Date:   Mon, 24 Feb 2020 11:55:35 +0530
From:   Bhupesh Sharma <bhsharma@...hat.com>
To:     Amit Kachhap <amit.kachhap@....com>
Cc:     Dave Anderson <anderson@...hat.com>,
        James Morse <james.morse@....com>,
        Mark Rutland <mark.rutland@....com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        Catalin Marinas <catalin.marinas@....com>, x86@...nel.org,
        kexec mailing list <kexec@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linuxppc-dev@...ts.ozlabs.org,
        Kazuhito Hagio <k-hagio@...jp.nec.com>,
        bhupesh linux <bhupesh.linux@...il.com>,
        Will Deacon <will@...nel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Steve Capper <steve.capper@....com>
Subject: Re: [RESEND PATCH v5 2/5] arm64/crash_core: Export TCR_EL1.T1SZ in vmcoreinfo

Hi Amit,

On Fri, Feb 21, 2020 at 2:36 PM Amit Kachhap <amit.kachhap@....com> wrote:
>
> Hi Bhupesh,
>
> On 1/13/20 5:44 PM, Bhupesh Sharma wrote:
> > Hi James,
> >
> > On 01/11/2020 12:30 AM, Dave Anderson wrote:
> >>
> >> ----- Original Message -----
> >>> Hi Bhupesh,
> >>>
> >>> On 25/12/2019 19:01, Bhupesh Sharma wrote:
> >>>> On 12/12/2019 04:02 PM, James Morse wrote:
> >>>>> On 29/11/2019 19:59, Bhupesh Sharma wrote:
> >>>>>> vabits_actual variable on arm64 indicates the actual VA space size,
> >>>>>> and allows a single binary to support both 48-bit and 52-bit VA
> >>>>>> spaces.
> >>>>>>
> >>>>>> If the ARMv8.2-LVA optional feature is present, and we are running
> >>>>>> with a 64KB page size; then it is possible to use 52-bits of address
> >>>>>> space for both userspace and kernel addresses. However, any kernel
> >>>>>> binary that supports 52-bit must also be able to fall back to 48-bit
> >>>>>> at early boot time if the hardware feature is not present.
> >>>>>>
> >>>>>> Since TCR_EL1.T1SZ indicates the size offset of the memory region
> >>>>>> addressed by TTBR1_EL1 (and hence can be used for determining the
> >>>>>> vabits_actual value) it makes more sense to export the same in
> >>>>>> vmcoreinfo rather than vabits_actual variable, as the name of the
> >>>>>> variable can change in future kernel versions, but the architectural
> >>>>>> constructs like TCR_EL1.T1SZ can be used better to indicate intended
> >>>>>> specific fields to user-space.
> >>>>>>
> >>>>>> User-space utilities like makedumpfile and crash-utility, need to
> >>>>>> read/write this value from/to vmcoreinfo
> >>>>>
> >>>>> (write?)
> >>>>
> >>>> Yes, also write so that the vmcoreinfo from an (crashing) arm64
> >>>> system can
> >>>> be used for
> >>>> analysis of the root-cause of panic/crash on say an x86_64 host using
> >>>> utilities like
> >>>> crash-utility/gdb.
> >>>
> >>> I read this as as "User-space [...] needs to write to vmcoreinfo".
> >
> > That's correct. But for writing to vmcore dump in the kdump kernel, we
> > need to read the symbols from the vmcoreinfo in the primary kernel.
> >
> >>>>>> for determining if a virtual address lies in the linear map range.
> >>>>>
> >>>>> I think this is a fragile example. The debugger shouldn't need to know
> >>>>> this.
> >>>>
> >>>> Well that the current user-space utility design, so I am not sure we
> >>>> can
> >>>> tweak that too much.
> >>>>
> >>>>>> The user-space computation for determining whether an address lies in
> >>>>>> the linear map range is the same as we have in kernel-space:
> >>>>>>
> >>>>>>     #define __is_lm_address(addr)    (!(((u64)addr) &
> >>>>>> BIT(vabits_actual -
> >>>>>>     1)))
> >>>>>
> >>>>> This was changed with 14c127c957c1 ("arm64: mm: Flip kernel VA
> >>>>> space"). If
> >>>>> user-space
> >>>>> tools rely on 'knowing' the kernel memory layout, they must have to
> >>>>> constantly be fixed
> >>>>> and updated. This is a poor argument for adding this to something that
> >>>>> ends up as ABI.
> >>>>
> >>>> See above. The user-space has to rely on some ABI/guaranteed
> >>>> hardware-symbols which can be
> >>>> used for 'determining' the kernel memory layout.
> >>>
> >>> I disagree. Everything and anything in the kernel will change. The
> >>> ABI rules apply to
> >>> stuff exposed via syscalls and kernel filesystems. It does not apply
> >>> to kernel internals,
> >>> like the memory layout we used yesterday. 14c127c957c1 is a case in
> >>> point.
> >>>
> >>> A debugger trying to rely on this sort of thing would have to play
> >>> catchup whenever it
> >>> changes.
> >>
> >> Exactly.  That's the whole point.
> >>
> >> The crash utility and makedumpfile are not in the same league as other
> >> user-space tools.
> >> They have always had to "play catchup" precisely because they depend
> >> upon kernel internals,
> >> which constantly change.
> >
> > I agree with you and DaveA here. Software user-space debuggers are
> > dependent on kernel internals (which can change from time-to-time) and
> > will have to play catch-up (which has been the case since the very start).
> >
> > Unfortunately we don't have any clear ABI for software debugging tools -
> > may be something to look for in future.
> >
> > A case in point is gdb/kgdb, which still needs to run with KASLR
> > turned-off (nokaslr) for debugging, as it confuses gdb which resolve
> > kernel symbol address from symbol table of vmlinux. But we can
> > work-around the same in makedumpfile/crash by reading the 'kaslr_offset'
> > value. And I have several users telling me now they cannot use gdb on
> > KASLR enabled kernel to debug panics, but can makedumpfile + crash
> > combination to achieve the same.
> >
> > So, we should be looking to fix these utilities which are broken since
> > the 52-bit changes for arm64. Accordingly, I will try to send the v6
> > soon while incorporating the comments posted on the v5.
>
> Any update on the next v6 version. Since this patch series is fixing the
> current broken kdump so need this series to add some more fields in
> vmcoreinfo for Pointer Authentication work.

Sorry for the delay. I was caught up in some other urgent arm64
user-space issues.
I am preparing the v6 now and hopefully will be able to post it out
for review later today.

Thanks,
Bhupesh

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