linux-kernel - Re: [PATCH 2/2] x86/entry: Fix unwinding from kprobe on PUSH/POP instruction

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Date:   Thu, 16 Feb 2023 11:46:30 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...nel.org>
Cc:     Masami Hiramatsu <mhiramat@...nel.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        Chen Zhongjin <chenzhongjin@...wei.com>,
        "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
        Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH 2/2] x86/entry: Fix unwinding from kprobe on PUSH/POP
 instruction

On Wed, Feb 15, 2023 at 03:16:37PM -0800, Josh Poimboeuf wrote:
> On Wed, Feb 15, 2023 at 11:25:54AM +0100, Peter Zijlstra wrote:

> > Well, I was specifically thinking about #DB, why don't we need to
> > decrement when we put a hardware breakpoint on a stack modifying op?
> 
> I assume you mean the INT1 instruction.  Yeah, maybe we should care
> about that.

Nah, I was thinking #DB from DR7, but ...

> I'm struggling to come up with any decent ideas about how to implement
> that.  Presumably the #DB handler would have to communicate to the
> unwinder somehow whether the given frame is a trap.

... I had forgotten that #DB is not unconditionally trap :/ The worst
part seems to be that code breakpoints are faults while data breakpoints
are traps.

And you so don't want to go decode the DR registers in the unwinder,
quality mess this :/

Put a breakpoint on the stack and you've got PUSH doing a trap, put a
breakpoint on the PUSH instruction and you get a fault, and lo and
behold, you get a different unwind :-(