[PATCH] kdump, x86: report actual value of phys_base in VMCOREINFO

HATAYAMA Daisuke d.hatayama at jp.fujitsu.com
Tue Nov 11 22:40:42 PST 2014

Currently, VMCOREINFO note information reports the virtual address of
phys_base that is assigned to symbol phys_base. But this doesn't make
sense because to refer to value of the phys_base, it's necessary to
get the value of phys_base itself we are now about to refer to.

Userland tools related to kdump such as makedumpfile and crash utility
so far have made some efforts to calculate phys_base from memory
mapping information on a variety of crash dump formats. But there's no
guarantee to keep maintaining it in the future.

This is also useful for crash dump mechanism running outside Linux
kernel such as virtual machine hypervisor such as qemu dump, which
ordinary users use via virsh dump, or ones implemented on vendor
specific firmware. They cannot get phys_base without special mechanism
because phys_base is kernel information.

To get VMCOREINFO in vmcore, it's easy to use strings and grep
commands like this; VMCOREINFO consists of simple string:

$ strings vmcore-3.10.0-121.el7.x86_64 | grep -E ".*VMCOREINFO.*" -A 100

Similarly, this is also useful to get value of phys_base in kdump 2nd
kernel contained in vmcore using the above-mentioned external crash
dump mechanism; kdump 2nd kernel is an inherently relocated kernel.

This commit doesn't remove VMCOREINFO_SYMBOL(phys_base) line because
makedumpfile refers to it and if removing it, old versions
makedumpfile doesn't work well.

Signed-off-by: HATAYAMA Daisuke <d.hatayama at jp.fujitsu.com>
 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 4859810..e6d00a4 100644
--- a/arch/x86/kernel/machine_kexec_64.c
+++ b/arch/x86/kernel/machine_kexec_64.c
@@ -334,6 +334,7 @@ void arch_crash_save_vmcoreinfo(void)
 			      (unsigned long)&_text - __START_KERNEL);
+	VMCOREINFO_LENGTH(phys_base, phys_base);
 /* arch-dependent functionality related to kexec file-based syscall */

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