linux-kernel - Re: [PATCH] x86: kprobes: orc: Fix ORC walks in kretprobes

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Date:   Fri, 5 Mar 2021 12:45:19 -0800
From:   Daniel Xu <dxu@...uu.xyz>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Masami Hiramatsu <mhiramat@...nel.org>, rostedt@...dmis.org,
        kuba@...nel.org, ast@...nel.org, tglx@...utronix.de,
        mingo@...hat.com, x86@...nel.org, linux-kernel@...r.kernel.org,
        bpf@...r.kernel.org, kernel-team@...com, yhs@...com
Subject: Re: [PATCH] x86: kprobes: orc: Fix ORC walks in kretprobes

On Fri, Mar 05, 2021 at 01:32:44PM -0600, Josh Poimboeuf wrote:
> On Fri, Mar 05, 2021 at 11:25:15AM -0800, Daniel Xu wrote:
> > > BTW, is this a regression? or CONFIG_UNWINDER_ORC has this issue before?
> > > It seems that the above commit just changed the default unwinder. This means
> > > OCR stack unwinder has this bug before that commit.
> > 
> > I see your point -- I suppose it depends on point of view. Viewed from
> > userspace, a change in kernel defaults means that one kernel worked and
> > the next one didn't -- all without the user doing anything. Consider it
> > from the POV of a typical linux user who just takes whatever the distro
> > gives them and doesn't compile their own kernels.
> > 
> > From the kernel point of view, you're also right. ORC didn't regress, it
> > was always broken for this particular use case. But as a primarily
> > userspace developer, I would consider this a kernel regression.
> 
> Either way, if the bug has always existed in the ORC unwinder, the Fixes
> tag needs to reference the original ORC commit:
> 
>   Fixes: ee9f8fce9964 ("x86/unwind: Add the ORC unwinder")
> 
> That makes it possible for stable kernels to identify the source of the
> bug and corresponding fix.  Many people used the ORC unwinder before it
> became the default.

Got it. I'll change it in the next version if we get to V2 (another
ongoing discussion in Masami's patchset).

Thanks,
Daniel