Re: [RFC PATCH 2/4] trace: Allow kprobes to override livepatched functions

Yafang Shao
2026-04-03T13:27:30Z

On Fri, Apr 3, 2026 at 6:26 PM Menglong Dong <menglong.dong@linux.dev> wrote:
>
> On 2026/4/2 21:20 Yafang Shao <laoar.shao@gmail.com> write:
> > On Thu, Apr 2, 2026 at 8:48 PM Menglong Dong <menglong.dong@linux.dev> wrote:
> > >
> > > On 2026/4/2 17:26, Yafang Shao wrote:
> > > > Introduce the ability for kprobes to override the return values of
> > > > functions that have been livepatched. This functionality is guarded by the
> > > > CONFIG_KPROBE_OVERRIDE_KLP_FUNC configuration option.
> > >
> > > Hi, Yafang. This is a interesting idea.
> > >
> [...]
> >
> > +/* noclone to avoid bond_get_slave_hook.constprop.0 */
> > +__attribute__((__noclone__, __noinline__))
> > +int bond_get_slave_hook(struct sk_buff *skb, u32 hash, unsigned int count)
> > +{
> > + return -1;
> > +}
>
> Hi, yafang.
>
> I see what you mean now. So you want to allow BPF program override
> the return of all the kernel functions in a KLP module.
>
> I think the security problem is a big issue. Image that we have a KLP
> in our environment. Any users can crash the kernel by hook a BPF
> program on it with the calling of bpf_override_write().

This feature is guarded by the CONFIG_KPROBE_OVERRIDE_KLP_FUNC
configuration option, which is disabled by default. Consequently, the
user must explicitly enable this option to utilize the feature.

>
> What's more, this is a little weird for me. If we allow to use bpf_override_return()
> for the kernel functions in a KLP, why not we allow it in a common kernel
> module, as KLP is a kind of kernel module. Then, why not we allow to
> use it for all the kernel functions?

By leveraging KLP, we can rapidly deploy new features without
interrupting production workloads. Accordingly, this feature is
specifically targeted at KLP-patched functions to maintain that
seamless delivery model.

>
> Can we mark the "bond_get_slave_hook" with ALLOW_ERROR_INJECTION() in
> your example? Then we can override its return directly. This is a more
> reasonable for me. With ALLOW_ERROR_INJECTION(), we are telling people that
> anyone can modify the return of this function safely.

It is unfortunate that ALLOW_ERROR_INJECTION() is incompatible with
KLP-patched functions, as this limits our ability to perform fault
injection on livepatched code

>
> WDYT?
>
> BTW, this is a BPF modification, so maybe we can use "bpf: xxx" for the title
> of this patch. Then, the BPF maintainers can notice this patch ;)

I agree with the suggestion. I will update the subject prefix to
"trace, bpf:" in the next version.

--
Regards
Yafang