Python-lang gdb script to extract x86_64 guest vmcore from qemu coredump

When qemu dies unexpectedly, for example in response to an explicit
abort() call, or (more importantly) when an external signal is delivered
to it that results in a coredump, sometimes it is useful to extract the
guest vmcore from the qemu process' memory image. The guest vmcore might
help understand an emulation problem in qemu, or help debug the guest.

This script reimplements (and cuts many features of) the
qmp_dump_guest_memory() command in gdb/Python,

  https://sourceware.org/gdb/current/onlinedocs/gdb/Python-API.html

working off the saved memory image of the qemu process. The docstring in
the patch (serving as gdb help text) describes the limitations relative to
the QMP command.

Dependencies of qmp_dump_guest_memory() have been reimplemented as needed.
I sought to follow the general structure, sticking to original function
names where possible. However, keeping it simple prevailed in some places.

The patch has been tested with a 4 VCPU, 768 MB, RHEL-6.4
(2.6.32-358.el6.x86_64) guest:

- The script printed

> guest RAM blocks:
> target_start     target_end       host_addr        message count
> ---------------- ---------------- ---------------- ------- -----
> 0000000000000000 00000000000a0000 00007f95d0000000 added       1
> 00000000000a0000 00000000000b0000 00007f960ac00000 added       2
> 00000000000c0000 00000000000ca000 00007f95d00c0000 added       3
> 00000000000ca000 00000000000cd000 00007f95d00ca000 joined      3
> 00000000000cd000 00000000000d0000 00007f95d00cd000 joined      3
> 00000000000d0000 00000000000f0000 00007f95d00d0000 joined      3
> 00000000000f0000 0000000000100000 00007f95d00f0000 joined      3
> 0000000000100000 0000000030000000 00007f95d0100000 joined      3
> 00000000fc000000 00000000fc800000 00007f960ac00000 added       4
> 00000000fffe0000 0000000100000000 00007f9618800000 added       5
> dumping range at 00007f95d0000000 for length 00000000000a0000
> dumping range at 00007f960ac00000 for length 0000000000010000
> dumping range at 00007f95d00c0000 for length 000000002ff40000
> dumping range at 00007f960ac00000 for length 0000000000800000
> dumping range at 00007f9618800000 for length 0000000000020000

- The vmcore was checked with "readelf", comparing the results against a
  vmcore written by qmp_dump_guest_memory():

> --- theirs      2013-09-12 17:38:59.797289404 +0200
> +++ mine        2013-09-12 17:39:03.820289404 +0200
> @@ -27,16 +27,16 @@
>    Type           Offset             VirtAddr           PhysAddr
>                   FileSiz            MemSiz              Flags  Align
>    NOTE           0x0000000000000190 0x0000000000000000 0x0000000000000000
> -                 0x0000000000000ca0 0x0000000000000ca0         0
> -  LOAD           0x0000000000000e30 0x0000000000000000 0x0000000000000000
> +                 0x000000000000001c 0x000000000000001c         0
> +  LOAD           0x00000000000001ac 0x0000000000000000 0x0000000000000000
>                   0x00000000000a0000 0x00000000000a0000         0
> -  LOAD           0x00000000000a0e30 0x0000000000000000 0x00000000000a0000
> +  LOAD           0x00000000000a01ac 0x0000000000000000 0x00000000000a0000
>                   0x0000000000010000 0x0000000000010000         0
> -  LOAD           0x00000000000b0e30 0x0000000000000000 0x00000000000c0000
> +  LOAD           0x00000000000b01ac 0x0000000000000000 0x00000000000c0000
>                   0x000000002ff40000 0x000000002ff40000         0
> -  LOAD           0x000000002fff0e30 0x0000000000000000 0x00000000fc000000
> +  LOAD           0x000000002fff01ac 0x0000000000000000 0x00000000fc000000
>                   0x0000000000800000 0x0000000000800000         0
> -  LOAD           0x00000000307f0e30 0x0000000000000000 0x00000000fffe0000
> +  LOAD           0x00000000307f01ac 0x0000000000000000 0x00000000fffe0000
>                   0x0000000000020000 0x0000000000020000         0
>
>  There is no dynamic section in this file.
> @@ -47,13 +47,6 @@
>
>  No version information found in this file.
>
> -Notes at offset 0x00000190 with length 0x00000ca0:
> +Notes at offset 0x00000190 with length 0x0000001c:
>    Owner                Data size       Description
> -  CORE         0x00000150      NT_PRSTATUS (prstatus structure)
> -  CORE         0x00000150      NT_PRSTATUS (prstatus structure)
> -  CORE         0x00000150      NT_PRSTATUS (prstatus structure)
> -  CORE         0x00000150      NT_PRSTATUS (prstatus structure)
> -  QEMU         0x000001b0      Unknown note type: (0x00000000)
> -  QEMU         0x000001b0      Unknown note type: (0x00000000)
> -  QEMU         0x000001b0      Unknown note type: (0x00000000)
> -  QEMU         0x000001b0      Unknown note type: (0x00000000)
> +  NONE         0x00000005      Unknown note type: (0x00000000)

