On Fri, Oct 24, 2025 at 12:12:56AM -0700, Song Liu wrote:
> ftrace_hash_ipmodify_enable() checks IPMODIFY and DIRECT ftrace_ops on
> the same kernel function. When needed, ftrace_hash_ipmodify_enable()
> calls ops->ops_func() to prepare the direct ftrace (BPF trampoline) to
> share the same function as the IPMODIFY ftrace (livepatch).
>
> ftrace_hash_ipmodify_enable() is called in register_ftrace_direct() path,
> but not called in modify_ftrace_direct() path. As a result, the following
> operations will break livepatch:
>
> 1. Load livepatch to a kernel function;
> 2. Attach fentry program to the kernel function;
> 3. Attach fexit program to the kernel function.
>
> After 3, the kernel function being used will not be the livepatched
> version, but the original version.
>
> Fix this by adding ftrace_hash_ipmodify_enable() to modify_ftrace_direct()
> and adjust some logic around the call.
>
> Signed-off-by: Song Liu <[email protected]>
> ---
> kernel/bpf/trampoline.c | 12 +++++++-----
> kernel/trace/ftrace.c | 12 ++++++++++--
> 2 files changed, 17 insertions(+), 7 deletions(-)
>
> diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
> index 5949095e51c3..8015f5dc3169 100644
> --- a/kernel/bpf/trampoline.c
> +++ b/kernel/bpf/trampoline.c
> @@ -221,6 +221,13 @@ static int register_fentry(struct bpf_trampoline *tr,
> void *new_addr)
>
> if (tr->func.ftrace_managed) {
> ftrace_set_filter_ip(tr->fops, (unsigned long)ip, 0, 1);
> + /*
> + * Clearing fops->trampoline_mutex and fops->NULL is
s/trampoline_mutex/trampoline/
> + * needed by the "goto again" case in
> + * bpf_trampoline_update().
> + */
> + tr->fops->trampoline = 0;
> + tr->fops->func = NULL;
IIUC you move this because if modify_fentry returns -EAGAIN
we don't want to reset the trampoline, right?
> ret = register_ftrace_direct(tr->fops, (long)new_addr);
> } else {
> ret = bpf_arch_text_poke(ip, BPF_MOD_CALL, NULL, new_addr);
> @@ -479,11 +486,6 @@ static int bpf_trampoline_update(struct bpf_trampoline
> *tr, bool lock_direct_mut
> * BPF_TRAMP_F_SHARE_IPMODIFY is set, we can generate the
> * trampoline again, and retry register.
> */
> - /* reset fops->func and fops->trampoline for re-register */
> - tr->fops->func = NULL;
> - tr->fops->trampoline = 0;
> -
> - /* free im memory and reallocate later */
> bpf_tramp_image_free(im);
> goto again;
> }
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index 7f432775a6b5..370f620734cf 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -2020,8 +2020,6 @@ static int __ftrace_hash_update_ipmodify(struct
> ftrace_ops *ops,
> if (is_ipmodify)
> goto rollback;
>
> - FTRACE_WARN_ON(rec->flags & FTRACE_FL_DIRECT);
why is this needed?
thanks,
jirka
> -
> /*
> * Another ops with IPMODIFY is already
> * attached. We are now attaching a direct
> @@ -6128,6 +6126,15 @@ __modify_ftrace_direct(struct ftrace_ops *ops,
> unsigned long addr)
> if (err)
> return err;
>
> + /*
> + * Call ftrace_hash_ipmodify_enable() here, so that we can call
> + * ops->ops_func for the ops. This is needed because the above
> + * register_ftrace_function_nolock() worked on tmp_ops.
> + */
> + err = ftrace_hash_ipmodify_enable(ops);
> + if (err)
> + goto out;
> +
> /*
> * Now the ftrace_ops_list_func() is called to do the direct callers.
> * We can safely change the direct functions attached to each entry.
> @@ -6149,6 +6156,7 @@ __modify_ftrace_direct(struct ftrace_ops *ops, unsigned
> long addr)
>
> mutex_unlock(&ftrace_lock);
>
> +out:
> /* Removing the tmp_ops will add the updated direct callers to the
> functions */
> unregister_ftrace_function(&tmp_ops);
>
> --
> 2.47.3
>
>
