'off' member of ksym_desc.
> >
> > Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
> [...]
>
> Everything above (trimmed) makes sense to me.
>
> > +/* Expects:
> > + * BPF_REG_8 - pointer to instruction
> > + */
> > +static void emit_relo_ksym_btf(struct bpf_gen *gen, struct ksym_relo_desc *relo, int insn)
> > +{
>
> But I don't quite follow why we need these changes to emit_relo_ksym_btf.
> Maybe we should have these changes in a separate patch and add some
> more explanations?
>

Before, if the bpf_btf_find_by_name_kind call failed, we just bailed out due to
the emit_check_err. Now, if it is weak, the error check is conditional, so
we set 0 as the default values and skip the store for btf_id and btf_fd if the
btf lookup failed. Till here, it is similar to the case for emit_relo_kfunc_btf.

Note that we only reach this path once for each unique symbol: the next time, we
enter the kdesc->ref > 1 branch, which copies from the existing insn.

Regarding src_reg stuff: in bpf_object__relocate_data, for obj->gen_loader,
ext->is_set is always true. For the normal libbpf case, it is only true if the
lookup succeeded for BTF (in bpf_object__resolve_ksym_var_btf_id). So depending
on if ext->is_set, it skips assigning BPF_PSEUDO_BTF_ID to src_reg and zeroes
out insn[0].imm and insn[1].imm. Also, the case for ext->is_set = false for
libbpf is only reached if we don't fail on lookup error, and that depends on
ext->is_weak. TLDR; ext->is_weak and lookup failure means src_reg is not
assigned.

For gen_loader, since this src_reg assignment is always there, we need to clear
it for the case where lookup failed, hence the:
-log:
+	emit(gen, BPF_JMP_IMM(BPF_JSGE, BPF_REG_7, 0, 3));

otherwise we end up with src_reg = BPF_PSEUDO_BTF_ID, imm[0] = 0, imm[1] = 0,
which ends up failing the load.

Similarly, we jump over the src_reg adjustment from the kdesc->ref > 1 case if
imm is not equal to 0 (if it were 0, then this is weak ksym). Error check
ensures this instruction is only reached if relo->is_weak (for the same symbol),
so we don't need to check it again there.

Doing it the other way around (not assigning BPF_PSEUDO_BTF_ID by default for
gen_loader) would still involve writing to it in the success case, so IMO
touching it seems unavoidable. If there are better ideas, please lmk.

I added the debug statements so that the selftest reloc result can be inspected
easily, but not sure I can/should verify it from the selftest itself.  I'll
split typeless and weak ksym support into separate patches next time, and
explain this in the commit message.

> Thanks,
> Song
>
> [...]

--
Kartikeya