- The vmcore was checked with "crash" too, again comparing the results
  against a vmcore written by qmp_dump_guest_memory():

> --- guest.vmcore.log2   2013-09-12 17:52:27.074289201 +0200
> +++ example.dump.log2   2013-09-12 17:52:15.904289203 +0200
> @@ -22,11 +22,11 @@
>  This GDB was configured as "x86_64-unknown-linux-gnu"...
>
>       KERNEL: /usr/lib/debug/lib/modules/2.6.32-358.el6.x86_64/vmlinux
> -    DUMPFILE: /home/lacos/tmp/guest.vmcore
> +    DUMPFILE: /home/lacos/tmp/example.dump
>          CPUS: 4
> -        DATE: Thu Sep 12 17:16:11 2013
> -      UPTIME: 00:01:09
> -LOAD AVERAGE: 0.07, 0.03, 0.00
> +        DATE: Thu Sep 12 17:17:41 2013
> +      UPTIME: 00:00:38
> +LOAD AVERAGE: 0.18, 0.05, 0.01
>         TASKS: 130
>      NODENAME: localhost.localdomain
>       RELEASE: 2.6.32-358.el6.x86_64
> @@ -38,12 +38,12 @@
>       COMMAND: "swapper"
>          TASK: ffffffff81a8d020  (1 of 4)  [THREAD_INFO: ffffffff81a00000]
>           CPU: 0
> -       STATE: TASK_RUNNING (PANIC)
> +       STATE: TASK_RUNNING (ACTIVE)
> +     WARNING: panic task not found
>
>  crash> bt
>  PID: 0      TASK: ffffffff81a8d020  CPU: 0   COMMAND: "swapper"
> - #0 [ffffffff81a01ed0] default_idle at ffffffff8101495d
> - #1 [ffffffff81a01ef0] cpu_idle at ffffffff81009fc6
> + #0 [ffffffff81a01ef0] cpu_idle at ffffffff81009fc6
>  crash> task ffffffff81a8d020
>  PID: 0      TASK: ffffffff81a8d020  CPU: 0   COMMAND: "swapper"
>  struct task_struct {
> @@ -75,7 +75,7 @@
>        prev = 0xffffffff81a8d080
>      },
>      on_rq = 0,
> -    exec_start = 8618466836,
> +    exec_start = 7469214014,
>      sum_exec_runtime = 0,
>      vruntime = 0,
>      prev_sum_exec_runtime = 0,
> @@ -149,7 +149,7 @@
>    },
>    tasks = {
>      next = 0xffff88002d621948,
> -    prev = 0xffff880029618f28
> +    prev = 0xffff880023b74488
>    },
>    pushable_tasks = {
>      prio = 140,
> @@ -165,7 +165,7 @@
>      }
>    },
>    mm = 0x0,
> -  active_mm = 0xffff88002929b780,
> +  active_mm = 0xffff8800297eb980,
>    exit_state = 0,
>    exit_code = 0,
>    exit_signal = 0,
> @@ -177,7 +177,7 @@
>    sched_reset_on_fork = 0,
>    pid = 0,
>    tgid = 0,
> -  stack_canary = 2483693585637059287,
> +  stack_canary = 7266362296181431986,
>    real_parent = 0xffffffff81a8d020,
>    parent = 0xffffffff81a8d020,
>    children = {
> @@ -224,14 +224,14 @@
>    set_child_tid = 0x0,
>    clear_child_tid = 0x0,
>    utime = 0,
> -  stime = 3,
> +  stime = 2,
>    utimescaled = 0,
> -  stimescaled = 3,
> +  stimescaled = 2,
>    gtime = 0,
>    prev_utime = 0,
>    prev_stime = 0,
>    nvcsw = 0,
> -  nivcsw = 1000,
> +  nivcsw = 1764,
>    start_time = {
>      tv_sec = 0,
>      tv_nsec = 0

