scripts/gdb/symbols: determine KASLR offset on s390

Use QEMU's qemu.PhyMemMode [1] functionality to read vmcore from the
physical memory the same way the existing dump tooling does this. 
Gracefully handle non-QEMU targets, early boot, and memory corruptions;
print a warning if such situation is detected.

[1] https://qemu-project.gitlab.io/qemu/system/gdb.html#examining-physical-memory

Link: https://lkml.kernel.org/r/20250303110437.79070-1-iii@linux.ibm.com
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Acked-by: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: Andrew Donnellan <ajd@linux.ibm.com>
Cc: Christian Borntraeger <borntraeger@linux.ibm.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Kieran Bingham <kbingham@kernel.org>
Cc: Nina Schoetterl-Glausch <nsg@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: iii-i

scripts/gdb/linux/symbols.py

@@ -14,6 +14,7 @@
 import gdb
 import os
 import re
+import struct
 
 from itertools import count
 from linux import modules, utils, constants
@@ -54,6 +55,29 @@ def stop(self):
             return False
 
 
+def get_vmcore_s390():
+    with utils.qemu_phy_mem_mode():
+        vmcore_info = 0x0e0c
+        paddr_vmcoreinfo_note = gdb.parse_and_eval("*(unsigned long long *)" +
+                                                   hex(vmcore_info))
+        inferior = gdb.selected_inferior()
+        elf_note = inferior.read_memory(paddr_vmcoreinfo_note, 12)
+        n_namesz, n_descsz, n_type = struct.unpack(">III", elf_note)
+        desc_paddr = paddr_vmcoreinfo_note + len(elf_note) + n_namesz + 1
+        return gdb.parse_and_eval("(char *)" + hex(desc_paddr)).string()
+
+
+def get_kerneloffset():
+    if utils.is_target_arch('s390'):
+        try:
+            vmcore_str = get_vmcore_s390()
+        except gdb.error as e:
+            gdb.write("{}\n".format(e))
+            return None
+        return utils.parse_vmcore(vmcore_str).kerneloffset
+    return None
+
+
 class LxSymbols(gdb.Command):
     """(Re-)load symbols of Linux kernel and currently loaded modules.
 
@@ -160,7 +184,12 @@ def load_all_symbols(self):
                 obj.filename.endswith('vmlinux.debug')):
                 orig_vmlinux = obj.filename
         gdb.execute("symbol-file", to_string=True)
-        gdb.execute("symbol-file {0}".format(orig_vmlinux))
+        kerneloffset = get_kerneloffset()
+        if kerneloffset is None:
+            offset_arg = ""
+        else:
+            offset_arg = " -o " + hex(kerneloffset)
+        gdb.execute("symbol-file {0}{1}".format(orig_vmlinux, offset_arg))
 
         self.loaded_modules = []
         module_list = modules.module_list()

scripts/gdb/linux/utils.py

@@ -11,6 +11,11 @@
 # This work is licensed under the terms of the GNU GPL version 2.
 #
 
+import contextlib
+import dataclasses
+import re
+import typing
+
 import gdb
 
 
@@ -216,3 +221,33 @@ def gdb_eval_or_none(expresssion):
         return gdb.parse_and_eval(expresssion)
     except gdb.error:
         return None
+
+
+@contextlib.contextmanager
+def qemu_phy_mem_mode():
+    connection = gdb.selected_inferior().connection
+    orig = connection.send_packet("qqemu.PhyMemMode")
+    if orig not in b"01":
+        raise gdb.error("Unexpected qemu.PhyMemMode")
+    orig = orig.decode()
+    if connection.send_packet("Qqemu.PhyMemMode:1") != b"OK":
+        raise gdb.error("Failed to set qemu.PhyMemMode")
+    try:
+        yield
+    finally:
+        if connection.send_packet("Qqemu.PhyMemMode:" + orig) != b"OK":
+            raise gdb.error("Failed to restore qemu.PhyMemMode")
+
+
+@dataclasses.dataclass
+class VmCore:
+    kerneloffset: typing.Optional[int]
+
+
+def parse_vmcore(s):
+    match = re.search(r"KERNELOFFSET=([0-9a-f]+)", s)
+    if match is None:
+        kerneloffset = None
+    else:
+        kerneloffset = int(match.group(1), 16)
+    return VmCore(kerneloffset=kerneloffset)