Re: [PATCH 5/7] ftrace: Store direct called addresses in their ops

Jiri Olsa
2023-03-19T15:29:32Z

On Thu, Mar 16, 2023 at 06:38:09PM +0100, Florent Revest wrote:
> All direct calls are now registered using the register_ftrace_direct API
> so each ops can jump to only one direct-called trampoline.
>
> By storing the direct called trampoline address directly in the ops we
> can save one hashmap lookup in the direct call ops and implement arm64
> direct calls on top of call ops.
>
> Signed-off-by: Florent Revest <revest@chromium.org>
> ---
> include/linux/ftrace.h | 3 +++
> kernel/trace/ftrace.c | 7 +++++--
> 2 files changed, 8 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
> index abee60865fc7..6a532dd6789e 100644
> --- a/include/linux/ftrace.h
> +++ b/include/linux/ftrace.h
> @@ -321,6 +321,9 @@ struct ftrace_ops {
> unsigned long trampoline_size;
> struct list_head list;
> ftrace_ops_func_t ops_func;
> +#ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
> + unsigned long direct_call;
> +#endif
> #endif
> };
>
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index 33530198d1ca..66c91fa4b6ab 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -2582,9 +2582,8 @@ ftrace_add_rec_direct(unsigned long ip, unsigned long addr,
> static void call_direct_funcs(unsigned long ip, unsigned long pip,
> struct ftrace_ops *ops, struct ftrace_regs *fregs)
> {
> - unsigned long addr;
> + unsigned long addr = ops->direct_call;

nice, should it be read with READ_ONCE ?

jirka

>
> - addr = ftrace_find_rec_direct(ip);
> if (!addr)
> return;
>
> @@ -5380,6 +5379,7 @@ int register_ftrace_direct(struct ftrace_ops *ops, unsigned long addr)
> ops->func = call_direct_funcs;
> ops->flags = MULTI_FLAGS;
> ops->trampoline = FTRACE_REGS_ADDR;
> + ops->direct_call = addr;
>
> err = register_ftrace_function_nolock(ops);
>
> @@ -5454,6 +5454,7 @@ __modify_ftrace_direct(struct ftrace_ops *ops, unsigned long addr)
> /* Enable the tmp_ops to have the same functions as the direct ops */
> ftrace_ops_init(&tmp_ops);
> tmp_ops.func_hash = ops->func_hash;
> + tmp_ops.direct_call = addr;
>
> err = register_ftrace_function_nolock(&tmp_ops);
> if (err)
> @@ -5475,6 +5476,8 @@ __modify_ftrace_direct(struct ftrace_ops *ops, unsigned long addr)
> entry->direct = addr;
> }
> }
> + /* Prevent store tearing if a trampoline concurrently accesses the value */
> + WRITE_ONCE(ops->direct_call, addr);
>
> mutex_unlock(&ftrace_lock);
>
> --
> 2.40.0.rc2.332.ga46443480c-goog
>