- <name_dropping>I asked for Dave Anderson's help with verifying the
  extracted vmcore, and his comments make me think I should post
  this.</name_dropping>
Signed-off-by: NLaszlo Ersek <lersek@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

scripts/dump-guest-memory.py

+# This python script adds a new gdb command, "dump-guest-memory". It
+# should be loaded with "source dump-guest-memory.py" at the (gdb)
+# prompt.
+#
+# Copyright (C) 2013, Red Hat, Inc.
+#
+# Authors:
+#   Laszlo Ersek <lersek@redhat.com>
+#
+# This work is licensed under the terms of the GNU GPL, version 2 or later. See
+# the COPYING file in the top-level directory.
+#
+# The leading docstring doesn't have idiomatic Python formatting. It is
+# printed by gdb's "help" command (the first line is printed in the
+# "help data" summary), and it should match how other help texts look in
+# gdb.
+
+import struct
+
+class DumpGuestMemory(gdb.Command):
+    """Extract guest vmcore from qemu process coredump.
+
+The sole argument is FILE, identifying the target file to write the
+guest vmcore to.
+
+This GDB command reimplements the dump-guest-memory QMP command in
+python, using the representation of guest memory as captured in the qemu
+coredump. The qemu process that has been dumped must have had the
+command line option "-machine dump-guest-core=on".
+
+For simplicity, the "paging", "begin" and "end" parameters of the QMP
+command are not supported -- no attempt is made to get the guest's
+internal paging structures (ie. paging=false is hard-wired), and guest
+memory is always fully dumped.
+
+Only x86_64 guests are supported.
+
+The CORE/NT_PRSTATUS and QEMU notes (that is, the VCPUs' statuses) are
+not written to the vmcore. Preparing these would require context that is
+only present in the KVM host kernel module when the guest is alive. A
+fake ELF note is written instead, only to keep the ELF parser of "crash"
+happy.
+
+Dependent on how busted the qemu process was at the time of the
+coredump, this command might produce unpredictable results. If qemu
+deliberately called abort(), or it was dumped in response to a signal at
+a halfway fortunate point, then its coredump should be in reasonable
+shape and this command should mostly work."""
+
+    TARGET_PAGE_SIZE = 0x1000
+    TARGET_PAGE_MASK = 0xFFFFFFFFFFFFF000
+
+    # Various ELF constants
+    EM_X86_64   = 62        # AMD x86-64 target machine
+    ELFDATA2LSB = 1         # little endian
+    ELFCLASS64  = 2
+    ELFMAG      = "\x7FELF"
+    EV_CURRENT  = 1
+    ET_CORE     = 4
+    PT_LOAD     = 1
+    PT_NOTE     = 4
+
+    # Special value for e_phnum. This indicates that the real number of
+    # program headers is too large to fit into e_phnum. Instead the real
+    # value is in the field sh_info of section 0.
+    PN_XNUM = 0xFFFF
+
+    # Format strings for packing and header size calculation.
+    ELF64_EHDR = ("4s" # e_ident/magic
+                  "B"  # e_ident/class
+                  "B"  # e_ident/data
+                  "B"  # e_ident/version
+                  "B"  # e_ident/osabi
+                  "8s" # e_ident/pad
+                  "H"  # e_type
+                  "H"  # e_machine
+                  "I"  # e_version
+                  "Q"  # e_entry
+                  "Q"  # e_phoff
+                  "Q"  # e_shoff
+                  "I"  # e_flags
+                  "H"  # e_ehsize
+                  "H"  # e_phentsize
+                  "H"  # e_phnum
+                  "H"  # e_shentsize
+                  "H"  # e_shnum
+                  "H"  # e_shstrndx
+                 )
+    ELF64_PHDR = ("I"  # p_type
+                  "I"  # p_flags
+                  "Q"  # p_offset
+                  "Q"  # p_vaddr
+                  "Q"  # p_paddr
+                  "Q"  # p_filesz
+                  "Q"  # p_memsz
+                  "Q"  # p_align
+                 )
+
+    def __init__(self):
+        super(DumpGuestMemory, self).__init__("dump-guest-memory",
+                                              gdb.COMMAND_DATA,
+                                              gdb.COMPLETE_FILENAME)
+        self.uintptr_t     = gdb.lookup_type("uintptr_t")
+        self.elf64_ehdr_le = struct.Struct("<%s" % self.ELF64_EHDR)
+        self.elf64_phdr_le = struct.Struct("<%s" % self.ELF64_PHDR)
+
+    def int128_get64(self, val):
+        assert (val["hi"] == 0)
+        return val["lo"]
+
+    def qtailq_foreach(self, head, field_str):
+        var_p = head["tqh_first"]
+        while (var_p != 0):
+            var = var_p.dereference()
+            yield var
+            var_p = var[field_str]["tqe_next"]
+
+    def qemu_get_ram_block(self, ram_addr):
+        ram_blocks = gdb.parse_and_eval("ram_list.blocks")
+        for block in self.qtailq_foreach(ram_blocks, "next"):
+            if (ram_addr - block["offset"] < block["length"]):
+                return block
+        raise gdb.GdbError("Bad ram offset %x" % ram_addr)
+
