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

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Date:	Fri, 26 Feb 2016 10:10:17 -0500
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	Brian Gerst <brgerst@...il.com>
Cc:	Roger Pau Monné <roger.pau@...rix.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	David Vrabel <david.vrabel@...rix.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 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.


-boris