linux-kernel - Re: [PATCH 04/29] x86/livepatch: Validate __fentry_

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Date:   Fri, 18 Feb 2022 13:08:31 -0800
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, joao@...rdrivepizza.com, hjl.tools@...il.com,
        andrew.cooper3@...rix.com, linux-kernel@...r.kernel.org,
        ndesaulniers@...gle.com, keescook@...omium.org,
        samitolvanen@...gle.com, mark.rutland@....com,
        alyssa.milburn@...el.com, Miroslav Benes <mbenes@...e.cz>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH 04/29] x86/livepatch: Validate __fentry__ location

On Fri, Feb 18, 2022 at 05:49:06PM +0100, Peter Zijlstra wrote:
> Currently livepatch assumes __fentry__ lives at func+0, which is most
> likely untrue with IBT on. Override the weak klp_get_ftrace_location()
> function with an arch specific version that's IBT aware.
> 
> Also make the weak fallback verify the location is an actual ftrace
> location as a sanity check.
> 
> Suggested-by: Miroslav Benes <mbenes@...e.cz>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  arch/x86/include/asm/livepatch.h |    9 +++++++++
>  kernel/livepatch/patch.c         |    2 +-
>  2 files changed, 10 insertions(+), 1 deletion(-)
> 
> --- a/arch/x86/include/asm/livepatch.h
> +++ b/arch/x86/include/asm/livepatch.h
> @@ -17,4 +17,13 @@ static inline void klp_arch_set_pc(struc
>  	ftrace_instruction_pointer_set(fregs, ip);
>  }
>  
> +#define klp_get_ftrace_location klp_get_ftrace_location
> +static inline unsigned long klp_get_ftrace_location(unsigned long faddr)
> +{
> +	unsigned long addr = ftrace_location(faddr);
> +	if (!addr && IS_ENABLED(CONFIG_X86_IBT))
> +		addr = ftrace_location(faddr + 4);
> +	return addr;

I'm kind of surprised this logic doesn't exist in ftrace itself.  Is
livepatch really the only user that needs to find the fentry for a given
function?

I had to do a double take for the ftrace_location() semantics, as I
originally assumed that's what it did, based on its name and signature.

Instead it apparently functions like a bool but returns its argument on
success.

Though the function comment tells a different story:

/**
 * ftrace_location - return true if the ip giving is a traced location

So it's all kinds of confusing...

-- 
Josh