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

[Brian Gerst]

On Fri, Feb 26, 2016 at 8:51 AM, Boris Ostrovsky
<boris.ostrovsky@xxxxxxxxxx> wrote:
> On 02/26/2016 05:53 AM, Roger Pau Monnà 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.

--
Brian Gerst