linux-kernel - Re: [PATCH 1/2] x86/kprobes: Fix kprobes instruction boudary check with CONFIG

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Date:   Wed, 7 Sep 2022 16:28:28 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...nel.org>,
        Suleiman Souhlal <suleiman@...gle.com>,
        bpf <bpf@...r.kernel.org>, linux-kernel@...r.kernel.org,
        Borislav Petkov <bp@...e.de>,
        Josh Poimboeuf <jpoimboe@...nel.org>, x86@...nel.org
Subject: Re: [PATCH 1/2] x86/kprobes: Fix kprobes instruction boudary check
 with CONFIG_RETHUNK

On Wed, Sep 07, 2022 at 10:49:13PM +0900, Masami Hiramatsu wrote:
> On Wed, 7 Sep 2022 14:56:52 +0200
> Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > On Wed, Sep 07, 2022 at 09:55:21AM +0900, Masami Hiramatsu (Google) wrote:
> > 
> > >  	if (!kallsyms_lookup_size_offset(paddr, NULL, &offset))
> > >  		return 0;
> > >  
> > 
> > One more thing:
> > 
> >   https://lkml.kernel.org/r/20220902130951.853460809@infradead.org
> > 
> > can result in negative offsets. The expression:
> > 
> > 	'paddr - offset'
> > 
> > will still get you to +0, but I might not have fully considered things
> > when I wrote that patch.
> 
> Hmm, isn't 'offset' unsigned? If 'paddr - offset' is still available
> to find the function entry address, it is OK to me.

Yeah, but the magic of 2s complement means it doesn't matter ;-)