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

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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.

--
Brian Gerst

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[PATCH v3 0/2] Clear .bss for VP guests Boris Ostrovsky <boris.ostrovsky@oracle.com> - 2016-02-25 16:20 +0100
  [PATCH v3 1/2] xen/x86: Zero out .bss for PV guests Boris Ostrovsky <boris.ostrovsky@oracle.com> - 2016-02-25 16:20 +0100
    Re: [PATCH v3 1/2] xen/x86: Zero out .bss for PV guests Brian Gerst <brgerst@gmail.com> - 2016-02-25 17:20 +0100
  [PATCH v3 2/2] xen/x86: Drop mode-selecting ifdefs in startup_xen() Boris Ostrovsky <boris.ostrovsky@oracle.com> - 2016-02-25 16:20 +0100
  Re: [Xen-devel] [PATCH v3 0/2] Clear .bss for VP guests Roger Pau Monné <roger.pau@citrix.com> - 2016-02-26 12:00 +0100
    Re: [Xen-devel] [PATCH v3 0/2] Clear .bss for VP guests Boris Ostrovsky <boris.ostrovsky@oracle.com> - 2016-02-26 15:00 +0100
      Re: [Xen-devel] [PATCH v3 0/2] Clear .bss for VP guests Brian Gerst <brgerst@gmail.com> - 2016-02-26 15:50 +0100
        Re: [Xen-devel] [PATCH v3 0/2] Clear .bss for VP guests Boris Ostrovsky <boris.ostrovsky@oracle.com> - 2016-02-26 16:20 +0100
          Re: [Xen-devel] [PATCH v3 0/2] Clear .bss for VP guests David Vrabel <david.vrabel@citrix.com> - 2016-02-26 16:20 +0100
          Re: [Xen-devel] [PATCH v3 0/2] Clear .bss for VP guests Roger Pau Monné <roger.pau@citrix.com> - 2016-02-26 16:30 +0100
            Re: [Xen-devel] [PATCH v3 0/2] Clear .bss for VP guests David Vrabel <david.vrabel@citrix.com> - 2016-02-26 16:30 +0100
              Re: [Xen-devel] [PATCH v3 0/2] Clear .bss for VP guests Boris Ostrovsky <boris.ostrovsky@oracle.com> - 2016-02-26 16:40 +0100
            Re: [Xen-devel] [PATCH v3 0/2] Clear .bss for VP guests Boris Ostrovsky <boris.ostrovsky@oracle.com> - 2016-02-26 16:40 +0100

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