linux-kernel - Re: [PATCH v2] x86_64, vmcoreinfo: Append 'page_offset

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Date:   Wed, 21 Nov 2018 13:07:57 +0530
From:   Bhupesh Sharma <bhsharma@...hat.com>
To:     Kazuhito Hagio <k-hagio@...jp.nec.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Borislav Petkov <bp@...en8.de>, Baoquan He <bhe@...hat.com>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dave Anderson <anderson@...hat.com>,
        James Morse <james.morse@....com>,
        Omar Sandoval <osandov@...com>, x86@...nel.org,
        kexec mailing list <kexec@...ts.infradead.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2] x86_64, vmcoreinfo: Append 'page_offset_base' to vmcoreinfo

Hi Kazu,

On Tue, Nov 20, 2018 at 2:47 AM Kazuhito Hagio <k-hagio@...jp.nec.com> wrote:
>
> On 11/15/2018 4:47 PM, Bhupesh Sharma wrote:
> > Adding 'page_offset_base' to the vmcoreinfo can be specially useful for
> > live-debugging of a running kernel via user-space utilities
> > like makedumpfile (see [1]).
>
> I agree.
>
> > Recently, I saw an issue with the 'makedumpfile' utility (see [2] for
> > details), whose live debugging feature is broken with newer kernels
> > (I tested the same with 4.19-rc8+ kernel), as KCORE_REMAP segments were
> > added to kcore, thus leading to an additional sections in the same, and
> > makedumpfile is not longer able to determine the start of direct
> > mapping of all physical memory, as it relies on traversing the PT_LOAD
> > segments inside kcore and using the last PT_LOAD segment
> > to determine the start of direct mapping.
>
> Actually, the KCORE_REMAP segments were already removed from kcore by
> commit bf904d2762ee ("x86/pti/64: Remove the SYSCALL64 entry trampoline")
> and kcore's program headers got back to the previous ones, but this
> fact shows that they are changeable.
>
> So I think that if we have this NUMBER(page_offset_base) in vmcoreinfo
> for x86_64, user-space tools (especially makedumpfile) would become
> more stable against changes in kcore and vmcore, rather than depending
> on their PT_LOAD values.

Thanks a lot for your review.

Hi x86 maintainers,

Ping, any comments on this patch. Since it is a useful addition to the
kernel-userspace ABI (as suggested by the user-space utility
maintainer as well), can we consider including this in the kernel?

Thanks,
Bhupesh