Re: [Qemu-devel] [PATCH v2] dump-guest-memory.py: fix python 2 support

[Eric Blake]

On 01/19/2018 01:18 PM, Laszlo Ersek wrote:
> On 01/19/18 17:25, Marc-André Lureau wrote:
>> Python GDB support may use Python 2 or 3.
>>
>> Inferior.read_memory() may return a 'buffer' with Python 2 or a
>> 'memoryview' with Python 3 (see also
>> https://sourceware.org/gdb/onlinedocs/gdb/Inferiors-In-Python.html)
>>
>> The elf.add_vmcoreinfo_note() method expects a "bytes" object. Wrap
>> the returned memory with bytes(), which works with both 'memoryview'
>> and 'buffer'.
>>

If I understand it, the issue stems from the fact that vmcoreinfo is a
bytes buffer under python2, but a memoryview under python3.  Let's
compare the patches:

The V1 patch:
https://lists.gnu.org/archive/html/qemu-devel/2018-01/msg04010.html

>
>          if vmcoreinfo:
> -            self.elf.add_vmcoreinfo_note(vmcoreinfo.tobytes())
> +            # Python 2.7 returns a buffer
> +            vmciview = memoryview(vmcoreinfo)
> +            self.elf.add_vmcoreinfo_note(vmciview.tobytes())

did an unconditional memoryview(vmcoreinfo) followed by a .tobytes()
call. Under python 2.7, vmcoreinfo is converted from a buffer to a
memoryview, then back to bytes; under python 3, memoryview(vmcoreinfo)
is a no-op (since it is already a memory view), then that is converted
to bytes; the double conversion was necessary because bytes.tobytes()
doesn't exist.  But python 2.6 doesn't have the memoryview conversion,
so we've excluded older python users.

Now looking at the V2 patch:


>> @@ -564,7 +564,7 @@ shape and this command should mostly work."""
>>  
>>          vmcoreinfo = self.phys_memory_read(addr, size)
>>          if vmcoreinfo:
>> -            self.elf.add_vmcoreinfo_note(vmcoreinfo.tobytes())
>> +            self.elf.add_vmcoreinfo_note(bytes(vmcoreinfo))

Under python2, and bytes(buffer) is still bytes; under python3,
vmcoreinfo is a memoryview but bytes(buffer) is equivalent to
buffer.to_bytes().  We've avoided the intermediate conversion through
memoryview, and thus work regardless of python version.

> Ugh, can you please point me to the v1->v2 difference?

Hope that helped (and hope I got it right, I'm learning as well from
this thread).

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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