Subject: Re: [PATCH 1/2 v2] ftrace: don't allow IPMODIFY without proper compiler support

From: Masami Hiramatsu <[email-masked]>
Date: Thu, 15 Jan 2015 12:34:33 +0900

(2015/01/14 17:47), Jiri Kosina wrote:
> 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]>
> ---
> 
> v1 -> v2: turn macro function into a plain macro
> 
>  arch/x86/include/asm/ftrace.h | 2 ++
>  include/linux/ftrace.h        | 5 +++++
>  kernel/trace/ftrace.c         | 5 +++++
>  3 files changed, 12 insertions(+)
> 
> diff --git a/arch/x86/include/asm/ftrace.h b/arch/x86/include/asm/ftrace.h
> index f45acad..f84eaaf 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 ftrace_ipmodify_supported 1
>  #else
>  # define MCOUNT_ADDR		((long)(mcount))
> +# define ftrace_ipmodify_supported 0
>  #endif
>  #define MCOUNT_INSN_SIZE	5 /* sizeof mcount call */
>  
> diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
> index 1da6029..837153a 100644
> --- a/include/linux/ftrace.h
> +++ b/include/linux/ftrace.h
> @@ -244,6 +244,11 @@ 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 ftrace_ipmodify_supported
> +/* let's not make any implicit assumptions about profiling call placement */
> +# define ftrace_ipmodify_supported 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..05af4cd 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 (!ftrace_ipmodify_supported) {
> +		WARN(1, "Your compiler doesn't support features necessary for IPMODIFY");
> +		return 0;
> +	}
> +

Hmm, if this binary doesn't support IPMODIFY, it should return -ENOTSUPP.
And also, IMHO, we'd better reject registering ftrace_ops with IPMODIFY
in this situation before updating hash table.

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]