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Subject[PATCH v2] x86_64, vmcoreinfo: Append 'page_offset_base' to vmcoreinfo
x86_64 kernel uses 'page_offset_base' variable to point to the
start of direct mapping of all physical memory. This variable
is also updated for KASLR boot cases, so this can be exported
via vmcoreinfo as a standard ABI between kernel and user-space,
to allow user-space utilities to use the same for calculating
the start of direct mapping of all physical memory.

'arch/x86/kernel/head64.c' sets the same as:
unsigned long page_offset_base __ro_after_init = __PAGE_OFFSET_BASE_L4;

and also uses the same to indicate the base of KASLR regions on x86_64:
static __initdata struct kaslr_memory_region {
unsigned long *base;
unsigned long size_tb;
} kaslr_regions[] = {
{ &page_offset_base, 0 },
.. snip ..

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]).

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.

Such user-space issues can be resolved if the user-space code instead
uses a standard ABI to read the kernel exposed machine specific
variables. With the kernel commit 23c85094fe1895caefdd
["proc/kcore: add vmcoreinfo note to /proc/kcore"]), it is
now possible to use the vmcoreinfo present inside kcore as the standard
ABI which can be used by the user-space utilities for reading
the machine specific information (and hence for debugging a
live kernel).

User-space utilities like makedumpfile, kexec-tools and crash
are either already using this ABI or are discussing patches
which look to add the same feature. This helps in simplifying the
overall code and also in reducing code-rewrite across the
user-space utilities for getting values of these kernel

Accordingly 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.

- 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
FREE 740346 yes Free pages
KERN_DATA 58152 no Dumpable kernel

page size: 4096
Total pages on system: 925640
Total size on system: 3791421440 Byte

[1]. MAN pages -> MAKEDUMPFILE(8) and CRASH(8)
[2]. makedumpfile issue with latest kernels ->

Cc: Boris Petkov <>
Cc: Baoquan He <>
Cc: Ingo Molnar <>
Cc: Thomas Gleixner <>
Cc: Kazuhito Hagio <>
Cc: Dave Anderson <>
Cc: James Morse <>
Cc: Omar Sandoval <>
Signed-off-by: Bhupesh Sharma <>
Changes since v1:
- Fixed the build issue reported by build bot and tested this version
with 'make allmodconfig'.
- Reworded most of the commit log to explain the intent behind the

arch/x86/kernel/machine_kexec_64.c | 3 +++
1 file changed, 3 insertions(+)

diff --git a/arch/x86/kernel/machine_kexec_64.c b/arch/x86/kernel/machine_kexec_64.c
index 4c8acdfdc5a7..6161d77c5bfb 100644
--- a/arch/x86/kernel/machine_kexec_64.c
+++ b/arch/x86/kernel/machine_kexec_64.c
@@ -356,6 +356,9 @@ void arch_crash_save_vmcoreinfo(void)
+ VMCOREINFO_NUMBER(page_offset_base);

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