scripts/gdb/symbols: determine KASLR offset on s390 during early boot

Using lx-symbols during s390 early boot fails with:

    Error occurred in Python: 'utf-8' codec can't decode byte 0xcb in position 0: invalid continuation byte

The reason is that s390 decompressor's startup_kernel() does not create
vmcoreinfo note, and sets vmcore_info to kernel's physical base.  This
confuses get_vmcore_s390().

Fix by handling this special case.  Extract vm_layout.kaslr_offset from
the kernel image in physical memory, which is placed there by the
decompressor using the __bootdata_preserved mechanism, and generate a
synthetic vmcoreinfo note from it.

Link: https://lkml.kernel.org/r/20250515155811.114392-4-iii@linux.ibm.com
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Kieran Bingham <kbingham@kernel.org>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: iii-i

scripts/gdb/linux/symbols.py

@@ -54,6 +54,18 @@ def get_vmcore_s390():
         vmcore_info = 0x0e0c
         paddr_vmcoreinfo_note = gdb.parse_and_eval("*(unsigned long long *)" +
                                                    hex(vmcore_info))
+        if paddr_vmcoreinfo_note == 0 or paddr_vmcoreinfo_note & 1:
+            # In the early boot case, extract vm_layout.kaslr_offset from the
+            # vmlinux image in physical memory.
+            if paddr_vmcoreinfo_note == 0:
+                kaslr_offset_phys = 0
+            else:
+                kaslr_offset_phys = paddr_vmcoreinfo_note - 1
+            with utils.pagination_off():
+                gdb.execute("symbol-file {0} -o {1}".format(
+                    utils.get_vmlinux(), hex(kaslr_offset_phys)))
+            kaslr_offset = gdb.parse_and_eval("vm_layout.kaslr_offset")
+            return "KERNELOFFSET=" + hex(kaslr_offset)[2:]
         inferior = gdb.selected_inferior()
         elf_note = inferior.read_memory(paddr_vmcoreinfo_note, 12)
         n_namesz, n_descsz, n_type = struct.unpack(">III", elf_note)