linux-kernel - Re: [RFC 1/6] vdso/extable: fix calculation of base

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Date:   Fri, 26 Feb 2021 09:47:25 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Nadav Amit <nadav.amit@...il.com>
Cc:     Linux-MM <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
        Hugh Dickins <hughd@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Andrew Morton <akpm@...ux-foundation.org>, x86@...nel.org
Subject: Re: [RFC 1/6] vdso/extable: fix calculation of base

On Fri, Feb 26, 2021, Nadav Amit wrote:
> 
> > On Feb 25, 2021, at 1:16 PM, Sean Christopherson <seanjc@...gle.com> wrote:
> > It's been literally years since I wrote this code, but I distinctly remember the
> > addresses being relative to the base.  I also remember testing multiple entries,
> > but again, that was a long time ago.
> > 
> > Assuming things have changed, or I was flat out wrong, the comment above the
> > macro magic should also be updated.
> > 
> > /*
> > * Inject exception fixup for vDSO code.  Unlike normal exception fixup,
> > * vDSO uses a dedicated handler the addresses are relative to the overall
> > * exception table, not each individual entry.
> > */
> 
> I will update the comment. I am not very familiar with pushsection stuff,
> but the offsets were wrong.
> 
> Since you say you checked it, I wonder whether it can somehow be caused
> by having exception table entries defined from multiple object files.

Oooh, I think that would do it.  Have you checked what happens if there are
multiple object files and multiple fixups within an object file?

> Anyhow, this change follows the kernel’s (not vDSO) exception table
> scheme.