Re: [PATCH bpf-next v5 8/8] resolve_btfids: Change in-place update with raw binary output

Ihor Solodrai
2025-12-19T01:37:41Z

On 12/18/25 4:53 PM, bot+bpf-ci@kernel.org wrote:
>> diff --git a/tools/bpf/resolve_btfids/main.c b/tools/bpf/resolve_btfids/main.c
>> index e721e20a2bbd..7127396c5f50 100644
>> --- a/tools/bpf/resolve_btfids/main.c
>> +++ b/tools/bpf/resolve_btfids/main.c
>
> [ ... ]
>
>> @@ -587,6 +563,19 @@ static int load_btf(struct object *obj)
>> obj->base_btf = base_btf;
>> obj->btf = btf;
>>
>> + if (obj->base_btf && obj->distill_base) {
>> + err = btf__distill_base(obj->btf, &base_btf, &btf);
>> + if (err) {
>> + pr_err("FAILED to distill base BTF: %s\n", strerror(errno));
>> + goto out_err;
>> + }
>> +
>> + btf__free(obj->btf);
>> + btf__free(obj->base_btf);
>> + obj->btf = btf;
>> + obj->base_btf = base_btf;
>> + }
>> +
>> return 0;
>>
>> out_err:
>> + btf__free(base_btf);
>> + btf__free(btf);
>> return err;
>> }
>
> Can this lead to a double free if btf__distill_base() fails?
>
> At the point of calling btf__distill_base(), the local variables base_btf
> and btf have already been assigned to obj->base_btf and obj->btf. If
> btf__distill_base() fails, it does not modify its output parameters, so
> base_btf and btf still point to the same memory as obj->base_btf and
> obj->btf.
>
> The out_err path then frees base_btf and btf (the original BTF objects).
> But obj->base_btf and obj->btf still point to this freed memory. When
> load_btf() returns an error to main(), the cleanup at the out: label
> calls btf__free(obj.base_btf) and btf__free(obj.btf), which would free
> the same memory a second time.
>
> The execution path would be:
> main() -> load_btf() -> btf__distill_base() fails
> -> out_err frees base_btf and btf
> -> return err to main()
> -> main() goto out
> -> btf__free(obj.base_btf) and btf__free(obj.btf) -> double free
>
> Perhaps the out_err path should set obj->base_btf = NULL and obj->btf =
> NULL after freeing, or load_btf() should clear obj->base_btf/obj->btf
> before going to out_err?

I confirmed this is indeed a double free, although in a corner case.

I agree, we should set obj->btf and obj->btf_base to NULL at out_err.

Sending a v6 in a bit.

>
>
> ---
> AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
> See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md
>
> CI run summary: https://github.com/kernel-patches/bpf/actions/runs/20355860150