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

From: Steven Rostedt <[email-masked]>
Date: Tue, 13 Jan 2015 21:12:23 -0500

On Tue, 13 Jan 2015 23:47:57 +0100 (CET)
Jiri Kosina <[email-masked]> wrote:
> 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; })
>  #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; }

Is there any reason that this is a macro function? Why not just define:

#define ftrace_ipmodify_supported 0

?

-- Steve

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