Re: [PATCH] x86_64, vmcoreinfo: Append 'page_offset_base' to vmcoreinfo

From: Baoquan He
Date: Sat Oct 27 2018 - 06:02:50 EST

Hi Bhupesh,

Sorry for top posting. Because I don't know which line at below I should
add comment into.

So could you plese tell what problem you have met in user space tools?
Which user space tool is broken so that we need export page_offset_base
to vmcoreinfo?

Sorry I didn't get what problem this patch is trying to fix from the
patch log.

About this, I have replied to you in
You might miss it.

About this exporting, I ever posted patch to upstream and we have had
discussion, please check

In makedumpfile and crash, we have had a clear method to analyze and
deduce it from kcore or vmcore.


On 10/27/18 at 04:13am, Bhupesh Sharma wrote:
> Since commit 23c85094fe1895caefdd
> ["proc/kcore: add vmcoreinfo note to /proc/kcore"]), '/proc/kcore'
> contains a new PT_NOTE which carries the VMCOREINFO information.
> If the same is available, one can use it in user-land to
> retrieve machine specific symbols or strings being appended to the
> vmcoreinfo even for live-debugging of the primary kernel as a
> standard interface exposed by kernel for sharing machine specific
> details with the user-land.
> In the past I had a discussion with James, where he suggested this
> approach (please see [0]) and I really liked the idea. Since then I
> have been working on unifying the implementations of
> (atleast) the commonly used user-space utilities that provide
> live-debugging capabilities (tools like 'makedumpfile' and
> 'crash-utility', see [1] for details of these tools).
> For the same, when live debugging on x86_64 machines, user-space
> tools currently rely on different mechanisms to determine
> the 'page_offset_base' value (i.e. start of direct mapping of all
> physical memory). One of the approach used by 'makedumpfile'
> user-space tool for e.g. is to calculate the same from the last
> PT_LOAD available in '/proc/kcore', which can be flaky as and when
> new sections (for e.g. KCORE_REMAP which was added
> to recent kernels) are added to kcore.
> For other architectures like arm64, I have already proposed using
> the vmcoreinfo note (in '/proc/kcore') in the user-space utilities to
> determine machine specific details like VA_BITS, PAGE_OFFSET,
> kasrl_offset() (see [2] for details), for which different user-space
> tools earlier used different (and at times flaky) approaches like:
> - Reading kernel CONFIGs from user-space and determining CONFIG values
> like VA_BITS from there.
> - Reading symbols from '/proc/kallsyms' and determining their values
> via '/dev/mem' interface.
> - Reading symbols from 'vmlinux' and determing their values from
> reading memory.
> This patch allows appending 'page_offset_base' for x86_64 platforms
> to vmcoreinfo, so that user-space tools can use the same as a standard
> interface to determine the start of direct mapping of all physical
> memory.
> Testing:
> -------
> - I tested this patch (rebased on 'linux-next') on a x86_64 machine
> using the modified 'makedumpfile' user-space code (see [3] for my
> github tree which contains the same) for determining how many pages
> are dumpable when different dump_level is specified (which is
> one use-case of live-debugging via 'makedumpfile').
> - I tested both the KASLR and non-KASLR boot cases with this patch.
> - Here is one sample log (for KASLR boot case) on my x86_64 machine:
> < snip..>
> The kernel doesn't support mmap(),read() will be used instead.
> ----------------------------------------------------------------------
> ZERO 21299 yes Pages filled
> with zero
> NON_PRI_CACHE 91785 yes Cache
> pages without private flag
> PRI_CACHE 1 yes Cache pages with
> private flag
> USER 14057 yes User process
> pages
> FREE 740346 yes Free pages
> KERN_DATA 58152 no Dumpable kernel
> data
> page size: 4096
> Total pages on system: 925640
> Total size on system: 3791421440 Byte
> I understand that there might be some reservations about exporting
> such machine-specific details in the vmcoreinfo, but to unify
> the implementations across user-land and archs, perhaps this would be
> good starting point to start a discussion.
> [0].
> [1]. MAN pages -> MAKEDUMPFILE(8) and CRASH(8)
> [2].
> [3].
> Cc: Boris Petkov <bp@xxxxxxxxx>
> Cc: Baoquan He <bhe@xxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Kazuhito Hagio <k-hagio@xxxxxxxxxxxxx>
> Cc: Dave Anderson <anderson@xxxxxxxxxx>
> Cc: James Morse <james.morse@xxxxxxx>
> Cc: Omar Sandoval <osandov@xxxxxx>
> Cc: x86@xxxxxxxxxx
> Cc: kexec@xxxxxxxxxxxxxxxxxxx
> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> Signed-off-by: Bhupesh Sharma <bhsharma@xxxxxxxxxx>
> ---
> arch/x86/kernel/machine_kexec_64.c | 1 +
> 1 file changed, 1 insertion(+)
> diff --git a/arch/x86/kernel/machine_kexec_64.c b/arch/x86/kernel/machine_kexec_64.c
> index 4c8acdfdc5a7..834ccefef867 100644
> --- a/arch/x86/kernel/machine_kexec_64.c
> +++ b/arch/x86/kernel/machine_kexec_64.c
> @@ -356,6 +356,7 @@ void arch_crash_save_vmcoreinfo(void)
> VMCOREINFO_SYMBOL(init_top_pgt);
> vmcoreinfo_append_str("NUMBER(pgtable_l5_enabled)=%d\n",
> pgtable_l5_enabled());
> + VMCOREINFO_NUMBER(page_offset_base);
> #ifdef CONFIG_NUMA
> --
> 2.7.4