Subject: Re: [PATCH] ftrace: don't allow IPMODIFY without proper compiler support (was Re: Re: livepatching tree for linux-next)

From: Masami Hiramatsu <[email-masked]>
Date: Wed, 14 Jan 2015 11:34:33 +0900

(2015/01/14 7:47), Jiri Kosina wrote:
> On Mon, 12 Jan 2015, Masami Hiramatsu wrote:
> 
>>> In any case, Masami, I really think you would like to do something 
>>> like that for IPMODIFY as well ... or are you deliberately defering 
>>> the responsibility to handle the possible mcount fallout to the 
>>> ftrace_ops owner?
>>
>> Ah, good point. I just tried to use ftrace and WARN if not possible
>> to use it. I'll see it tomorrow. Anyway, I'd prefer to have this
>> kind of checking functionality in ftrace.
> 
> Okay, so how about something like this, for example ... ?

Thanks! Could you read my comments?

> From: Jiri Kosina <[email-masked]>
> Subject: [PATCH] ftrace: don't allow IPMODIFY without proper compiler support
> 
> Using IPMODIFY needs to be allowed only with compilers which are 
> guaranteed to generate function prologues compatible with function 
> redirection through changing instruction pointer in saved regs.
> 
> For example changing regs->ip on x86_64 in cases when gcc is using mcount 
> (and not fentry) is not allowed, because at the time mcount call is 
> issued, the original function's prologue has already been executed, which 
> leads to all kinds of inconsistent havoc.
> 
> There is currently no way to express dependency on gcc features in 
> Kconfig, (CC_USING_FENTRY is defined only during build, so it's not 
> possible for Kconfig symbol to depend on it) so this needs to be checked 
> in runtime.
> 
> Mark x86_64 with fentry supported for now. Other archs can be added 
> gradually.
> 
> Signed-off-by: Jiri Kosina <[email-masked]>
> ---
>  arch/x86/include/asm/ftrace.h | 2 ++
>  include/linux/ftrace.h        | 4 ++++
>  kernel/trace/ftrace.c         | 5 +++++
>  3 files changed, 11 insertions(+)
> 
> diff --git a/arch/x86/include/asm/ftrace.h b/arch/x86/include/asm/ftrace.h
> index f45acad..29fa417 100644
> --- a/arch/x86/include/asm/ftrace.h
> +++ b/arch/x86/include/asm/ftrace.h
> @@ -4,8 +4,10 @@
>  #ifdef CONFIG_FUNCTION_TRACER
>  #ifdef CC_USING_FENTRY
>  # define MCOUNT_ADDR		((long)(__fentry__))
> +# define arch_ftrace_ipmodify_compiler_support(void) ({ 1; })
>  #else
>  # define MCOUNT_ADDR		((long)(mcount))
> +#define arch_ftrace_ipmodify_compiler_support(void) ({ 0; })

Hmm, can we just define ARCH_FTRACE_SUPPORT_IPMODIFY here?

>  #endif
>  #define MCOUNT_INSN_SIZE	5 /* sizeof mcount call */
>  
> diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
> index 1da6029..655ba99 100644
> --- a/include/linux/ftrace.h
> +++ b/include/linux/ftrace.h
> @@ -244,6 +244,10 @@ static inline int ftrace_function_local_disabled(struct ftrace_ops *ops)
>  extern void ftrace_stub(unsigned long a0, unsigned long a1,
>  			struct ftrace_ops *op, struct pt_regs *regs);
>  
> +#ifndef arch_ftrace_ipmodify_compiler_support
> +/* let's not make any implicit assumptions about profiling call placement */
> +# define arch_ftrace_ipmodify_compiler_support() { 0; }
> +#endif
>  #else /* !CONFIG_FUNCTION_TRACER */
>  /*
>   * (un)register_ftrace_function must be a macro since the ops parameter
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index 929a733..11370fd 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -1809,6 +1809,11 @@ static int __ftrace_hash_update_ipmodify(struct ftrace_ops *ops,
>  	if (!(ops->flags & FTRACE_OPS_FL_IPMODIFY))
>  		return 0;
>  
> +	if (!arch_ftrace_ipmodify_compiler_support()) {
> +		WARN(1, "Your compiler doesn't support features necessary for IPMODIFY");
> +		return 0;
> +	}

Actually, if ftrace doesn't support IPMODIFY, I would like to just drop
the entire code for CONFIG_KPROBES_ON_FTRACE(this is a hidden config),
instead of checking at runtime.

So, there are 2 ifdefs of code in kernel/kprobes.c for
CONFIG_KPROBES_ON_FTRACE, those should also check
ARCH_FTRACE_SUPPORT_IPMODIFY too.

Thank you,

> +
>  	/*
>  	 * Since the IPMODIFY is a very address sensitive action, we do not
>  	 * allow ftrace_ops to set all functions to new hash.
> 

-- 
Masami HIRAMATSU
Software Platform Research Dept. Linux Technology Research Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: [email-masked]