Re: [Qemu-devel] dump-guest-memory enhancement.

[Phi Debian]

Hi All,

Sorry Laszlo for flooding your mailbox, I missed the 'reply to all' so
I redo the post here.

And I added some more comment at the end to answer your questions.
Phi

==========================================================
CU82$ /usr/bin/readelf -a vmcore
ELF Header:
  Magic:   7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00
  Class:                             ELF32
  Data:                              2's complement, little endian
  Version:                           1 (current)
  OS/ABI:                            UNIX - System V
  ABI Version:                       0
  Type:                              CORE (Core file)
  Machine:                           ARM
  Version:                           0x1
  Entry point address:               0x0
  Start of program headers:          52 (bytes into file)
  Start of section headers:          0 (bytes into file)
  Flags:                             0x0
  Size of this header:               52 (bytes)
  Size of program headers:           32 (bytes)
  Number of program headers:         2
  Size of section headers:           0 (bytes)
  Number of section headers:         0
  Section header string table index: 0

There are no sections in this file.

There are no sections to group in this file.

Program Headers:
  Type     Offset      VirtAddr        PhysAddr   FileSiz    MemSiz  Flg Align
  NOTE   0x000074 0x00000000 0x00000000 0x000a0 0x000a0     0
  LOAD   0x000114 0x60000000 0x60000000 0x40000000 0x40000000     0

There is no dynamic section in this file.

There are no relocations in this file.

No version information found in this file.

Notes at offset 0x00000074 with length 0x000000a0:
  Owner                 Data size       Description
  CORE                 0x0000008c       NT_PRSTATUS (prstatus structure)


========================================================

The Align fot the PT_LOAD is ZERO, then the offset is 0x114, having an
Align set to TARGET_PAGE_BITS, (or at least 4Kb) would provide a
chance for any debugger to do page align copy (either lseek/read, or
mmap) as they trip on the core, marginal detail, but may help.

As an example, a userland main(){abort();} kind of prog would produce
a core file like this.

CM01$ readelf -a core.2000
...
    LOAD off    0x0000000000001000 vaddr 0x0000000000400000 paddr
0x0000000000000000 align 2**12
Program Headers:
  Type           Offset   VirtAddr   PhysAddr   FileSiz MemSiz  Flg Align
  NOTE           0x0001d4 0x00000000 0x00000000 0x001d8 0x00000     0
  LOAD           0x001000 0x00a42000 0x00000000 0x00000 0x1b000 R E 0x1000
  LOAD           0x002000 0x00a5e000 0x00000000 0x01000 0x01000 RW  0x1000

The align for LOAD's is 0x1000 thus the file offset is 0x01000, 0x2000 etc...

==========================================================

I guess dump-guest-memory is of a marginal use, yet it can be usefull
when kexec/kdump is broken or non existant on some new architecture
(os/arch bring up).

So to answer your question, the content of the PT_LOAD is ok, only its
offset is non aligned.

I got to precise I obtained this vmcore by implementing the arc_arm
part of the qemu dump-guest-memory, and planing to do the same for
arm64, I may have mis-used the QEMU API's, but for what I can read,
the align member is left non initialised after a memset(zero) of the
phdr/shdr i.e it is left at zero, and I got the impression that the
wayt the elf is produced, section/progs alignment was not in mind. So
I guess other arch are not aligned either, I did not test that.

Cheers,
Phi