+    def qemu_get_ram_ptr(self, ram_addr):
+        block = self.qemu_get_ram_block(ram_addr)
+        return block["host"] + (ram_addr - block["offset"])
+
+    def memory_region_get_ram_ptr(self, mr):
+        if (mr["alias"] != 0):
+            return (self.memory_region_get_ram_ptr(mr["alias"].dereference()) +
+                    mr["alias_offset"])
+        return self.qemu_get_ram_ptr(mr["ram_addr"] & self.TARGET_PAGE_MASK)
+
+    def guest_phys_blocks_init(self):
+        self.guest_phys_blocks = []
+
+    def guest_phys_blocks_append(self):
+        print "guest RAM blocks:"
+        print ("target_start     target_end       host_addr        message "
+               "count")
+        print ("---------------- ---------------- ---------------- ------- "
+               "-----")
+
+        current_map_p = gdb.parse_and_eval("address_space_memory.current_map")
+        current_map = current_map_p.dereference()
+        for cur in range(current_map["nr"]):
+            flat_range   = (current_map["ranges"] + cur).dereference()
+            mr           = flat_range["mr"].dereference()
+
+            # we only care about RAM
+            if (not mr["ram"]):
+                continue
+
+            section_size = self.int128_get64(flat_range["addr"]["size"])
+            target_start = self.int128_get64(flat_range["addr"]["start"])
+            target_end   = target_start + section_size
+            host_addr    = (self.memory_region_get_ram_ptr(mr) +
+                            flat_range["offset_in_region"])
+            predecessor = None
+
+            # find continuity in guest physical address space
+            if (len(self.guest_phys_blocks) > 0):
+                predecessor = self.guest_phys_blocks[-1]
+                predecessor_size = (predecessor["target_end"] -
+                                    predecessor["target_start"])
+
+                # the memory API guarantees monotonically increasing
+                # traversal
+                assert (predecessor["target_end"] <= target_start)
+
+                # we want continuity in both guest-physical and
+                # host-virtual memory
+                if (predecessor["target_end"] < target_start or
+                    predecessor["host_addr"] + predecessor_size != host_addr):
+                    predecessor = None
+
+            if (predecessor is None):
+                # isolated mapping, add it to the list
+                self.guest_phys_blocks.append({"target_start": target_start,
+                                               "target_end"  : target_end,
+                                               "host_addr"   : host_addr})
+                message = "added"
+            else:
+                # expand predecessor until @target_end; predecessor's
+                # start doesn't change
+                predecessor["target_end"] = target_end
+                message = "joined"
+
+            print ("%016x %016x %016x %-7s %5u" %
+                   (target_start, target_end, host_addr.cast(self.uintptr_t),
+                    message, len(self.guest_phys_blocks)))
+
+    def cpu_get_dump_info(self):
+        # We can't synchronize the registers with KVM post-mortem, and
+        # the bits in (first_x86_cpu->env.hflags) seem to be stale; they
+        # may not reflect long mode for example. Hence just assume the
+        # most common values. This also means that instruction pointer
+        # etc. will be bogus in the dump, but at least the RAM contents
+        # should be valid.
+        self.dump_info = {"d_machine": self.EM_X86_64,
+                          "d_endian" : self.ELFDATA2LSB,
+                          "d_class"  : self.ELFCLASS64}
+
+    def encode_elf64_ehdr_le(self):
+        return self.elf64_ehdr_le.pack(
+                                 self.ELFMAG,                 # e_ident/magic
+                                 self.dump_info["d_class"],   # e_ident/class
+                                 self.dump_info["d_endian"],  # e_ident/data
+                                 self.EV_CURRENT,             # e_ident/version
+                                 0,                           # e_ident/osabi
+                                 "",                          # e_ident/pad
+                                 self.ET_CORE,                # e_type
+                                 self.dump_info["d_machine"], # e_machine
+                                 self.EV_CURRENT,             # e_version
+                                 0,                           # e_entry
+                                 self.elf64_ehdr_le.size,     # e_phoff
+                                 0,                           # e_shoff
+                                 0,                           # e_flags
+                                 self.elf64_ehdr_le.size,     # e_ehsize
+                                 self.elf64_phdr_le.size,     # e_phentsize
+                                 self.phdr_num,               # e_phnum
