Re: [Xen-devel] [PATCH v3 0/2] Clear .bss for VP guests

[Boris Ostrovsky <boris.ostrovsky@oracle.com>]

On 02/26/2016 10:22 AM, Roger Pau Monné wrote:
> El 26/2/16 a les 16:10, Boris Ostrovsky ha escrit:
>> On 02/26/2016 09:42 AM, Brian Gerst wrote:
>>> On Fri, Feb 26, 2016 at 8:51 AM, Boris Ostrovsky
>>> <boris.ostrovsky@oracle.com> wrote:
>>>> On 02/26/2016 05:53 AM, Roger Pau Monné wrote:
>>>>> El 25/2/16 a les 16:16, Boris Ostrovsky ha escrit:
>>>>>> PV guests need to have their .bss zeroed out since it is not
>>>>>> guaranteed
>>>>>> to be cleared by Xen's domain builder
>>>>> I guess I'm missing something, but elf_load_image (in libelf-loader.c)
>>>>> seems to be able to clear segments (it will zero the memory between
>>>>> p_paddr + p_filesz and p_paddr + p_memsz) while loading the ELF into
>>>>> memory, so if the program headers are correctly setup the .bss
>>>>> should be
>>>>> zeroed out AFAICT.
>>>> Right, but I don't think this is guaranteed. It's uninitialized data
>>>> so in
>>>> principle it can be anything.
>>>>
>>>> The ELF spec says "the system initializes the data with zero when the
>>>> program begins to run" which I read as it's up to runtime and not the
>>>> loader
>>>> to do so.
>>>>
>>>> And since kernel does it explicitly on baremetal path I think it's a
>>>> good
>>>> idea for PV to do the same.
>>> It does it on bare metal because bzImage is a raw binary image, not ELF.
>> OK, I didn't think about this.
>>
>> But nevertheless, is it guaranteed that .bss is cleared by the loader?
>> My reading of the spec is that it's not.
> I think this is very blur in general. The copy of the spec I have says:
>
> "the system initializes the data with zeros when the program begins to run"
>
> What is "the system" here, Xen or the guest kernel?
>
> Just to be clear, I'm not opposing to this change in any way, but the
> message in patch 1/2 needs to be fixed:
>
> "They have been able to run without problems because Xen domain builder
> happens to give out zeroed pages."
>
> This is wrong IMHO, .bss is not cleared because we are using zeroed
> pages, but because elf_load_image explicitly zeroes the space between
> p_filesz and p_memsz in ELF program headers (which is were .bss resides
> on properly arranged ELF binaries) when loading them.

That's what I meant --- that the builder/loader gives out zeroed pages, 
not that Xen's allocator clears them in general. I'll update the commit 
message.

>
> I'm quite sure NetBSD also relies on this, so I would say it's
> intrinsically part of the Xen boot ABI now, and this change just adds
> seatbelts to Linux.

Maybe NetBSD should drive carefully then ;-)

-boris