Re: [PATCH bpf-next 2/3] ftrace: bpf: Fix IPMODIFY + DIRECT in modify_ftrace_direct()

[Jiri Olsa <olsajiri@gmail.com>]

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 <song@kernel.org>
> ---
>  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
> 
>