Re: [PATCH 4/8] ftrace: Store direct called addresses in their ops

Mark Rutland
2023-02-02T15:32:26Z

On Wed, Feb 01, 2023 at 05:34:16PM +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 | 6 ++++--
> 2 files changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
> index a7dbd307c3a4..84f717f8959e 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 cb77a0a208c7..b0426de11c45 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -2577,9 +2577,8 @@ 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;
>
> - addr = ftrace_find_rec_direct(ip);
> if (!addr)
> return;
>
> @@ -5375,6 +5374,7 @@ 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);
>
> @@ -5445,6 +5445,7 @@ __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)
> @@ -5466,6 +5467,7 @@ __modify_ftrace_direct(struct ftrace_ops *ops, unsigned long addr)
> entry->direct = addr;
> }
> }
> + ops->direct_call = addr;

AFAICT we don't synchronize threads when installing the tmp_ops, so IIUC on
arches with call_ops, there could be a a thread in the middle of ftrace_caller
which has loaded the ops pointer from the patch-site, but hasn't yet loaded the
ops::direct_func pointer, and could race with this assignment.

Given that, I think this needs to be:

WRITE_ONCE(ops->direct_call, addr);

... in order to avoid the risk of the store being torn, and the ftrace_caller
trampoline loading a corrupted pointer.

Other than that, this looks good to me!

Thanks,
Mark.

>
> mutex_unlock(&ftrace_lock);
>
> --
> 2.39.1.519.gcb327c4b5f-goog