linux-kernel - Re: [PATCH -tip v7 09/13] kprobes: Setup instruction pointer in __kretprobe_trampoline

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Message-ID: <20210617044032.txng4enhiduacvt6@treble>
Date:   Wed, 16 Jun 2021 23:40:32 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...nel.org>, X86 ML <x86@...nel.org>,
        Daniel Xu <dxu@...uu.xyz>, linux-kernel@...r.kernel.org,
        bpf@...r.kernel.org, kuba@...nel.org, mingo@...hat.com,
        ast@...nel.org, tglx@...utronix.de, kernel-team@...com, yhs@...com,
        linux-ia64@...r.kernel.org,
        Abhishek Sagar <sagar.abhishek@...il.com>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>
Subject: Re: [PATCH -tip v7 09/13] kprobes: Setup instruction pointer in
 __kretprobe_trampoline_handler

On Wed, Jun 16, 2021 at 11:39:11PM -0500, Josh Poimboeuf wrote:
> On Thu, May 27, 2021 at 03:40:29PM +0900, Masami Hiramatsu wrote:
> > To simplify the stacktrace with pt_regs from kretprobe handler,
> > set the correct return address to the instruction pointer in
> > the pt_regs before calling kretprobe handlers.
> > 
> > Suggested-by: Josh Poimboeuf <jpoimboe@...hat.com>
> > Signed-off-by: Masami Hiramatsu <mhiramat@...nel.org>
> > Tested-by: Andrii Nakryik <andrii@...nel.org>
> > ---
> >  Changes in v3:
> >   - Cast the correct_ret_addr to unsigned long.
> > ---
> >  kernel/kprobes.c |    3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> > index 54e5b89aad67..1598aca375c9 100644
> > --- a/kernel/kprobes.c
> > +++ b/kernel/kprobes.c
> > @@ -1914,6 +1914,9 @@ unsigned long __kretprobe_trampoline_handler(struct pt_regs *regs,
> >  		BUG_ON(1);
> >  	}
> >  
> > +	/* Set the instruction pointer to the correct address */
> > +	instruction_pointer_set(regs, (unsigned long)correct_ret_addr);
> > +
> >  	/* Run them. */
> >  	first = current->kretprobe_instances.first;
> >  	while (first) {
> > 
> 
> Hi Masami,
> 
> I know I suggested this patch, but I believe it would only be useful in
> combination with the use of UNWIND_HINT_REGS in SAVE_REGS_STRING.  But I
> think that would be tricky to pull off correctly.  Instead, we have
> UNWIND_HINT_FUNC, which is working fine.
> 
> So I'd suggest dropping this patch, as the unwinder isn't actually
> reading regs->ip after all.

... and I guess this means patches 6-8 are no longer necessary.

-- 
Josh