linux-kernel - Re: [PATCH -tip v3 05/11] x86/kprobes: Add UNWIND_HINT_FUNC on kretprobe

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Message-Id: <20210322090846.515f8c0a9b71acadfc186b7f@kernel.org>
Date:   Mon, 22 Mar 2021 09:08:46 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
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
Subject: Re: [PATCH -tip v3 05/11] x86/kprobes: Add UNWIND_HINT_FUNC on
 kretprobe_trampoline code

On Sun, 21 Mar 2021 12:52:03 -0500
Josh Poimboeuf <jpoimboe@...hat.com> wrote:

> On Sat, Mar 20, 2021 at 10:05:43PM +0900, Masami Hiramatsu wrote:
> > On Sat, 20 Mar 2021 21:16:16 +0900
> > Masami Hiramatsu <mhiramat@...nel.org> wrote:
> > 
> > > On Fri, 19 Mar 2021 21:22:39 +0900
> > > Masami Hiramatsu <mhiramat@...nel.org> wrote:
> > > 
> > > > From: Josh Poimboeuf <jpoimboe@...hat.com>
> > > > 
> > > > Add UNWIND_HINT_FUNC on kretporbe_trampoline code so that ORC
> > > > information is generated on the kretprobe_trampoline correctly.
> > > > 
> > > 
> > > Test bot also found a new warning for this.
> > > 
> > > > >> arch/x86/kernel/kprobes/core.o: warning: objtool: kretprobe_trampoline()+0x25: call without frame pointer save/setup
> > > 
> > > With CONFIG_FRAME_POINTER=y.
> > > 
> > > Of course this can be fixed with additional "push %bp; mov %sp, %bp" before calling
> > > trampoline_handler. But actually we know that this function has a bit special
> > > stack frame too. 
> > > 
> > > Can I recover STACK_FRAME_NON_STANDARD(kretprobe_trampoline) when CONFIG_FRAME_POINTER=y ?
> > 
> > So something like this. Does it work?
> > 
> > diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
> > index b31058a152b6..651f337dc880 100644
> > --- a/arch/x86/kernel/kprobes/core.c
> > +++ b/arch/x86/kernel/kprobes/core.c
> > @@ -760,6 +760,10 @@ int kprobe_int3_handler(struct pt_regs *regs)
> >  }
> >  NOKPROBE_SYMBOL(kprobe_int3_handler);
> >  
> > +#ifdef CONFIG_FRAME_POINTER
> > +#undef UNWIND_HINT_FUNC
> > +#define UNWIND_HINT_FUNC
> > +#endif
> 
> This hunk isn't necessary.  The unwind hints don't actually have an
> effect with frame pointers.

Without this, objtool shows the following warning with CONFIG_FRAME_POINTER=y.

arch/x86/kernel/kprobes/core.o: warning: objtool: kretprobe_trampoline()+0x1: BUG: why am I validating an ignored function?

It seems to complain about putting UNWIND_HINT on STACK_FRAME_NON_STANDARD function.

> 
> >  /*
> >   * When a retprobed function returns, this code saves registers and
> >   * calls trampoline_handler() runs, which calls the kretprobe's handler.
> > @@ -797,7 +801,14 @@ asm(
> >  	".size kretprobe_trampoline, .-kretprobe_trampoline\n"
> >  );
> >  NOKPROBE_SYMBOL(kretprobe_trampoline);
> > -
> > +#ifdef CONFIG_FRAME_POINTER
> > +/*
> > + * kretprobe_trampoline skips updating frame pointer. The frame pointer
> > + * saved in trampoline_handler points to the real caller function's
> > + * frame pointer.
> > + */
> > +STACK_FRAME_NON_STANDARD(kretprobe_trampoline);
> > +#endif
> >  
> >  /*
> >   * Called from kretprobe_trampoline
> 
> Ack.

Thank you!

> 
> -- 
> Josh
> 


-- 
Masami Hiramatsu <mhiramat@...nel.org>