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Message-Id: <20251118141934.ddf14aabf371d0939415b588@kernel.org>
Date: Tue, 18 Nov 2025 14:19:34 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Menglong Dong <menglong8.dong@...il.com>
Cc: ast@...nel.org, rostedt@...dmis.org, daniel@...earbox.net,
john.fastabend@...il.com, andrii@...nel.org, martin.lau@...ux.dev,
eddyz87@...il.com, song@...nel.org, yonghong.song@...ux.dev,
kpsingh@...nel.org, sdf@...ichev.me, haoluo@...gle.com, jolsa@...nel.org,
mhiramat@...nel.org, mark.rutland@....com, mathieu.desnoyers@...icios.com,
jiang.biao@...ux.dev, bpf@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next v2 1/6] ftrace: introduce FTRACE_OPS_FL_JMP
On Mon, 17 Nov 2025 11:49:01 +0800
Menglong Dong <menglong8.dong@...il.com> wrote:
> For now, the "nop" will be replaced with a "call" instruction when a
> function is hooked by the ftrace. However, sometimes the "call" can break
> the RSB and introduce extra overhead. Therefore, introduce the flag
> FTRACE_OPS_FL_JMP, which indicate that the ftrace_ops should be called
> with a "jmp" instead of "call". For now, it is only used by the direct
> call case.
>
> When a direct ftrace_ops is marked with FTRACE_OPS_FL_JMP, the last bit of
> the ops->direct_call will be set to 1. Therefore, we can tell if we should
> use "jmp" for the callback in ftrace_call_replace().
>
nit: Is it sure the last bit is always 0?
At least register_ftrace_direct() needs to reject if @addr
parameter has the last bit.
Thanks,
> Signed-off-by: Menglong Dong <dongml2@...natelecom.cn>
> ---
> include/linux/ftrace.h | 33 +++++++++++++++++++++++++++++++++
> kernel/trace/Kconfig | 12 ++++++++++++
> kernel/trace/ftrace.c | 9 ++++++++-
> 3 files changed, 53 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
> index 07f8c309e432..015dd1049bea 100644
> --- a/include/linux/ftrace.h
> +++ b/include/linux/ftrace.h
> @@ -359,6 +359,7 @@ enum {
> FTRACE_OPS_FL_DIRECT = BIT(17),
> FTRACE_OPS_FL_SUBOP = BIT(18),
> FTRACE_OPS_FL_GRAPH = BIT(19),
> + FTRACE_OPS_FL_JMP = BIT(20),
> };
>
> #ifndef CONFIG_DYNAMIC_FTRACE_WITH_ARGS
> @@ -577,6 +578,38 @@ static inline void arch_ftrace_set_direct_caller(struct ftrace_regs *fregs,
> unsigned long addr) { }
> #endif /* CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS */
>
> +#ifdef CONFIG_DYNAMIC_FTRACE_WITH_JMP
> +static inline bool ftrace_is_jmp(unsigned long addr)
> +{
> + return addr & 1;
> +}
> +
> +static inline unsigned long ftrace_jmp_set(unsigned long addr)
> +{
> + return addr | 1UL;
> +}
> +
> +static inline unsigned long ftrace_jmp_get(unsigned long addr)
> +{
> + return addr & ~1UL;
> +}
> +#else
> +static inline bool ftrace_is_jmp(unsigned long addr)
> +{
> + return false;
> +}
> +
> +static inline unsigned long ftrace_jmp_set(unsigned long addr)
> +{
> + return addr;
> +}
> +
> +static inline unsigned long ftrace_jmp_get(unsigned long addr)
> +{
> + return addr;
> +}
> +#endif /* CONFIG_DYNAMIC_FTRACE_WITH_JMP */
> +
> #ifdef CONFIG_STACK_TRACER
>
> int stack_trace_sysctl(const struct ctl_table *table, int write, void *buffer,
> diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
> index d2c79da81e4f..4661b9e606e0 100644
> --- a/kernel/trace/Kconfig
> +++ b/kernel/trace/Kconfig
> @@ -80,6 +80,12 @@ config HAVE_DYNAMIC_FTRACE_NO_PATCHABLE
> If the architecture generates __patchable_function_entries sections
> but does not want them included in the ftrace locations.
>
> +config HAVE_DYNAMIC_FTRACE_WITH_JMP
> + bool
> + help
> + If the architecture supports to replace the __fentry__ with a
> + "jmp" instruction.
> +
> config HAVE_SYSCALL_TRACEPOINTS
> bool
> help
> @@ -330,6 +336,12 @@ config DYNAMIC_FTRACE_WITH_ARGS
> depends on DYNAMIC_FTRACE
> depends on HAVE_DYNAMIC_FTRACE_WITH_ARGS
>
> +config DYNAMIC_FTRACE_WITH_JMP
> + def_bool y
> + depends on DYNAMIC_FTRACE
> + depends on DYNAMIC_FTRACE_WITH_DIRECT_CALLS
> + depends on HAVE_DYNAMIC_FTRACE_WITH_JMP
> +
> config FPROBE
> bool "Kernel Function Probe (fprobe)"
> depends on HAVE_FUNCTION_GRAPH_FREGS && HAVE_FTRACE_GRAPH_FUNC
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index 59cfacb8a5bb..a6c060a4f50b 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -5951,7 +5951,8 @@ static void remove_direct_functions_hash(struct ftrace_hash *hash, unsigned long
> for (i = 0; i < size; i++) {
> hlist_for_each_entry(entry, &hash->buckets[i], hlist) {
> del = __ftrace_lookup_ip(direct_functions, entry->ip);
> - if (del && del->direct == addr) {
> + if (del && ftrace_jmp_get(del->direct) ==
> + ftrace_jmp_get(addr)) {
> remove_hash_entry(direct_functions, del);
> kfree(del);
> }
> @@ -6018,6 +6019,9 @@ int register_ftrace_direct(struct ftrace_ops *ops, unsigned long addr)
>
> mutex_lock(&direct_mutex);
>
> + if (ops->flags & FTRACE_OPS_FL_JMP)
> + addr = ftrace_jmp_set(addr);
> +
> /* Make sure requested entries are not already registered.. */
> size = 1 << hash->size_bits;
> for (i = 0; i < size; i++) {
> @@ -6138,6 +6142,9 @@ __modify_ftrace_direct(struct ftrace_ops *ops, unsigned long addr)
>
> lockdep_assert_held_once(&direct_mutex);
>
> + if (ops->flags & FTRACE_OPS_FL_JMP)
> + addr = ftrace_jmp_set(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;
> --
> 2.51.2
>
--
Masami Hiramatsu (Google) <mhiramat@...nel.org>
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