On Sun, Oct 26, 2025 at 01:54:43PM -0700, Song Liu wrote:

When livepatch is attached to the same function as bpf trampoline with a fexit program, bpf trampoline code calls register_ftrace_direct() twice. The first time will fail with -EAGAIN, and the second time it will succeed. This requires register_ftrace_direct() to unregister the address on the first attempt. Otherwise, the bpf trampoline cannot attach. Here is an easy way to reproduce this issue:

insmod samples/livepatch/livepatch-sample.ko bpftrace -e 'fexit:cmdline_proc_show {}' ERROR: Unable to attach probe: fexit:vmlinux:cmdline_proc_show...

Fix this by cleaning up the hash when register_ftrace_function_nolock hits errors.

Also, move the code that resets ops->func and ops->trampoline to the error path of register_ftrace_direct().

Fixes: d05cb470663a ("ftrace: Fix modification of direct_function hash while in use") Cc: stable@vger.kernel.org # v6.6+ Reported-by: Andrey Grodzovsky andrey.grodzovsky@crowdstrike.com Closes: https://lore.kernel.org/live-patching/c5058315a39d4615b333e485893345be@crowd... Cc: Steven Rostedt (Google) rostedt@goodmis.org Cc: Masami Hiramatsu (Google) mhiramat@kernel.org Acked-and-tested-by: Andrey Grodzovsky andrey.grodzovsky@crowdstrike.com Signed-off-by: Song Liu song@kernel.org

---

kernel/bpf/trampoline.c | 5 ----- kernel/trace/ftrace.c | 6 ++++++ 2 files changed, 6 insertions(+), 5 deletions(-)

diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c index 5949095e51c3..f2cb0b097093 100644 --- a/kernel/bpf/trampoline.c +++ b/kernel/bpf/trampoline.c @@ -479,11 +479,6 @@ 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 42bd2ba68a82..725c224fb4e6 100644 --- a/kernel/trace/ftrace.c +++ b/kernel/trace/ftrace.c @@ -6048,6 +6048,12 @@ int register_ftrace_direct(struct ftrace_ops *ops, unsigned long addr) ops->direct_call = addr; err = register_ftrace_function_nolock(ops);

• if (err) {

• /* cleanup for possible another register call */
  

• ops->func = NULL;
  

• ops->trampoline = 0;
  

nit, we could cleanup also flags and direct_call just to be complete, but at the same time it does not seem to affect anything

jirka

• remove_direct_functions_hash(hash, addr);
  

• }

out_unlock: mutex_unlock(&direct_mutex); -- 2.47.3

On Sun, 26 Oct 2025 13:54:43 -0700 Song Liu song@kernel.org wrote:

--- a/kernel/trace/ftrace.c +++ b/kernel/trace/ftrace.c @@ -6048,6 +6048,12 @@ int register_ftrace_direct(struct ftrace_ops *ops, unsigned long addr) ops->direct_call = addr; err = register_ftrace_function_nolock(ops);

• if (err) {

• /* cleanup for possible another register call */
  

• ops->func = NULL;
  

• ops->trampoline = 0;
  

• remove_direct_functions_hash(hash, addr);
  

• }

As you AI bot noticed that it was missing what unregister_ftrace_direct() does, instead, can we make a helper function that both use? This way it will not get out of sync again.

static void reset_direct(struct ftrace_ops *ops, unsigned long addr) { struct ftrace_hash *hash = ops->func_hash->filter_hash;

ops->func = NULL; ops->trampoline = 0; remove_direct_functions_hash(hash, addr); }

Then we could have:

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c index 0c91247a95ab..51c3f5d46fde 100644 --- a/kernel/trace/ftrace.c +++ b/kernel/trace/ftrace.c @@ -6062,7 +6062,7 @@ int register_ftrace_direct(struct ftrace_ops *ops, unsigned long addr)

err = register_ftrace_function_nolock(ops); if (err) - remove_direct_functions_hash(hash, addr); + reset_direct(ops, addr);

out_unlock: mutex_unlock(&direct_mutex); @@ -6095,7 +6095,6 @@ EXPORT_SYMBOL_GPL(register_ftrace_direct); int unregister_ftrace_direct(struct ftrace_ops *ops, unsigned long addr, bool free_filters) { - struct ftrace_hash *hash = ops->func_hash->filter_hash; int err;

if (check_direct_multi(ops)) @@ -6105,13 +6104,9 @@ int unregister_ftrace_direct(struct ftrace_ops *ops, unsigned long addr,

mutex_lock(&direct_mutex); err = unregister_ftrace_function(ops); - remove_direct_functions_hash(hash, addr); + reset_direct(ops, addr); mutex_unlock(&direct_mutex);

- /* cleanup for possible another register call */ - ops->func = NULL; - ops->trampoline = 0; - if (free_filters) ftrace_free_filter(ops); return err;

-- Steve