linux-kernel - Re: [PATCH 03/15] x86/kprobes: Fix frame pointer annotations

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Date:   Sat, 8 Jun 2019 00:21:09 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Andy Lutomirski <luto@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jason Baron <jbaron@...mai.com>, Jiri Kosina <jkosina@...e.cz>,
        David Laight <David.Laight@...LAB.COM>,
        Borislav Petkov <bp@...en8.de>,
        Julia Cartwright <julia@...com>, Jessica Yu <jeyu@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>, Nadav Amit <namit@...are.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Edward Cree <ecree@...arflare.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>
Subject: Re: [PATCH 03/15] x86/kprobes: Fix frame pointer annotations

On Fri, 7 Jun 2019 09:36:02 -0400
Josh Poimboeuf <jpoimboe@...hat.com> wrote:

> On Fri, Jun 07, 2019 at 10:02:10PM +0900, Masami Hiramatsu wrote:
> > On Wed, 05 Jun 2019 15:07:56 +0200
> > Peter Zijlstra <peterz@...radead.org> wrote:
> > 
> > > The kprobe trampolines have a FRAME_POINTER annotation that makes no
> > > sense. It marks the frame in the middle of pt_regs, at the place of
> > > saving BP.
> > 
> > commit ee213fc72fd67 introduced this code, and this is for unwinder which
> > uses frame pointer. I think current code stores the address of previous
> > (original context's) frame pointer into %rbp. So with that, if unwinder
> > tries to decode frame pointer, it can get the original %rbp value,
> > instead of &pt_regs from current %rbp.
> > 
> > > 
> > > Change it to mark the pt_regs frame as per the ENCODE_FRAME_POINTER
> > > from the respective entry_*.S.
> > > 
> > 
> > With this change, I think stack unwinder can not get the original %rbp
> > value. Peter, could you check the above commit?
> 
> The unwinder knows how to decode the encoded frame pointer.  So it can
> find regs by decoding the new rbp value, and it also knows that regs->bp
> is the original rbp value.
> 
> Reviewed-by: Josh Poimboeuf <jpoimboe@...hat.com>
> 

Ah, OK. My misunderstood. So this encode framepointer as same as other
interrupt entry stack.
Then, it looks good to me too.

Acked-by: Masami Hiramatsu <mhiramat@...nel.org>

Thank you Josh!




> -- 
> Josh


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