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: [email protected] # v6.6+
> Reported-by: Andrey Grodzovsky <[email protected]>
> Closes:
> https://lore.kernel.org/live-patching/[email protected]/
> Cc: Steven Rostedt (Google) <[email protected]>
> Cc: Masami Hiramatsu (Google) <[email protected]>
> Acked-and-tested-by: Andrey Grodzovsky <[email protected]>
> Signed-off-by: Song Liu <[email protected]>
> ---
> 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
>