+                                 0,                           # e_shentsize
+                                 0,                           # e_shnum
+                                 0                            # e_shstrndx
+                                )
+
+    def encode_elf64_note_le(self):
+        return self.elf64_phdr_le.pack(self.PT_NOTE,         # p_type
+                                       0,                    # p_flags
+                                       (self.memory_offset -
+                                        len(self.note)),     # p_offset
+                                       0,                    # p_vaddr
+                                       0,                    # p_paddr
+                                       len(self.note),       # p_filesz
+                                       len(self.note),       # p_memsz
+                                       0                     # p_align
+                                      )
+
+    def encode_elf64_load_le(self, offset, start_hwaddr, range_size):
+        return self.elf64_phdr_le.pack(self.PT_LOAD, # p_type
+                                       0,            # p_flags
+                                       offset,       # p_offset
+                                       0,            # p_vaddr
+                                       start_hwaddr, # p_paddr
+                                       range_size,   # p_filesz
+                                       range_size,   # p_memsz
+                                       0             # p_align
+                                      )
+
+    def note_init(self, name, desc, type):
+        # name must include a trailing NUL
+        namesz = (len(name) + 1 + 3) / 4 * 4
+        descsz = (len(desc)     + 3) / 4 * 4
+        fmt = ("<"   # little endian
+               "I"   # n_namesz
+               "I"   # n_descsz
+               "I"   # n_type
+               "%us" # name
+               "%us" # desc
+               % (namesz, descsz))
+        self.note = struct.pack(fmt,
+                                len(name) + 1, len(desc), type, name, desc)
+
+    def dump_init(self):
+        self.guest_phys_blocks_init()
+        self.guest_phys_blocks_append()
+        self.cpu_get_dump_info()
+        # we have no way to retrieve the VCPU status from KVM
+        # post-mortem
+        self.note_init("NONE", "EMPTY", 0)
+
+        # Account for PT_NOTE.
+        self.phdr_num = 1
+
+        # We should never reach PN_XNUM for paging=false dumps: there's
+        # just a handful of discontiguous ranges after merging.
+        self.phdr_num += len(self.guest_phys_blocks)
+        assert (self.phdr_num < self.PN_XNUM)
+
+        # Calculate the ELF file offset where the memory dump commences:
+        #
+        #   ELF header
+        #   PT_NOTE
+        #   PT_LOAD: 1
+        #   PT_LOAD: 2
+        #   ...
+        #   PT_LOAD: len(self.guest_phys_blocks)
+        #   ELF note
+        #   memory dump
+        self.memory_offset = (self.elf64_ehdr_le.size +
+                              self.elf64_phdr_le.size * self.phdr_num +
+                              len(self.note))
+
+    def dump_begin(self, vmcore):
+        vmcore.write(self.encode_elf64_ehdr_le())
+        vmcore.write(self.encode_elf64_note_le())
+        running = self.memory_offset
+        for block in self.guest_phys_blocks:
+            range_size = block["target_end"] - block["target_start"]
+            vmcore.write(self.encode_elf64_load_le(running,
+                                                   block["target_start"],
+                                                   range_size))
+            running += range_size
+        vmcore.write(self.note)
+
+    def dump_iterate(self, vmcore):
+        qemu_core = gdb.inferiors()[0]
+        for block in self.guest_phys_blocks:
+            cur  = block["host_addr"]
+            left = block["target_end"] - block["target_start"]
+            print ("dumping range at %016x for length %016x" %
+                   (cur.cast(self.uintptr_t), left))
+            while (left > 0):
+                chunk_size = min(self.TARGET_PAGE_SIZE, left)
+                chunk = qemu_core.read_memory(cur, chunk_size)
+                vmcore.write(chunk)
+                cur  += chunk_size
+                left -= chunk_size
+
+    def create_vmcore(self, filename):
+        vmcore = open(filename, "wb")
+        self.dump_begin(vmcore)
+        self.dump_iterate(vmcore)
+        vmcore.close()
+
+    def invoke(self, args, from_tty):
+        # Unwittingly pressing the Enter key after the command should
+        # not dump the same multi-gig coredump to the same file.
+        self.dont_repeat()
+
+        argv = gdb.string_to_argv(args)
+        if (len(argv) != 1):
+            raise gdb.GdbError("usage: dump-guest-memory FILE")
+
+        self.dump_init()
+        self.create_vmcore(argv[0])
+
+DumpGuestMemory()