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

[Bhupesh Sharma]

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.

   TYPE		PAGES			EXCLUDABLE	DESCRIPTION
   ----------------------------------------------------------------------
   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]. https://www.mail-archive.com/kexec@xxxxxxxxxxxxxxxxxxx/msg20300.html
[1]. MAN pages -> MAKEDUMPFILE(8) and CRASH(8)
[2]. https://www.spinics.net/lists/kexec/msg21608.html
     http://lists.infradead.org/pipermail/kexec/2018-October/021725.html
[3]. https://github.com/bhupesh-sharma/makedumpfile/tree/add-page-offset-base-to-vmcore-v1

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
 	VMCOREINFO_SYMBOL(node_data);
-- 
2.7.4