linux-kernel - Re: [PATCH v2 12/39] x86/ibt,ftrace: Search for __fentry_

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Date:   Thu, 03 Mar 2022 20:09:33 +0530
From:   "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     alexei.starovoitov@...il.com, alyssa.milburn@...el.com,
        andrew.cooper3@...rix.com, hjl.tools@...il.com,
        joao@...rdrivepizza.com, jpoimboe@...hat.com,
        keescook@...omium.org, linux-kernel@...r.kernel.org,
        mark.rutland@....com, mbenes@...e.cz,
        Masami Hiramatsu <mhiramat@...nel.org>,
        ndesaulniers@...gle.com, Steven Rostedt <rostedt@...dmis.org>,
        samitolvanen@...gle.com, x86@...nel.org
Subject: Re: [PATCH v2 12/39] x86/ibt,ftrace: Search for __fentry__ location

Peter Zijlstra wrote:
> 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..)

With the changes here, we always promote probes at +0 offset to the LEP 
at +8.  So, we get the old behavior of ftrace_location(). But, we also 
have functions that do not have a GEP. For those, we allow probing at an 
offset of +0, resulting in ftrace_location() giving us the actual ftrace 
site, which on ppc64le will be at +4.

On ppc32, this will affect all probes since the ftrace site is usually 
at an offset of +8.

I don't think x86 has this issue since you will always have ftrace site 
at +0 if a function does not have the endbr instruction. And if you do 
have the endbr instruction, then you adjust the probe address so it 
won't be at an offset of 0 into the function.

> 
>> 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.

Sure. We do have ftrace_text_reserved(), but that only returns a 
boolean. Masami wanted to ensure that the probe is at an instruction 
boundary, hence this check.

> 
> 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.

Thanks, I'll take a look.

- Naveen