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

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Date:	Fri, 26 Feb 2016 15:12:04 +0000
From:	David Vrabel <david.vrabel@...rix.com>
To:	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Brian Gerst <brgerst@...il.com>
CC:	Roger Pau Monné <roger.pau@...rix.com>,
	"Konrad Rzeszutek Wilk" <konrad.wilk@...cle.com>,
	<andrew.cooper3@...rix.com>,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
	<mcgrof@...nel.org>, <xen-devel@...ts.xenproject.org>
Subject: Re: [Xen-devel] [PATCH v3 0/2] Clear .bss for VP guests

On 26/02/16 15:10, Boris Ostrovsky wrote:
> On 02/26/2016 09:42 AM, Brian Gerst wrote:
>> On Fri, Feb 26, 2016 at 8:51 AM, Boris Ostrovsky
>> <boris.ostrovsky@...cle.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'm going to apply it once you post a final version with the last
suggestion from Brian.

I'll drop the tag for stable though.

David