On Oct 24, 2025, at 4:43 AM, Jiri Olsa [off-list ref] wrote:

On Fri, Oct 24, 2025 at 12:12:56AM -0700, Song Liu wrote:

quoted

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/

Good catch!

quoted

+ * 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?

Right, we don’t want to reset this in the modify_fentry path. 
We can add a check before “goto again” so that we only do the
reset for register_fentry, but I think it is cleaner this way. 
I can be convinced to change it. 

quoted

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?

This is needed for the modify_ftrace_direct case, because 
the record already have a direct function (BPF trampoline)
attached. 

Thanks,
Song