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Date:   Thu, 3 Mar 2022 14:04:52 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>
Cc:     Steven Rostedt <rostedt@goodmis.org>, alexei.starovoitov@gmail.com,
        alyssa.milburn@intel.com, andrew.cooper3@citrix.com,
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        keescook@chromium.org, linux-kernel@vger.kernel.org,
        mark.rutland@arm.com, mbenes@suse.cz,
        Masami Hiramatsu <mhiramat@kernel.org>,
        ndesaulniers@google.com, samitolvanen@google.com, x86@kernel.org
Subject: Re: [PATCH v2 12/39] x86/ibt,ftrace: Search for __fentry__ location
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On Thu, Mar 03, 2022 at 03:15:14PM +0530, Naveen N. Rao wrote:

> > diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
> > index 7f0ce42f8ff9..4c13406e0bc4 100644
> > --- a/arch/x86/kernel/kprobes/core.c
> > +++ b/arch/x86/kernel/kprobes/core.c
> > @@ -198,13 +198,14 @@ __recover_probed_insn(kprobe_opcode_t *buf, unsigned long addr)
> > 
> >  	kp = get_kprobe((void *)addr);
> >  	faddr = ftrace_location(addr);
> > +
> >  	/*
> > -	 * Addresses inside the ftrace location are refused by
> > -	 * arch_check_ftrace_location(). Something went terribly wrong
> > -	 * if such an address is checked here.
> > +	 * In case addr maps to sym+0 ftrace_location() might return something
> > +	 * other than faddr. In that case consider it the same as !faddr.
> >  	 */
> > -	if (WARN_ON(faddr && faddr != addr))
> > -		return 0UL;
> > +	if (faddr && faddr != addr)
> > +		faddr = 0;
> > +
> >  	/*
> >  	 * Use the current code if it is not modified by Kprobe
> >  	 * and it cannot be modified by ftrace.
> 
> I hit this issue yesterday in kprobe generic code in
> check_ftrace_location().

What exactly where you running to trigger this? (so that I can extend my
test coverage etc..)

> In both these scenarios, we just want to check if a
> particular instruction is reserved by ftrace.  ftrace_location_range()
> should still work for that purpose, so that may be the easier fix:
> 
> diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> index 066fa644e9dfa3..ee3cd035403ca2 100644
> --- a/kernel/kprobes.c
> +++ b/kernel/kprobes.c
> @@ -1596,7 +1596,7 @@ static int check_ftrace_location(struct kprobe *p)
> {
> 	unsigned long ftrace_addr;
> 
> -	ftrace_addr = ftrace_location((unsigned long)p->addr);
> +	ftrace_addr = ftrace_location_range((unsigned long)p->addr, (unsigned long)p->addr);

Yes, although perhaps a new helper. I'll go ponder during lunch.

PS. I posted v3 but forgot to Cc you:

  https://lkml.kernel.org/r/20220303112321.422525803@infradead.org

I think the above hunk ended up in the kprobe patch, but on second
thought I should've put it in the ftrace one. I'll go ammend and add
this other site you found.