Re: [PATCH bpf-next v2 1/6] ftrace: introduce FTRACE_OPS_FL_JMP

[Menglong Dong <menglong.dong@linux.dev>]

On 2025/11/18 13:19, Masami Hiramatsu wrote:
> On Mon, 17 Nov 2025 11:49:01 +0800
> Menglong Dong <menglong8.dong@gmail.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?

AFAIK, the function address is 16-bytes aligned for x86_64, and
8-bytes aligned for arm64, so I guess it is.

In the feature, if there is a exception, we can make ftrace_jmp_set,
ftrace_jmp_get arch-specification.

> At least register_ftrace_direct() needs to reject if @addr
> parameter has the last bit.

That make sense, I'll add such checking in the next version.

Thanks!
Menglong Dong

> 
> Thanks,
> 
> 
> > Signed-off-by: Menglong Dong <dongml2@chinatelecom.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;
> 
> 
>