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

[Boris Ostrovsky]

On 02/26/2016 09:42 AM, Brian Gerst wrote:
> 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.

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