On Tue, Oct 5, 2021 at 5:29 PM Kumar Kartikeya Dwivedi [off-list ref] 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?

Thanks,
Song

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