linux-kernel - Re: [PATCH v2 09/13] powerpc/ftrace: Implement CONFIG_DYNAMIC_FTRACE_WITH

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Date:   Tue, 15 Feb 2022 08:00:33 +0000
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>,
        Jiri Kosina <jikos@...nel.org>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Miroslav Benes <mbenes@...e.cz>,
        Ingo Molnar <mingo@...hat.com>, Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "live-patching@...r.kernel.org" <live-patching@...r.kernel.org>
Subject: Re: [PATCH v2 09/13] powerpc/ftrace: Implement
 CONFIG_DYNAMIC_FTRACE_WITH_ARGS



Le 14/02/2022 à 16:25, Naveen N. Rao a écrit :
> Christophe Leroy wrote:
>> Implement CONFIG_DYNAMIC_FTRACE_WITH_ARGS. It accelerates the call
>> of livepatching.
>>
>> Also note that powerpc being the last one to convert to
>> CONFIG_DYNAMIC_FTRACE_WITH_ARGS, it will now be possible to remove
>> klp_arch_set_pc() on all architectures.
>>
>> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
>> ---
>>  arch/powerpc/Kconfig                 |  1 +
>>  arch/powerpc/include/asm/ftrace.h    | 17 +++++++++++++++++
>>  arch/powerpc/include/asm/livepatch.h |  4 +---
>>  3 files changed, 19 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
>> index cdac2115eb00..e2b1792b2aae 100644
>> --- a/arch/powerpc/Kconfig
>> +++ b/arch/powerpc/Kconfig
>> @@ -210,6 +210,7 @@ config PPC
>>      select HAVE_DEBUG_KMEMLEAK
>>      select HAVE_DEBUG_STACKOVERFLOW
>>      select HAVE_DYNAMIC_FTRACE
>> +    select HAVE_DYNAMIC_FTRACE_WITH_ARGS    if MPROFILE_KERNEL || PPC32
>>      select HAVE_DYNAMIC_FTRACE_WITH_REGS    if MPROFILE_KERNEL || PPC32
>>      select HAVE_EBPF_JIT
>>      select HAVE_EFFICIENT_UNALIGNED_ACCESS    if !(CPU_LITTLE_ENDIAN 
>> && POWER7_CPU)
>> diff --git a/arch/powerpc/include/asm/ftrace.h 
>> b/arch/powerpc/include/asm/ftrace.h
>> index b3f6184f77ea..45c3d6f11daa 100644
>> --- a/arch/powerpc/include/asm/ftrace.h
>> +++ b/arch/powerpc/include/asm/ftrace.h
>> @@ -22,6 +22,23 @@ static inline unsigned long 
>> ftrace_call_adjust(unsigned long addr)
>>  struct dyn_arch_ftrace {
>>      struct module *mod;
>>  };
>> +
>> +#ifdef CONFIG_DYNAMIC_FTRACE_WITH_ARGS
>> +struct ftrace_regs {
>> +    struct pt_regs regs;
>> +};
>> +
>> +static __always_inline struct pt_regs *arch_ftrace_get_regs(struct 
>> ftrace_regs *fregs)
>> +{
>> +    return &fregs->regs;
>> +}
> 
> I think this is wrong. We need to differentiate between ftrace_caller() 
> and ftrace_regs_caller() here, and only return pt_regs if coming in 
> through ftrace_regs_caller() (i.e., FL_SAVE_REGS is set).

Not sure I follow you.

This is based on 5740a7c71ab6 ("s390/ftrace: add 
HAVE_DYNAMIC_FTRACE_WITH_ARGS support")

It's all the point of HAVE_DYNAMIC_FTRACE_WITH_ARGS, have the regs also 
with ftrace_caller().

Sure you only have the params, but that's the same on s390, so what did 
I miss ?


> 
>> +
>> +static __always_inline void ftrace_instruction_pointer_set(struct 
>> ftrace_regs *fregs,
>> +                               unsigned long ip)
>> +{
>> +    regs_set_return_ip(&fregs->regs, ip);
> 
> Should we use that helper here? regs_set_return_ip() also updates some 
> other state related to taking interrupts and I don't think it makes 
> sense for use with ftrace.
> 


Today we have:

	static inline void klp_arch_set_pc(struct ftrace_regs *fregs, unsigned 
long ip)
	{
		struct pt_regs *regs = ftrace_get_regs(fregs);

		regs_set_return_ip(regs, ip);
	}


Which like x86 and s390 becomes:

	static inline void klp_arch_set_pc(struct ftrace_regs *fregs, unsigned 
long ip)
	{
		ftrace_instruction_pointer_set(fregs, ip);
	}



That's the reason why I've been using regs_set_return_ip(). Do you think 
it was wrong to use regs_set_return_ip() in klp_arch_set_pc() ?

That was added by 59dc5bfca0cb ("powerpc/64s: avoid reloading (H)SRR 
registers if they are still valid")

Christophe