[PATCH v3 0/2] powerpc/crash: backup region offset update to eflcorehdr

[Sourabh Jain <sourabhjain@linux.ibm.com>]

On a kernel crash, purgatory copies the first 64 KB of the crashed
kernel memory to the backup memory region reserved within crashkernel
memory for the kdump kernel. This ensures that the capture kernel can
use the first 64 KB of memory to place the exception vectors and other
required data.

However, in the elfcorehdr prepared using the kexec_file_load syscall,
or the one recreated during a memory hotplug event, the offset of the
program header representing the first 64 KB of memory is set to 0,
whereas it should point to the backup region.

This has not caused issues so far because the first 64 KB is usually
identical in both the crashed and capture kernels. However, this is
only an assumption and is not guaranteed to always hold true.

Therefore, update the offset of the program header representing the
first 64 KB of the crashed kernel memory in the elfcorehdr prepared for
the kdump kernel.

Testing:
=======

Git Tree: https://github.com/sourabhjains/linux/commits/kdump-backup-region-offset/

The above tree contains an additional patch that introduces a new sysfs
interface to dump the kdump elfcorehdr to the dmesg/console.

$> cat /sys/kernel/kexec/crash_elfcorehdr_print 
ELF Program Header Summary:
  Total Program Headers: 13 (0xd)
  PT_LOAD segments:      4 (0x4)
  PT_NOTE segments:      9 (0x9)

Program Headers:
Index Type               Offset             VirtAddr           PhysAddr     FileSize     MemSize  Flags   
================================================================================
0x0   PT_NOTE            0x3fd966400        0x0                0x3fd966400        0x218              0x218              ---
0x1   PT_NOTE            0x3fd996400        0x0                0x3fd996400        0x218              0x218              ---
0x2   PT_NOTE            0x3fd9c6400        0x0                0x3fd9c6400        0x218              0x218              ---
0x3   PT_NOTE            0x3fd9f6400        0x0                0x3fd9f6400        0x218              0x218              ---
0x4   PT_NOTE            0x3fda26400        0x0                0x3fda26400        0x218              0x218              ---
0x5   PT_NOTE            0x3fda56400        0x0                0x3fda56400        0x218              0x218              ---
0x6   PT_NOTE            0x3fda86400        0x0                0x3fda86400        0x218              0x218              ---
0x7   PT_NOTE            0x3fdab6400        0x0                0x3fdab6400        0x218              0x218              ---
0x8   PT_NOTE            0xa2c0000          0x0                0xa2c0000          0x10024            0x10024            ---
0x9   PT_LOAD            0x1af40000         0xc000000000000000 0x0                0x10000            0x10000            RWX
0xa   PT_LOAD            0x10000            0xc000000000010000 0x10000            0x17ff0000         0x17ff0000         RWX
0xb   PT_LOAD            0x2ec70000         0xc00000002ec70000 0x2ec70000         0x1390000          0x1390000          RWX
0xc   PT_LOAD            0x58000000         0xc000000058000000 0x58000000         0x3a8000000        0x3a8000000        RWX

So, once kdump is successfully loaded using the kexec command,
for example:

$ kexec --initrd=/boot/initramfs-`uname -r`.img /boot/vmlinuz-`uname -r` --append="`cat /proc/cmdline`" -pds

Running cat /sys/kernel/kexec/crash_elfcorehdr_print prints the
elfcorehdr in the above format. The offset of the program header
representing the first 64 KB can be easily observed.

Changlog:

v1:
https://lore.kernel.org/all/20260303134722.2814049-1-sourabhjain@linux.ibm.com/

v2:
https://lore.kernel.org/all/20260307052102.944952-1-sourabhjain@linux.ibm.com/
 - Fix build error: 'struct kimage' has no member named 'arch'
   How?
   Define ARCH_HAS_KIMAGE_ARCH and struct kimage_arch when
   CONFIG_KEXEC_FILE or CONFIG_CRASH_DUMP is enabled so that
   kimage->arch.backup_start is available with the kexec_load syscall.

v3:
- Fix a typo in a commit message 01/02
- Add a comment for a helper function in 02/02
- Add Reviewed-by tag
- No functional changes

Cc: Hari Bathini <hbathini@linux.ibm.com>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Mahesh Salgaonkar <mahesh@linux.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Cc: Shivang Upadhyay <shivangu@linux.ibm.com>
Cc: Sourabh Jain <sourabhjain@linux.ibm.com>
Cc: Aditya Gupta <adityag@linux.ibm.com>

Sourabh Jain (2):
  powerpc/crash: fix backup region offset update to elfcorehdr
  powerpc/crash: Update backup region offset in elfcorehdr on memory
    hotplug

 arch/powerpc/include/asm/kexec.h  | 14 +++++--
 arch/powerpc/kexec/crash.c        | 64 +++++++++++++++++++++++++++++++
 arch/powerpc/kexec/file_load_64.c | 29 +-------------
 3 files changed, 76 insertions(+), 31 deletions(-)

-- 
2.52.0