On Fri, Oct 8, 2021 at 10:07 AM Kumar Kartikeya Dwivedi
<memxor@xxxxxxxxx> wrote:
>
> The helper function returns a pointer that in the failure case encodes
> an error in the struct btf pointer. The current code lead to Coverity
> warning about the use of the invalid pointer:
>
> *** CID 1507963: Memory - illegal accesses (USE_AFTER_FREE)
> /kernel/bpf/verifier.c: 1788 in find_kfunc_desc_btf()
> 1782 return ERR_PTR(-EINVAL);
> 1783 }
> 1784
> 1785 kfunc_btf = __find_kfunc_desc_btf(env, offset, btf_modp);
> 1786 if (IS_ERR_OR_NULL(kfunc_btf)) {
> 1787 verbose(env, "cannot find module BTF for func_id %u\n", func_id);
> >>> CID 1507963: Memory - illegal accesses (USE_AFTER_FREE)
> >>> Using freed pointer "kfunc_btf".
> 1788 return kfunc_btf ?: ERR_PTR(-ENOENT);
> 1789 }
> 1790 return kfunc_btf;
> 1791 }
> 1792 return btf_vmlinux ?: ERR_PTR(-ENOENT);
> 1793 }
>
> Daniel suggested the use of ERR_CAST so that the intended use is clear
> to Coverity, but on closer look it seems that we never return NULL from
> the helper, hence it can just be switched to checking for IS_ERR and
> returning the pointer, similar to the cases elsewhere in the kernel.
>
> Signed-off-by: Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx>
> ---
> kernel/bpf/verifier.c | 4 +---
> 1 file changed, 1 insertion(+), 3 deletions(-)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 20900a1bac12..2551b6be8d42 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -1783,10 +1783,8 @@ static struct btf *find_kfunc_desc_btf(struct bpf_verifier_env *env,
> }
>
> kfunc_btf = __find_kfunc_desc_btf(env, offset, btf_modp);
Seems like __find_kfunc_desc_btf() already logs most of possible
reasons for the failure, with possibly btf_get_by_fd failure being the
biggest user-visible omission. If you complete __find_kfunc_desc_btf
logging and just pass through its result here without extra check,
wouldn't it work?
> - if (IS_ERR_OR_NULL(kfunc_btf)) {
> + if (IS_ERR(kfunc_btf))
> verbose(env, "cannot find module BTF for func_id %u\n", func_id);
> - return kfunc_btf ?: ERR_PTR(-ENOENT);
> - }
> return kfunc_btf;
> }
> return btf_vmlinux ?: ERR_PTR(-ENOENT);
> --
> 2.33.0
>
